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Abstract:
[Objective] The organic-enriched black bedded cherts in the Kuhfeng Formation were deposited in the Lower Yangtze during the Middle Permian. The hydrothermal cherts may be the key part for the relationship analysis between chert deposits and the extra-ordinarily high organic matter enrichment. This would be the basic theories for the shale gas exploration in this area. [Methods] This work uses major, minor and rare-earth elements, combined with the petrology, to analyze the origin of chert and their tectonic setting for the Kuhfeng Formation in the Chaohu and Tongling areas. [Results] The black bedded cherts in the Chaohu area contain abundant siliceous sponge spicules and radiolarians. The cinerous cherts in the Kuhfeng Formation in Tongling area contain very rare siliceous fossil, and consist of micro-quartz. Hydrothermal proxies, such as Al/(Al+Fe+Mn), Eu/Eu*, Al-Fe-Mn diagram and LuN/LaN ratios, indicate a hydrothermal origin and a biotic origin for the chert in the Kuhfeng Formation in Tongling and Chaohu areas, respectively. La-Th-Sc diagram and crossplot between La/Sc and Ti/Zr indicate oceanic island arc and active continental margin for the Tongling and Chaohu areas. [Conclusion] The chert in the Kuhfeng Formation in Tongling area was primary deposited by hydrothermal activity. The silicic acid was brought by hydrothermal vent near the slope environments. The black bedded cherts in the Lower Yangtze area are biotic origin, but the source of silicon was related to the hydrothermal activity. The organic matter enrichment in the Kuhfeng Formation in the Lower Yangtze area may be related to the phytoplankton flourishment induced by the nutrient element input from hydrothermal activity.
[Objective] The organic-enriched black bedded cherts in the Kuhfeng Formation were deposited in the Lower Yangtze during the Middle Permian. The hydrothermal cherts may be the key part for the relationship analysis between chert deposits and the extra-ordinarily high organic matter enrichment. This would be the basic theories for the shale gas exploration in this area. [Methods] This work uses major, minor and rare-earth elements, combined with the petrology, to analyze the origin of chert and their tectonic setting for the Kuhfeng Formation in the Chaohu and Tongling areas. [Results] The black bedded cherts in the Chaohu area contain abundant siliceous sponge spicules and radiolarians. The cinerous cherts in the Kuhfeng Formation in Tongling area contain very rare siliceous fossil, and consist of micro-quartz. Hydrothermal proxies, such as Al/(Al+Fe+Mn), Eu/Eu*, Al-Fe-Mn diagram and LuN/LaN ratios, indicate a hydrothermal origin and a biotic origin for the chert in the Kuhfeng Formation in Tongling and Chaohu areas, respectively. La-Th-Sc diagram and crossplot between La/Sc and Ti/Zr indicate oceanic island arc and active continental margin for the Tongling and Chaohu areas. [Conclusion] The chert in the Kuhfeng Formation in Tongling area was primary deposited by hydrothermal activity. The silicic acid was brought by hydrothermal vent near the slope environments. The black bedded cherts in the Lower Yangtze area are biotic origin, but the source of silicon was related to the hydrothermal activity. The organic matter enrichment in the Kuhfeng Formation in the Lower Yangtze area may be related to the phytoplankton flourishment induced by the nutrient element input from hydrothermal activity.
Abstract:
[Objective]Mixed sediments were widely developed in Lower First Member of Shahejie Formation in Lixian Slope, clarify the mixed sedimentary characteristics and its control on reservoir is of great significance to determines the target area for exploration and development in the next step. [Methods]Based on core, thin section, well logging seismic, as well as laboratory analysis data, the mixed sedimentary types, distribution regularity, control factors, sedimentary model and high-quality reservoir’s formation mechanism in Lower First Member of Shahejie Formation in Lixian Slope were analyzed. [Results]There are two types of mixed sedimentary in the Lower First Member of Shahejie Formation in the study area, mixed sedimentary rock, mixed sedimentary strata, the mixed sedimentary rock can be divided into 6 types in three major categories, mixed sandstones, mixed mudstones and mixed carbonate rocks, the mixed sedimentary strata can be divided into 16 types in four?lithofacies assemblages, terrigenous rock-carbonate rock, terrigenous rock- mixosedimentite, carbonate rock-mixosedimentite, mixosedimentite. The Lower First Member of Shahejie Formation in Lixian Slope have typical mixed features, which can be divided into 8 mixed sedimentary microfacies, including underwater distributary channel, mouth bar sheet underwater diversion bay, mud flat,?sandy beach bar, carbonate beach bar, inner beach and the semi-deep lake mud,?the mixed sediments have the characteristics of rapid phase transformation in the lateral direction, multiple types of rock superimpose frequently in the vertical, the mixed sediments have different distribution characteristics?of different intervals. The mixed sediments were controlled by climatic and water environment, tectonic background and palaeogeomorphology, sediment supply?and lake level changes, two kinds of mixed depositional model are developed,lowstand period and high-stand periods. The mixed sediments plays a significant role in reservoir controlling factors, the sandy beach bar and carbonate beach bar where developed in high-energy facies zone are the favorable mixed sedimentary microfacies, the closed saline water environment is the foundation of reservoir formation, controlled the formation and distribution of high-quality reservoirs, sedimentary cycles controlled the development degree and distribution palace of high-quality reservoirs. [conclusion] the palaeogeomorphology highland in the southwestern are favorable areas for mixed beach bars development and exploration practice.
[Objective]Mixed sediments were widely developed in Lower First Member of Shahejie Formation in Lixian Slope, clarify the mixed sedimentary characteristics and its control on reservoir is of great significance to determines the target area for exploration and development in the next step. [Methods]Based on core, thin section, well logging seismic, as well as laboratory analysis data, the mixed sedimentary types, distribution regularity, control factors, sedimentary model and high-quality reservoir’s formation mechanism in Lower First Member of Shahejie Formation in Lixian Slope were analyzed. [Results]There are two types of mixed sedimentary in the Lower First Member of Shahejie Formation in the study area, mixed sedimentary rock, mixed sedimentary strata, the mixed sedimentary rock can be divided into 6 types in three major categories, mixed sandstones, mixed mudstones and mixed carbonate rocks, the mixed sedimentary strata can be divided into 16 types in four?lithofacies assemblages, terrigenous rock-carbonate rock, terrigenous rock- mixosedimentite, carbonate rock-mixosedimentite, mixosedimentite. The Lower First Member of Shahejie Formation in Lixian Slope have typical mixed features, which can be divided into 8 mixed sedimentary microfacies, including underwater distributary channel, mouth bar sheet underwater diversion bay, mud flat,?sandy beach bar, carbonate beach bar, inner beach and the semi-deep lake mud,?the mixed sediments have the characteristics of rapid phase transformation in the lateral direction, multiple types of rock superimpose frequently in the vertical, the mixed sediments have different distribution characteristics?of different intervals. The mixed sediments were controlled by climatic and water environment, tectonic background and palaeogeomorphology, sediment supply?and lake level changes, two kinds of mixed depositional model are developed,lowstand period and high-stand periods. The mixed sediments plays a significant role in reservoir controlling factors, the sandy beach bar and carbonate beach bar where developed in high-energy facies zone are the favorable mixed sedimentary microfacies, the closed saline water environment is the foundation of reservoir formation, controlled the formation and distribution of high-quality reservoirs, sedimentary cycles controlled the development degree and distribution palace of high-quality reservoirs. [conclusion] the palaeogeomorphology highland in the southwestern are favorable areas for mixed beach bars development and exploration practice.
Abstract:
[Objective] The Qigebrak Formation developed abundant microbial carbonates, which are the favorable target for deep to ultra-deep oil and gas exploration. However, the existing stratigraphic division scheme of the Qigebrak Formation remains controversial, hindering the analysis of reservoir depositional evolution and distribution prediction. Further in-depth research and clarification are urgently needed. [Methods] This study focused on the Shiairik section in the northwest Aksu area of the Tarim Basin to define the depositional environment and sea-level change, and conduct a sequence stratigraphic division of the Qigebrak Formation based on the analysis of high-precision depositional facies and the Fischer plot. [Results] Ten main facies are recognized from the Qigebrak Formation and grouped into a carbonate ramp platform. Two subfacies; i.e., inner and middle ramp, are developed under this setting. The inner ramp consists of the tidal flat, lagoon, tidal channel, and grain shoals. Eight subtypes of peritidal cycles, two subtypes of shallow subtidal cycles and two subtypes of middle-ramp cycles are identified. Based on the stacking patterns reflected in the Fischer diagrams, analysis of orders of depositional facies and proportion of subtidal facies, the Qigebrak is divided into four third-order T-R sequences (SQ1–SQ4). Among these sequences, SQ1 only records the regressive system tract in the Qigebrak Formation, whereas SQ4 only preserves the transgressive system tract. [Conclusions] This study suggests that (1) it is reasonable to divide the Qigebrak Formation into four sequences; (2) the top of the Qigebrak Formation could have experienced the million-year-scale exposure and erosion, which would favor the formation of scaled reservoirs.
[Objective] The Qigebrak Formation developed abundant microbial carbonates, which are the favorable target for deep to ultra-deep oil and gas exploration. However, the existing stratigraphic division scheme of the Qigebrak Formation remains controversial, hindering the analysis of reservoir depositional evolution and distribution prediction. Further in-depth research and clarification are urgently needed. [Methods] This study focused on the Shiairik section in the northwest Aksu area of the Tarim Basin to define the depositional environment and sea-level change, and conduct a sequence stratigraphic division of the Qigebrak Formation based on the analysis of high-precision depositional facies and the Fischer plot. [Results] Ten main facies are recognized from the Qigebrak Formation and grouped into a carbonate ramp platform. Two subfacies; i.e., inner and middle ramp, are developed under this setting. The inner ramp consists of the tidal flat, lagoon, tidal channel, and grain shoals. Eight subtypes of peritidal cycles, two subtypes of shallow subtidal cycles and two subtypes of middle-ramp cycles are identified. Based on the stacking patterns reflected in the Fischer diagrams, analysis of orders of depositional facies and proportion of subtidal facies, the Qigebrak is divided into four third-order T-R sequences (SQ1–SQ4). Among these sequences, SQ1 only records the regressive system tract in the Qigebrak Formation, whereas SQ4 only preserves the transgressive system tract. [Conclusions] This study suggests that (1) it is reasonable to divide the Qigebrak Formation into four sequences; (2) the top of the Qigebrak Formation could have experienced the million-year-scale exposure and erosion, which would favor the formation of scaled reservoirs.
Abstract:
[Objective] The collision of the Arabian-Eurasian plates led to the formation of Zagros orogenic belt and the related foreland basin. The gradual filling of the Zagros foreland basin resulted in the retreat of the Neo-Tethys Ocean in the Zagros region, which causes the transition from the marine to terrestrial environments. This process provides the possible chance to explore the demise of the Neo-tethys Ocean in the Middle East. [Methods] In this study, we conducted detailed stratigraphy, sedimentology, sandstone petrolography, and detrital zircon U-Pb age on the Miocene sedimentary strata in the Lurestan and Khuzestan regions of the Zagros Mountains of southern Iran, which provides new constraints on the filling of the Zagros foreland basin and the regression process of the Neo-Tethys Ocean in the Zagros area. [Results] The Agha Jari Formation in the Lurestan region is characterized by large-scale, medium-to-thick layered sandstones interbedded with mudstones. Sedimentary environment analysis suggests that it has been formed in a fluvial environment. The sandstone petrography shows a lot of metamorphic clasts. In combination of the detrital zircon U-Pb age peak of Jurassic ~170 Ma, it is constrained the the Agha Jari Formation in the Lurestan region has mainly derived from the Sanandaj-Sirjan Zone to the north. The Agha Jari Formation in the Khuzestan area is featured as a large set of thick mudstone beds, thick mudstone interbedded with sandstone, thick sheeted sandstone interbedded with mudstone and sand-mudstone interbedded. The sedimentary environment analysis shows that it has been formed in a transitional deltaic environment. The sandstone modal composition yields the clasts of the sedimentary rocks occurred. The detrital zircon ages show a lot of Mesozoic ages, comparable with the Zagros Fold-and-Thrust Belt. So it is concluded that they have been mainly recycled from the sedimentary strata in Zagros Fold-and-Thrust Belt. Based on the underlying Gachsaran-Mishan formations with composing of shallow marine limestone and evaporite, the Agha Jari Formation represents the regional youngest marine sedimentation, recording the transition from the marine to the terrestrial environments. The calcareous nannofossil analysis from the underlying Mishan Formation yield the youngest age of the late Miocene. In consideration of the other biostratigraphic studies, the age of the Agha Jari Formation is constrained by no earlier than the Miocene. Therefore, in according to the sedimentary and biostratigraphic studies on the Agha Jari Formation in the Khuzestan region, the demise of the Neo-tethyan Ocean in the Zagros region has been constrained by no earlier than the Miocene and been shown the trend from the northwest to the southeast. [Conclusions] Combined with the analysis of regional paleogeography and provenance analysis on the Miocene strata in the Zagros orogenic belt, the gradual uplift of the Zagros orogenic belt has occurred after the Arabia-Eurasia collision. The uplifted Zagros orogenic belt has been eroded so as to provide the clastic materials for the Zagros foreland basin. As a result, the regression of the Neo-Tethys Ocean in the Zagros area is generally controlled by the continual filling in the foreland basin. This paleogeography of northwest-southeast regression of the Neo-tethyan Ocean has been existed no later than the early Miocene.
[Objective] The collision of the Arabian-Eurasian plates led to the formation of Zagros orogenic belt and the related foreland basin. The gradual filling of the Zagros foreland basin resulted in the retreat of the Neo-Tethys Ocean in the Zagros region, which causes the transition from the marine to terrestrial environments. This process provides the possible chance to explore the demise of the Neo-tethys Ocean in the Middle East. [Methods] In this study, we conducted detailed stratigraphy, sedimentology, sandstone petrolography, and detrital zircon U-Pb age on the Miocene sedimentary strata in the Lurestan and Khuzestan regions of the Zagros Mountains of southern Iran, which provides new constraints on the filling of the Zagros foreland basin and the regression process of the Neo-Tethys Ocean in the Zagros area. [Results] The Agha Jari Formation in the Lurestan region is characterized by large-scale, medium-to-thick layered sandstones interbedded with mudstones. Sedimentary environment analysis suggests that it has been formed in a fluvial environment. The sandstone petrography shows a lot of metamorphic clasts. In combination of the detrital zircon U-Pb age peak of Jurassic ~170 Ma, it is constrained the the Agha Jari Formation in the Lurestan region has mainly derived from the Sanandaj-Sirjan Zone to the north. The Agha Jari Formation in the Khuzestan area is featured as a large set of thick mudstone beds, thick mudstone interbedded with sandstone, thick sheeted sandstone interbedded with mudstone and sand-mudstone interbedded. The sedimentary environment analysis shows that it has been formed in a transitional deltaic environment. The sandstone modal composition yields the clasts of the sedimentary rocks occurred. The detrital zircon ages show a lot of Mesozoic ages, comparable with the Zagros Fold-and-Thrust Belt. So it is concluded that they have been mainly recycled from the sedimentary strata in Zagros Fold-and-Thrust Belt. Based on the underlying Gachsaran-Mishan formations with composing of shallow marine limestone and evaporite, the Agha Jari Formation represents the regional youngest marine sedimentation, recording the transition from the marine to the terrestrial environments. The calcareous nannofossil analysis from the underlying Mishan Formation yield the youngest age of the late Miocene. In consideration of the other biostratigraphic studies, the age of the Agha Jari Formation is constrained by no earlier than the Miocene. Therefore, in according to the sedimentary and biostratigraphic studies on the Agha Jari Formation in the Khuzestan region, the demise of the Neo-tethyan Ocean in the Zagros region has been constrained by no earlier than the Miocene and been shown the trend from the northwest to the southeast. [Conclusions] Combined with the analysis of regional paleogeography and provenance analysis on the Miocene strata in the Zagros orogenic belt, the gradual uplift of the Zagros orogenic belt has occurred after the Arabia-Eurasia collision. The uplifted Zagros orogenic belt has been eroded so as to provide the clastic materials for the Zagros foreland basin. As a result, the regression of the Neo-Tethys Ocean in the Zagros area is generally controlled by the continual filling in the foreland basin. This paleogeography of northwest-southeast regression of the Neo-tethyan Ocean has been existed no later than the early Miocene.
Abstract:
Near-source sedimentary system of member 1 of Shawan formation is the main reservoir in Chepaizi area of Junggar basin. Neverthless, there have been different points on depositional characteristics of it. On the basis of previous studies， using the mud logging data， core data， logging data and seismic data, combining with modern fan delta sediment, the depositional characteristics of near-source sedimentary system within the 4th sequence stratigraphic framework of member 1 of Shanwan formation were investigated, and the sedimentary model of it was built up. The results are as follows: (1) Near-source system was made of two complex fan deltas, one is shoal complex fan deltas, the other is complex fan deltas. The main microfacies were distributary and sand bar of fan delta front. (2) The lithology of the distributary was changed from shoreline to lake by conglomerate - glutenite –medium-coarse lithic sandstone bearing conglomerate - medium-coarse feldspar lithic sandstone bearing conglomerate - feldspar lithic fine sandstone bearing conglomerate. The lithic composition was dominated by igneous lithic, while the gravel composition was dominated by igneous gravel and mud-gravel. (3) Near-source system of sand group 1 of member 1 of Shawan formation mainly distributed below the third slope break zone. It reached in the the middle of the project where the far-source system reached as well, and they interacted with each other. Lacustrine range was the smallest during this stage. Near-source system of sand group 2 of member 1 of Shawan formation mainly distributed above the slope break zone and the lacustrine range was the largest this period. Near-source system of sand group 3 of member 1 of Shawan formation mainly distributed near the slope break zone. The slope of the eastern wing was steeper, the gully scale was smaller, the complex fan delta scale was smaller, the slope of the western wing was slower, the valley scale was larger, the composite fan delta scale was larger. The conclusion is that paleogeomorphology and the relative lacustrine level change controlled the sedimentary characteristics of the near-source systems.
Near-source sedimentary system of member 1 of Shawan formation is the main reservoir in Chepaizi area of Junggar basin. Neverthless, there have been different points on depositional characteristics of it. On the basis of previous studies， using the mud logging data， core data， logging data and seismic data, combining with modern fan delta sediment, the depositional characteristics of near-source sedimentary system within the 4th sequence stratigraphic framework of member 1 of Shanwan formation were investigated, and the sedimentary model of it was built up. The results are as follows: (1) Near-source system was made of two complex fan deltas, one is shoal complex fan deltas, the other is complex fan deltas. The main microfacies were distributary and sand bar of fan delta front. (2) The lithology of the distributary was changed from shoreline to lake by conglomerate - glutenite –medium-coarse lithic sandstone bearing conglomerate - medium-coarse feldspar lithic sandstone bearing conglomerate - feldspar lithic fine sandstone bearing conglomerate. The lithic composition was dominated by igneous lithic, while the gravel composition was dominated by igneous gravel and mud-gravel. (3) Near-source system of sand group 1 of member 1 of Shawan formation mainly distributed below the third slope break zone. It reached in the the middle of the project where the far-source system reached as well, and they interacted with each other. Lacustrine range was the smallest during this stage. Near-source system of sand group 2 of member 1 of Shawan formation mainly distributed above the slope break zone and the lacustrine range was the largest this period. Near-source system of sand group 3 of member 1 of Shawan formation mainly distributed near the slope break zone. The slope of the eastern wing was steeper, the gully scale was smaller, the complex fan delta scale was smaller, the slope of the western wing was slower, the valley scale was larger, the composite fan delta scale was larger. The conclusion is that paleogeomorphology and the relative lacustrine level change controlled the sedimentary characteristics of the near-source systems.
Abstract:
The dispute in paleogeographic pattern of the carbonate platform and distribution of reservoir facies during the deposition of the Dengying Formation in the middle-upper area, hinders the expansion of the exploration domain from the Mianyang-Changning intracratonic sag to other areas. Thus, this study investigates more than 30 sections (wells), and further confirms that the shoal-reef is marked by massive peloidal/ooidal dolograinstone and columnar, domal dolostromatolite, which were deposited around the platform (or inner ramp) margin of the middle-upper Yangtze Platform gradually shifting outward into tempestite of the middle-outer ramp or slump dolobreccia, argillaceous dolomite and chert of slope-basin facies. Due to the shoal-reef barrier, its back was dominated by tidal flat and lagoon. Of these, the tidal flat facies were marked by microbial dololaminite, domal dolostromatolite, dolothrombolite with minor peloidal dolograinstone and dolomudstone, which are commonly arranged into meter-centimeter scale cycles. The lagoon facies were composed mainly of dolomudstone and peloidal dolowackestone. The spatio-temporal distributions of these facies show a generally progradational trend of the Dengying Formation on the middle-upper Yangtze Platform, which consists of 2.5 depositional sequences with 3 progradations (shallowing) and 2 retrogradations (deepening). Owing to the denuation of the Dengying Formation at the platform margin-slope area, and upper Yangtze Platform beyond the current plate boundary, the platform marginal shoal-reef facies are locally present in the Deng 4 Member. In this case, some areas (e.g., Songlin and the periphery of the middle Yangtze Platform) with platform marginal shoal-reef facies in the lower-middle Dengying Formation (Deng 1 to 2 or Hamajing to Shibantan members), are important exploration targets. Moreover, the lower Deng 2 Member and upper Deng 4 Member (or coeval strata) formed during progradations composed mainly of microbial dolomite throughout the middle-upper Yangtze Platform (or inner ramp) interior, are also important exploration targets. Because the microbial dolomite was widely developed throughout the middle-upper Yangtze Platform interior, decoding the formation mechanism and distribution pattern of high-quality reservoir is the key to find new hydrocarbon provinces.
The dispute in paleogeographic pattern of the carbonate platform and distribution of reservoir facies during the deposition of the Dengying Formation in the middle-upper area, hinders the expansion of the exploration domain from the Mianyang-Changning intracratonic sag to other areas. Thus, this study investigates more than 30 sections (wells), and further confirms that the shoal-reef is marked by massive peloidal/ooidal dolograinstone and columnar, domal dolostromatolite, which were deposited around the platform (or inner ramp) margin of the middle-upper Yangtze Platform gradually shifting outward into tempestite of the middle-outer ramp or slump dolobreccia, argillaceous dolomite and chert of slope-basin facies. Due to the shoal-reef barrier, its back was dominated by tidal flat and lagoon. Of these, the tidal flat facies were marked by microbial dololaminite, domal dolostromatolite, dolothrombolite with minor peloidal dolograinstone and dolomudstone, which are commonly arranged into meter-centimeter scale cycles. The lagoon facies were composed mainly of dolomudstone and peloidal dolowackestone. The spatio-temporal distributions of these facies show a generally progradational trend of the Dengying Formation on the middle-upper Yangtze Platform, which consists of 2.5 depositional sequences with 3 progradations (shallowing) and 2 retrogradations (deepening). Owing to the denuation of the Dengying Formation at the platform margin-slope area, and upper Yangtze Platform beyond the current plate boundary, the platform marginal shoal-reef facies are locally present in the Deng 4 Member. In this case, some areas (e.g., Songlin and the periphery of the middle Yangtze Platform) with platform marginal shoal-reef facies in the lower-middle Dengying Formation (Deng 1 to 2 or Hamajing to Shibantan members), are important exploration targets. Moreover, the lower Deng 2 Member and upper Deng 4 Member (or coeval strata) formed during progradations composed mainly of microbial dolomite throughout the middle-upper Yangtze Platform (or inner ramp) interior, are also important exploration targets. Because the microbial dolomite was widely developed throughout the middle-upper Yangtze Platform interior, decoding the formation mechanism and distribution pattern of high-quality reservoir is the key to find new hydrocarbon provinces.
Abstract:
The saturated hydrocarbons of 26 coal measure source rock samples in the Upper Paleozoic of Ordos Basin were analyzed by gas chromatography and chromatography-mass spectrometry. According to the distribution characteristics of tricyclic terpanes (TT), the source rock samples in the study area were divided into three different types of distribution patterns. The C19TT is the main peak of the type I source rock, and the content of C19TT, C20TT and C21TT gradually decreases. At the same time, the Pr/Ph ratio is high and the content of C24 tetracyclic terpane is also rich in this type of sample. This kind of samples are mainly humic coal and carbonaceous mudstone, revealing the characteristics of lacustrine facies sedimentary environment. On the contrary, the tricyclic terpanes of type II source rocks are dominated by C23TT, and the contents of C19TT, C20TT and C21TT are gradually increasing, while the Pr/Ph ratio is low, and the content of C24 tetracyclic terpane is not abundant. In addition to coal-bearing mudstone, there are also humic coal and carbonaceous mudstone in these samples, and their sedimentary environment is quite different from that of type I source rocks. The distribution of tricyclic terpanes in type III source rocks is between type I and type II source rocks. On the one hand, the main peak carbon of tricyclic terpane is C23TT, on the other hand, the relative content of C19TT, C20TT and C21TT decreases in turn and shows the characteristics of stepwise distribution. In addition, the Pr/Ph ratio and the content of C24 tetracyclic terpane are between type I and type II source rocks. The results also reveal that with the increase of the thermal evolution degree of source rocks, the total amount of tricyclic terpanes shows an gradually increasing trend, but the distribution patterns of tricyclic terpanes in different types of source rocks have not changed significantly, in other words, the different distribution patterns of tricyclic terpanes in coal-bearing source rocks in the study area are less affected by the thermal evolution degree of organic matter, but mainly controlled by sedimentary environment and parent material type.
The saturated hydrocarbons of 26 coal measure source rock samples in the Upper Paleozoic of Ordos Basin were analyzed by gas chromatography and chromatography-mass spectrometry. According to the distribution characteristics of tricyclic terpanes (TT), the source rock samples in the study area were divided into three different types of distribution patterns. The C19TT is the main peak of the type I source rock, and the content of C19TT, C20TT and C21TT gradually decreases. At the same time, the Pr/Ph ratio is high and the content of C24 tetracyclic terpane is also rich in this type of sample. This kind of samples are mainly humic coal and carbonaceous mudstone, revealing the characteristics of lacustrine facies sedimentary environment. On the contrary, the tricyclic terpanes of type II source rocks are dominated by C23TT, and the contents of C19TT, C20TT and C21TT are gradually increasing, while the Pr/Ph ratio is low, and the content of C24 tetracyclic terpane is not abundant. In addition to coal-bearing mudstone, there are also humic coal and carbonaceous mudstone in these samples, and their sedimentary environment is quite different from that of type I source rocks. The distribution of tricyclic terpanes in type III source rocks is between type I and type II source rocks. On the one hand, the main peak carbon of tricyclic terpane is C23TT, on the other hand, the relative content of C19TT, C20TT and C21TT decreases in turn and shows the characteristics of stepwise distribution. In addition, the Pr/Ph ratio and the content of C24 tetracyclic terpane are between type I and type II source rocks. The results also reveal that with the increase of the thermal evolution degree of source rocks, the total amount of tricyclic terpanes shows an gradually increasing trend, but the distribution patterns of tricyclic terpanes in different types of source rocks have not changed significantly, in other words, the different distribution patterns of tricyclic terpanes in coal-bearing source rocks in the study area are less affected by the thermal evolution degree of organic matter, but mainly controlled by sedimentary environment and parent material type.
Abstract:
Abstract: The Liushagang formation is one of the petroliferous reservoirs in the Fushan Sag, Beibuwan Basin. However, the controlling factors of reservoir quality are unclear and there are no matched prediction technology for reservoir quality, which restrict the process of oil and gas exploration and development. In our study, the controlling factors of reservoir quality are unraveled from the perspective of “three-element controlling reservoir” including sedimentary microfacies, diagenetic facies and fracture facies. In addition, the methods of logging characterization and identification standards are established. The results show that the sedimentary microfacies, diagenetic facies and fractures play important role in controlling reservoir quality in the Liushagang formation. By analyzing the relationships between different types of sedimentary microfacies, diagenetic facies, and fracture facies and parameters of reservoir quality, it can be concluded that the high reservoir quality reservoirs are formed in the dissolution facies in the underwater distributary channel and mouth bad microfacies under high-energy depositional environment. Additionally, the development of fractures has a significant improvement on reservoir quality. By integrating the three controlling factors (sedimentary microfacies + diagenetic facies + fracture facies) and combined with four important reservoir characteristic parameters, four types of reservoirs are classified. The conclusion of oil and gas interpretation and oil test data prove that the proposed method can effectively evaluate and predict the favorable reservoir layers. This research can provide theoretical guidance for the increase of reservoirs and production in the Liushagang formation in Fushan Sag, and provide insights into the fine characterization of reservoirs and the prediction and evaluation of high-quality reservoirs.
Abstract: The Liushagang formation is one of the petroliferous reservoirs in the Fushan Sag, Beibuwan Basin. However, the controlling factors of reservoir quality are unclear and there are no matched prediction technology for reservoir quality, which restrict the process of oil and gas exploration and development. In our study, the controlling factors of reservoir quality are unraveled from the perspective of “three-element controlling reservoir” including sedimentary microfacies, diagenetic facies and fracture facies. In addition, the methods of logging characterization and identification standards are established. The results show that the sedimentary microfacies, diagenetic facies and fractures play important role in controlling reservoir quality in the Liushagang formation. By analyzing the relationships between different types of sedimentary microfacies, diagenetic facies, and fracture facies and parameters of reservoir quality, it can be concluded that the high reservoir quality reservoirs are formed in the dissolution facies in the underwater distributary channel and mouth bad microfacies under high-energy depositional environment. Additionally, the development of fractures has a significant improvement on reservoir quality. By integrating the three controlling factors (sedimentary microfacies + diagenetic facies + fracture facies) and combined with four important reservoir characteristic parameters, four types of reservoirs are classified. The conclusion of oil and gas interpretation and oil test data prove that the proposed method can effectively evaluate and predict the favorable reservoir layers. This research can provide theoretical guidance for the increase of reservoirs and production in the Liushagang formation in Fushan Sag, and provide insights into the fine characterization of reservoirs and the prediction and evaluation of high-quality reservoirs.
Abstract:
Qiangtang Basin, located in the most important oil & gas accumulation belt worldwide - eastern section of Tethys tectonic domain, has been listed as a strategic preparation area for oil & gas resources. However, there had been great arguments on the potential of oil & gas resources, which restricted the overall understanding of oil & gas exploration in Qiangtang Basin. Restoring the denudation history of basin in key tectonic periods and analyzing the burial history of source rocks is of great significance to deepen the hydrocarbon potential of leading source rocks and oil & gas resources in Qiangtang Basin. Based on the current data and previous research results, the main tectonic activities were analyzed and the denudation history of Qiangtang Basin in key tectonic periods were restored by using the structural-sedimentary extrapolation method. At the same time, the burial history of three sets of leading source beds in the Upper Triassic-Jurassic was analyzed by using the TSM basin simulation system. The results showed that: 1) Since the Late Triassic, Qiangtang basin had experienced four periods of tectonic movements: one period from the late Triassic to early Jurassic (210-180Ma)、the late Early Cretaceous (120-110Ma) and the Paleocene to the early Eocene (60-45Ma) and since the early Miocene; 2) At the first period (210-110Ma), Most areas of the basin was uplifted to the surface. The top of the Late Triassic Formation was denuded to form an ancient weathering crust, and then the volcanic rocks were deposited. The central uplift zone of the basin and the Northern Qiangtang were denuded most strongly; Between 120 and 110Ma, the basin strata were folded strongly, and the denudation was most intense in the central uplift zone and its two sides, and in the eastern part of the basin. The denudation is relatively weak in the middle and west of the North Qiangtang depression, and the south of the South Qiangtang. From 60 to 45 Ma, influenced by the long-range effect of the collision between the Indian continent and the Asian continent, the basin shortening rate is 11.9%, and the average uplift and denuded is about 700m. Since 23Ma, the basin continues to be affected by the north-south extrusion stress, and the whole basin escapes to the southeast direction. The basin's thrust faults continue to be active generally and more uniformly, and a number of normal faults or grabens were developed. The uplift rate of all parts of the basin is similar, and the uplift erosion is about 700m to some extent. 3) Influenced by the sedimentary thickness of the stratum and the differential denudation of multi-stage tectonic uplift, the maximum burial of the Upper Triassic Jurassic source beds in Qiangtang Basin occurred twice after the deposition of the J3-K1 Xueshan Formation and after the deposition of the Neogene Nkangtuo Formation and Suonahu Formation, respectively. The two main hydrocarbon generation periods corresponded to the two maximum burial depths and the subsequent tectonic uplift.
Qiangtang Basin, located in the most important oil & gas accumulation belt worldwide - eastern section of Tethys tectonic domain, has been listed as a strategic preparation area for oil & gas resources. However, there had been great arguments on the potential of oil & gas resources, which restricted the overall understanding of oil & gas exploration in Qiangtang Basin. Restoring the denudation history of basin in key tectonic periods and analyzing the burial history of source rocks is of great significance to deepen the hydrocarbon potential of leading source rocks and oil & gas resources in Qiangtang Basin. Based on the current data and previous research results, the main tectonic activities were analyzed and the denudation history of Qiangtang Basin in key tectonic periods were restored by using the structural-sedimentary extrapolation method. At the same time, the burial history of three sets of leading source beds in the Upper Triassic-Jurassic was analyzed by using the TSM basin simulation system. The results showed that: 1) Since the Late Triassic, Qiangtang basin had experienced four periods of tectonic movements: one period from the late Triassic to early Jurassic (210-180Ma)、the late Early Cretaceous (120-110Ma) and the Paleocene to the early Eocene (60-45Ma) and since the early Miocene; 2) At the first period (210-110Ma), Most areas of the basin was uplifted to the surface. The top of the Late Triassic Formation was denuded to form an ancient weathering crust, and then the volcanic rocks were deposited. The central uplift zone of the basin and the Northern Qiangtang were denuded most strongly; Between 120 and 110Ma, the basin strata were folded strongly, and the denudation was most intense in the central uplift zone and its two sides, and in the eastern part of the basin. The denudation is relatively weak in the middle and west of the North Qiangtang depression, and the south of the South Qiangtang. From 60 to 45 Ma, influenced by the long-range effect of the collision between the Indian continent and the Asian continent, the basin shortening rate is 11.9%, and the average uplift and denuded is about 700m. Since 23Ma, the basin continues to be affected by the north-south extrusion stress, and the whole basin escapes to the southeast direction. The basin's thrust faults continue to be active generally and more uniformly, and a number of normal faults or grabens were developed. The uplift rate of all parts of the basin is similar, and the uplift erosion is about 700m to some extent. 3) Influenced by the sedimentary thickness of the stratum and the differential denudation of multi-stage tectonic uplift, the maximum burial of the Upper Triassic Jurassic source beds in Qiangtang Basin occurred twice after the deposition of the J3-K1 Xueshan Formation and after the deposition of the Neogene Nkangtuo Formation and Suonahu Formation, respectively. The two main hydrocarbon generation periods corresponded to the two maximum burial depths and the subsequent tectonic uplift.
Abstract:
[Objective] In the western Sichuan Basin, the Middle Permian extensively features marine carbonate rocks, with dolomite being a focal point of geological investigation. However, the diverse and irregular distribution of dolomite types in the Middle Permian results in significant variations in dolomite characteristics across different regions. The rich variety of dolomite types described above constitutes an excellent set of natural gas reservoirs in the Sichuan Basin. [Methodology] To unravel the fluid dynamics of dolomites in the study area and reconstruct their diagenetic environments, the study extensively reviewed a substantial body of previous literature and references. samples from 16 well cores and 9 sections underwent a comprehensive analysis. Utilizing techniques such as microscopic thin section observation, cathodoluminescence, carbon-oxygen isotopes, strontium isotopes, and ICP-MS rare earth element analysis, the petrological and geochemical features were thoroughly investigated. [Results] The research findings can be summarized as follows: (1) Dolomite Types: Dolomites in the region can be broadly categorized into homogeneous dolomite and zebra-like dolomite. The primary type of homogeneous dolomite is granular dolomite, while zebra-like dolomite includes homogeneous dolomite with dark bands, predominantly filled with hydrothermal saddle dolomite. (2) Isotopic Analysis: Carbon Isotopes: Samples from the study area exhibit positive anomalies in carbon isotopes. Oxygen Isotopes: Oxygen isotope values show significant differences, with the filling material (saddle dolomite) exhibiting notably lower values than homogeneous dolomite. Oxygen isotope values in samples from southwestern Sichuan are significantly lower than those from northwestern Sichuan. (3) Rare Earth Elements: Rare earth element analysis reveals a negative anomaly in δCe and a positive anomaly in δEu, indicating that the oxidation conditions of the products were influenced by later-stage hydrothermal alteration. (4) Strontium Isotopes: Strontium isotopic values of homogeneous dolomite fall within the range of contemporaneous seawater. However, the filling material in southwestern Sichuan exhibits strontium isotopic values higher than the seawater range and significantly higher than homogeneous dolomite. [Conclusion] (1) Hydrothermal Modification: The diagenetic fluids responsible for the formation of homogeneous dolomite in the western to northern Sichuan region are primarily derived from contemporaneous seawater. Subsequent hydrothermal activities lead to modifications, resulting in the formation of hydrothermal saddle dolomite. Notably, the intensity of hydrothermal activity is more pronounced in the southwestern region and relatively weaker in the northwestern part of western Sichuan. (2) Diagenetic Environments: The diagenetic environments of dolomites in the study area encompass four types: marine diagenetic settings, shallow-to-intermediate burial diagenetic environments, and intermediate-to-deep burial diagenetic environments. The marine environment refers to an open-sea, grain shoal environment, predominantly developing fine to medium-crystalline dolomite. Inclusions exhibit a uniform temperature below 85°C. Shallow-to-intermediate burial environments, with burial depths ranging from approximately 800m to 2000m, primarily foster homogeneous fine-crystalline dolomite and some medium-to-coarse-crystalline dolomite. Inclusions exhibit a uniform temperature above 112°C. Hydrothermal saddle dolomite mainly develops in intermediate-to-deep burial environments with burial depths exceeding 3000m, and inclusions exhibit a uniform temperature above 175°C. This comprehensive research provides nuanced insights into the diverse dolomite types and their diagenetic histories, contributing significantly to the broader understanding of sedimentary processes and geological evolution in the western Sichuan Basin during the Middle Permian period.
[Objective] In the western Sichuan Basin, the Middle Permian extensively features marine carbonate rocks, with dolomite being a focal point of geological investigation. However, the diverse and irregular distribution of dolomite types in the Middle Permian results in significant variations in dolomite characteristics across different regions. The rich variety of dolomite types described above constitutes an excellent set of natural gas reservoirs in the Sichuan Basin. [Methodology] To unravel the fluid dynamics of dolomites in the study area and reconstruct their diagenetic environments, the study extensively reviewed a substantial body of previous literature and references. samples from 16 well cores and 9 sections underwent a comprehensive analysis. Utilizing techniques such as microscopic thin section observation, cathodoluminescence, carbon-oxygen isotopes, strontium isotopes, and ICP-MS rare earth element analysis, the petrological and geochemical features were thoroughly investigated. [Results] The research findings can be summarized as follows: (1) Dolomite Types: Dolomites in the region can be broadly categorized into homogeneous dolomite and zebra-like dolomite. The primary type of homogeneous dolomite is granular dolomite, while zebra-like dolomite includes homogeneous dolomite with dark bands, predominantly filled with hydrothermal saddle dolomite. (2) Isotopic Analysis: Carbon Isotopes: Samples from the study area exhibit positive anomalies in carbon isotopes. Oxygen Isotopes: Oxygen isotope values show significant differences, with the filling material (saddle dolomite) exhibiting notably lower values than homogeneous dolomite. Oxygen isotope values in samples from southwestern Sichuan are significantly lower than those from northwestern Sichuan. (3) Rare Earth Elements: Rare earth element analysis reveals a negative anomaly in δCe and a positive anomaly in δEu, indicating that the oxidation conditions of the products were influenced by later-stage hydrothermal alteration. (4) Strontium Isotopes: Strontium isotopic values of homogeneous dolomite fall within the range of contemporaneous seawater. However, the filling material in southwestern Sichuan exhibits strontium isotopic values higher than the seawater range and significantly higher than homogeneous dolomite. [Conclusion] (1) Hydrothermal Modification: The diagenetic fluids responsible for the formation of homogeneous dolomite in the western to northern Sichuan region are primarily derived from contemporaneous seawater. Subsequent hydrothermal activities lead to modifications, resulting in the formation of hydrothermal saddle dolomite. Notably, the intensity of hydrothermal activity is more pronounced in the southwestern region and relatively weaker in the northwestern part of western Sichuan. (2) Diagenetic Environments: The diagenetic environments of dolomites in the study area encompass four types: marine diagenetic settings, shallow-to-intermediate burial diagenetic environments, and intermediate-to-deep burial diagenetic environments. The marine environment refers to an open-sea, grain shoal environment, predominantly developing fine to medium-crystalline dolomite. Inclusions exhibit a uniform temperature below 85°C. Shallow-to-intermediate burial environments, with burial depths ranging from approximately 800m to 2000m, primarily foster homogeneous fine-crystalline dolomite and some medium-to-coarse-crystalline dolomite. Inclusions exhibit a uniform temperature above 112°C. Hydrothermal saddle dolomite mainly develops in intermediate-to-deep burial environments with burial depths exceeding 3000m, and inclusions exhibit a uniform temperature above 175°C. This comprehensive research provides nuanced insights into the diverse dolomite types and their diagenetic histories, contributing significantly to the broader understanding of sedimentary processes and geological evolution in the western Sichuan Basin during the Middle Permian period.
Abstract:
[Objective] γ-carotane and β-carotane belong to carotenoid-derived alkanes, which are widely distributed in sediments and crude oil, but carotenoid-derived alkanes with low carbon number are rare in the study of source rocks. The purpose of this study is to explore the sources of γ-carotane，β-carotane and carotenoid-derived alkanes with low carbon number, indicating significance in geological carriers. [Methods] 12 representative source rock samples are from the well Malu1 in the Lucaogou Formation of the Malang Sag,and Santanghu Basin were collected. The geochemical characteristics were demonstrated in detail by Gas Chromatography-Mass Spectrometry, Rock Pyrolysis, Vitrinite Reflectance (Ro) and Total Organic Carbon (TOC) analysis, so as to obtain the biogenic information of the target compound. [Results] The source rocks of Lucaogou Formation from the Well Malu 1 are rich in organic matter, good in type and in mature stage.Moreover ,the source rocks are rich in γ- and β-carotane with high abundance and have carotenoid-derived alkanes with low carbon number (Carbon number ranges from C13 to C25). Based on the comprehensive analysis of the distribution characteristics of a series of biomarkers such as n-alkanes, terpanes and steranes and other geochemical parameters, it is considered that bacteria may be the main biological precursors of γ- and β-carotane. In addition, combining the evidence of thermal evolution of hydrocarbon source rocks, microbial fossils and volcanic eruptions, the present study also proposes that the carotenoid-derived alkanes with low carbon number may be the products of γ- and β-carotane affected by microorganisms or thermal evolution.This study can provide new ideas for the exploration of source rocks.
[Objective] γ-carotane and β-carotane belong to carotenoid-derived alkanes, which are widely distributed in sediments and crude oil, but carotenoid-derived alkanes with low carbon number are rare in the study of source rocks. The purpose of this study is to explore the sources of γ-carotane，β-carotane and carotenoid-derived alkanes with low carbon number, indicating significance in geological carriers. [Methods] 12 representative source rock samples are from the well Malu1 in the Lucaogou Formation of the Malang Sag,and Santanghu Basin were collected. The geochemical characteristics were demonstrated in detail by Gas Chromatography-Mass Spectrometry, Rock Pyrolysis, Vitrinite Reflectance (Ro) and Total Organic Carbon (TOC) analysis, so as to obtain the biogenic information of the target compound. [Results] The source rocks of Lucaogou Formation from the Well Malu 1 are rich in organic matter, good in type and in mature stage.Moreover ,the source rocks are rich in γ- and β-carotane with high abundance and have carotenoid-derived alkanes with low carbon number (Carbon number ranges from C13 to C25). Based on the comprehensive analysis of the distribution characteristics of a series of biomarkers such as n-alkanes, terpanes and steranes and other geochemical parameters, it is considered that bacteria may be the main biological precursors of γ- and β-carotane. In addition, combining the evidence of thermal evolution of hydrocarbon source rocks, microbial fossils and volcanic eruptions, the present study also proposes that the carotenoid-derived alkanes with low carbon number may be the products of γ- and β-carotane affected by microorganisms or thermal evolution.This study can provide new ideas for the exploration of source rocks.
Abstract:
The dolomite evaporite symbiotic system is widely distributed during the Middle Triassic period in the Sichuan Basin, but its sedimentary characteristics lack systematic characterization. The development and distribution patterns and main controlling factors are still unclear. Based on drilling, core, and seismic data, this article characterizes the sedimentary characteristics of the developed dolomite evaporite symbiotic system in the study area at multiple scales, elucidates the genetic mechanisms of different types of symbiotic systems, and further reveals their spatiotemporal distribution patterns and main controlling factors. The research results indicate that: (1) Four types of symbiotic systems are mainly developed in the Leikoupo Formation in the northwest Sichuan Basin: thick layer of dolomite with thin layer of evaporite, interlayer of dolomite and evaporite, thick layer of evaporite with thin layer of dolomite, and overlapping of thick layer of evaporite with thick layer of dolomite. They are respectively formed in gypsum containing lagoons, cloud containing gypsum containing lagoons, gypsum salt lakes, and gypsum salt basins. Among them, thick layer of dolomite mixed with thin layer of evaporite is widely distributed in the four periods of Leikoupo. Thick layer of evaporite mixed with thin layer of dolomite, and the overlap of thick layer of evaporite and thick layer of dolomite is the most developed in the central part of the Leisan and Leisi sedimentary periods of the basin. (3) The uplift of the Longmenshan Island Chain on the west side resulted in southeast compression of the basin, while the uplift of the Xuefeng Mountain on the east side restricted the southern migration of the Luzhou Kaijiang ancient uplift. The southern subduction of the Qinling Mountains jointly formed a nearly northeast southwest structural pattern in the Sichuan Basin, controlling the overall distribution of gypsum and symbiotic systems in a northeast southwest direction. (4) Affected by the significant uplift of the Luzhou Kaijiang ancient uplift, the later subsidence center of the Leikoupo Formation migrated to the west, and the gypsum sedimentary center and four symbiotic systems correspondingly developed from early dispersion to later concentration and migrated to the west. Under the constraints of different types of symbiotic systems, the paleogeography of the Leikoupo Formation was reconstructed for four periods. The above research provides a new understanding for the study of the symbiotic system of dolomite evaporite in the northwest Sichuan region, and also has a guiding role in the reconstruction of regional paleogeography.
The dolomite evaporite symbiotic system is widely distributed during the Middle Triassic period in the Sichuan Basin, but its sedimentary characteristics lack systematic characterization. The development and distribution patterns and main controlling factors are still unclear. Based on drilling, core, and seismic data, this article characterizes the sedimentary characteristics of the developed dolomite evaporite symbiotic system in the study area at multiple scales, elucidates the genetic mechanisms of different types of symbiotic systems, and further reveals their spatiotemporal distribution patterns and main controlling factors. The research results indicate that: (1) Four types of symbiotic systems are mainly developed in the Leikoupo Formation in the northwest Sichuan Basin: thick layer of dolomite with thin layer of evaporite, interlayer of dolomite and evaporite, thick layer of evaporite with thin layer of dolomite, and overlapping of thick layer of evaporite with thick layer of dolomite. They are respectively formed in gypsum containing lagoons, cloud containing gypsum containing lagoons, gypsum salt lakes, and gypsum salt basins. Among them, thick layer of dolomite mixed with thin layer of evaporite is widely distributed in the four periods of Leikoupo. Thick layer of evaporite mixed with thin layer of dolomite, and the overlap of thick layer of evaporite and thick layer of dolomite is the most developed in the central part of the Leisan and Leisi sedimentary periods of the basin. (3) The uplift of the Longmenshan Island Chain on the west side resulted in southeast compression of the basin, while the uplift of the Xuefeng Mountain on the east side restricted the southern migration of the Luzhou Kaijiang ancient uplift. The southern subduction of the Qinling Mountains jointly formed a nearly northeast southwest structural pattern in the Sichuan Basin, controlling the overall distribution of gypsum and symbiotic systems in a northeast southwest direction. (4) Affected by the significant uplift of the Luzhou Kaijiang ancient uplift, the later subsidence center of the Leikoupo Formation migrated to the west, and the gypsum sedimentary center and four symbiotic systems correspondingly developed from early dispersion to later concentration and migrated to the west. Under the constraints of different types of symbiotic systems, the paleogeography of the Leikoupo Formation was reconstructed for four periods. The above research provides a new understanding for the study of the symbiotic system of dolomite evaporite in the northwest Sichuan region, and also has a guiding role in the reconstruction of regional paleogeography.
Abstract:
Since 2019, CNPC has conducted exploration on the Upper Cretaceous Donga strata in the Trakes Slope of the Termit Basin, Niger. Several wells obtained industrial oil flows, demonstrating good exploration potential. Compared to the Paleogene fluvial-deltaic Sokor1 Formation, the detailed characteristics of the Upper Cretaceous marine sandstones of the Donga Formation have not yet investigated in detail. In the present study, based on seismic, wireline and mud logging data, 53 side wall cores and cutting samples from 4 wells were analyzed thin sections, casting thin sections, X-ray diffraction and scanning electron microscope, gamma-ray spectral logging, etc. This study investigate the reservoir characteristics of the DS1~DS3 members of Donga Formation, and analyze their controlling factors. The results show that rock types of DS1 member are fine-medium grained quartz sandstones with high component maturity, whereas that of DS3 member are fine grained lithic quartz sandstones with cements mainly of calcareous minerals. The quartz grains are sub-round to round with moderate to poor sorting. The matrix are mainly composed of kaolinite and calcites, and sandstones exhibit point and line contacts. The most common diagenetic process were compaction, cementation and dissolution, and compaction and cementation are obvious. The sandstones commonly experienced dissolution, and pore types are mainly intergranular pores with medium to ultra-low porosity and permeability. The high-quality reservoirs of the Donga Formation are mainly distributed in the DS1 member. Physical properties of sandstones become better from the west to east in Trakes Slope. The analysis results show that sandstones development in the Donga Formation is mainly controlled by three factors. Firstly, vertical distribution of sedimentary facies and sandstones were mainly controlled by sea level changes. The DS1 member, deposited in the early stage of marine transgression, mainly consists of distributary channel sandstones, whereas DS3 member, deposited during the highest sea level, is dominated by delta front sheet sandstones and distributary channel sandstones. The high salinity of sea water in this period resulted in high content of carbonate cement, and poor physical properties of reservoir rocks. Secondly, the stable gentle slope in the Late Cretaceous was favorable for the development of sand bodies, and the intense strike-slip fault activities in the Paleogene induced the formation of micro-fractures in reservoir rocks, effectively improving their physical properties. Thirdly, the rigid support of quartz sandstones was favorable for the preservation of primary pores, and the dissolution of unstable minerals such as feldspar and calcareous minerals improved the pore structure of reservoir rocks. The development of regional cap rocks of marine mudstones and shales is conducive for the formation of “self-generation and self-preservation” play in the Donga Formation. Compared with the low slope of the Trakes Slope, sandstones with better physical properties are well developed in the middle and high slope which are closer to the eastern provenance. They are favorable exploration areas for the Donga Formation.
Since 2019, CNPC has conducted exploration on the Upper Cretaceous Donga strata in the Trakes Slope of the Termit Basin, Niger. Several wells obtained industrial oil flows, demonstrating good exploration potential. Compared to the Paleogene fluvial-deltaic Sokor1 Formation, the detailed characteristics of the Upper Cretaceous marine sandstones of the Donga Formation have not yet investigated in detail. In the present study, based on seismic, wireline and mud logging data, 53 side wall cores and cutting samples from 4 wells were analyzed thin sections, casting thin sections, X-ray diffraction and scanning electron microscope, gamma-ray spectral logging, etc. This study investigate the reservoir characteristics of the DS1~DS3 members of Donga Formation, and analyze their controlling factors. The results show that rock types of DS1 member are fine-medium grained quartz sandstones with high component maturity, whereas that of DS3 member are fine grained lithic quartz sandstones with cements mainly of calcareous minerals. The quartz grains are sub-round to round with moderate to poor sorting. The matrix are mainly composed of kaolinite and calcites, and sandstones exhibit point and line contacts. The most common diagenetic process were compaction, cementation and dissolution, and compaction and cementation are obvious. The sandstones commonly experienced dissolution, and pore types are mainly intergranular pores with medium to ultra-low porosity and permeability. The high-quality reservoirs of the Donga Formation are mainly distributed in the DS1 member. Physical properties of sandstones become better from the west to east in Trakes Slope. The analysis results show that sandstones development in the Donga Formation is mainly controlled by three factors. Firstly, vertical distribution of sedimentary facies and sandstones were mainly controlled by sea level changes. The DS1 member, deposited in the early stage of marine transgression, mainly consists of distributary channel sandstones, whereas DS3 member, deposited during the highest sea level, is dominated by delta front sheet sandstones and distributary channel sandstones. The high salinity of sea water in this period resulted in high content of carbonate cement, and poor physical properties of reservoir rocks. Secondly, the stable gentle slope in the Late Cretaceous was favorable for the development of sand bodies, and the intense strike-slip fault activities in the Paleogene induced the formation of micro-fractures in reservoir rocks, effectively improving their physical properties. Thirdly, the rigid support of quartz sandstones was favorable for the preservation of primary pores, and the dissolution of unstable minerals such as feldspar and calcareous minerals improved the pore structure of reservoir rocks. The development of regional cap rocks of marine mudstones and shales is conducive for the formation of “self-generation and self-preservation” play in the Donga Formation. Compared with the low slope of the Trakes Slope, sandstones with better physical properties are well developed in the middle and high slope which are closer to the eastern provenance. They are favorable exploration areas for the Donga Formation.
Abstract:
Abstract: Located at the edge of the Sichuan Basin, the Zhaotong Demonstration Area of Northern Yunnan and Guizhou was relatively shallower and closer to the source area than the Changning-Weiyuan area in the basin during the Late Ordovician-Early Silurian period, so the sedimentation of shale of the Wufeng Formation-Longmaxi Formation in this area may be different from that in the basin. In order to clarify the siliceous genesis, material source and source background of the late Ordovician-Early Silurian black shale in the Zhaotong Demonstration Area of Northern Yunnan Province, relevant research was carried out based on the test data of principal elements, trace elements and rare earth elements of core samples in the study area. The results show that the siliceous in the black shale in the study area mainly comes from siliceous organisms and terrestrial clastic materials, and the biogenic silicon first increases and then decreases from bottom to top, showing the opposite change law of terrestrial source silicon. There was a brief glacial period in the Late Ordovician, until the melting of the Early Silurian glaciers, the occurrence of marine intrusions, and then the gradual decline of sea level, the continuous shallow of water bodies, the overall manifestation of terrestrial input is characterized by first decreasing and then increasing, and the sedimentation rate also shows the same law. The standardized distribution mode of rare earth elements reflects that there may be a mixture source during the deposition of shale in the Wufeng-Longmaxi Formation in the study area. The chart of ΣREE-La/Yb and La/Sc-Co/Th indicate that their original materials may mainly come from acid granite and sedimentary rocks, and are affected by certain seafloor hydrothermal fluids during sedimentation. Sc/Cr-La/Y diagrams, SiO2/Al2O3-K2O/Na2O bivariate discriminant diagrams, and some trace and rare earth element ratios comprehensively reflect that the tectonic background of the shale source area of the Wufeng Formation-Longmaxi Formation is mainly passive continental margin.
Abstract: Located at the edge of the Sichuan Basin, the Zhaotong Demonstration Area of Northern Yunnan and Guizhou was relatively shallower and closer to the source area than the Changning-Weiyuan area in the basin during the Late Ordovician-Early Silurian period, so the sedimentation of shale of the Wufeng Formation-Longmaxi Formation in this area may be different from that in the basin. In order to clarify the siliceous genesis, material source and source background of the late Ordovician-Early Silurian black shale in the Zhaotong Demonstration Area of Northern Yunnan Province, relevant research was carried out based on the test data of principal elements, trace elements and rare earth elements of core samples in the study area. The results show that the siliceous in the black shale in the study area mainly comes from siliceous organisms and terrestrial clastic materials, and the biogenic silicon first increases and then decreases from bottom to top, showing the opposite change law of terrestrial source silicon. There was a brief glacial period in the Late Ordovician, until the melting of the Early Silurian glaciers, the occurrence of marine intrusions, and then the gradual decline of sea level, the continuous shallow of water bodies, the overall manifestation of terrestrial input is characterized by first decreasing and then increasing, and the sedimentation rate also shows the same law. The standardized distribution mode of rare earth elements reflects that there may be a mixture source during the deposition of shale in the Wufeng-Longmaxi Formation in the study area. The chart of ΣREE-La/Yb and La/Sc-Co/Th indicate that their original materials may mainly come from acid granite and sedimentary rocks, and are affected by certain seafloor hydrothermal fluids during sedimentation. Sc/Cr-La/Y diagrams, SiO2/Al2O3-K2O/Na2O bivariate discriminant diagrams, and some trace and rare earth element ratios comprehensively reflect that the tectonic background of the shale source area of the Wufeng Formation-Longmaxi Formation is mainly passive continental margin.
Abstract:
As a kind of hydrocarbon organic matter remaining in source rocks or As a kind of hydrocarbon organic matter that oil or natural gas remains in source rocks or reservoirs, the formation and evolution of bitumen interstitials are closely related to the evolution history of oil reservoirs, which is an important symbol of oil and gas accumulation and transformation process. Previous studies have carried out many analyses on the types, genesis and thermal evolution of bitumen components in reservoirs, but the restriction mechanism of bitumen on reservoirs is still unclear. This study takes the Cretaceous Brazilian reconstructed reservoir in the Yingmai 467 well area in the west of Tabei as an example. With the help of casting thin sections, fluorescent thin sections, laser Raman experiments, combined with logging parameter identification, the relationship between bitumen and oil and gas reservoir stages is discussed, and the influence of different bitumen types on reservoir quality is identified. The results show that : ①According to the main components and formation stages, the bitumen interstitial materials in the study section are divided into two categories : type I is mainly intergranular filling, and most of them are yellow-brown and brown-black under the fluorescence microscope. The main components are oily and bituminous bitumen, and the bitumen reflectivity is more than 1 %. The type II is distributed on the edge of the pore in the form of bitumen lining. Under the fluorescence microscope, it is mostly orange and blue ( white ) color, with colloidal bitumen as the main component. The bitumen reflectivity is low, ranging from 0.42 % to 0.79 %. ② The bitumen interstitial material in Yingmai 467 well area is mainly type I bitumen. Because the oil and gas in the source rock of Huangshanjie Formation migrated through the unconformity at the bottom of the Shushanhe Formation, the fault was filled into the Brazilian reorganized massive sand layer 2 and the Brazilian reorganized thin sand layer, and then gas washing occurred. Type II bitumen is precipitated by retrograde condensation from the source rock oil and gas of the second phase of the Qiakemake Formation along the Cretaceous bottom and the Yingmai 7 fault zone into the Brazilian reorganization. It is affected by the thickness of the sand body and is mainly distributed in the first layer of massive sand. ③Type I bitumen has a strong effect on reservoir reconstruction and occupies part of pore space ; category II bitumen has little effect on reservoir porosity.
As a kind of hydrocarbon organic matter remaining in source rocks or As a kind of hydrocarbon organic matter that oil or natural gas remains in source rocks or reservoirs, the formation and evolution of bitumen interstitials are closely related to the evolution history of oil reservoirs, which is an important symbol of oil and gas accumulation and transformation process. Previous studies have carried out many analyses on the types, genesis and thermal evolution of bitumen components in reservoirs, but the restriction mechanism of bitumen on reservoirs is still unclear. This study takes the Cretaceous Brazilian reconstructed reservoir in the Yingmai 467 well area in the west of Tabei as an example. With the help of casting thin sections, fluorescent thin sections, laser Raman experiments, combined with logging parameter identification, the relationship between bitumen and oil and gas reservoir stages is discussed, and the influence of different bitumen types on reservoir quality is identified. The results show that : ①According to the main components and formation stages, the bitumen interstitial materials in the study section are divided into two categories : type I is mainly intergranular filling, and most of them are yellow-brown and brown-black under the fluorescence microscope. The main components are oily and bituminous bitumen, and the bitumen reflectivity is more than 1 %. The type II is distributed on the edge of the pore in the form of bitumen lining. Under the fluorescence microscope, it is mostly orange and blue ( white ) color, with colloidal bitumen as the main component. The bitumen reflectivity is low, ranging from 0.42 % to 0.79 %. ② The bitumen interstitial material in Yingmai 467 well area is mainly type I bitumen. Because the oil and gas in the source rock of Huangshanjie Formation migrated through the unconformity at the bottom of the Shushanhe Formation, the fault was filled into the Brazilian reorganized massive sand layer 2 and the Brazilian reorganized thin sand layer, and then gas washing occurred. Type II bitumen is precipitated by retrograde condensation from the source rock oil and gas of the second phase of the Qiakemake Formation along the Cretaceous bottom and the Yingmai 7 fault zone into the Brazilian reorganization. It is affected by the thickness of the sand body and is mainly distributed in the first layer of massive sand. ③Type I bitumen has a strong effect on reservoir reconstruction and occupies part of pore space ; category II bitumen has little effect on reservoir porosity.
Abstract:
【Objective】The carbonate-evaporite syngenetic system is widely distributed in the Cambrian of the Sichuan Basin, however, the developmental characteristics, depositional environments, depositional processes, and evolutionary modes of this syngenetic system are weakly studied, and the research methods need to be clarified urgently. 【Methods】The petrological characteristics of the symbiotic system, the state of the evaporites, the depositional environment of the symbiotic system, the depositional process and the evolutionary pattern of the symbiotic system are investigated based on the data of drilling cores, field sections, rock sheets and carbon and oxygen isotopes, and the isotopic signatures of Fe, Mn and S. 【Results】The results show that 1) the developmental characteristics of the symbiotic system, 2) the depositional environment, and 3) the evolutionary pattern of the symbiotic system are not well understood, but are not well understood. The results show that: 1) the carbonate-evaporite symbiotic system in the study area has developed five kinds of carbonate-evaporite symbiotic system rock assemblage sequences, including: carbonate rock and evaporite interbedded, evaporite sandwiched with carbonate rock, evaporite overlain carbonate rock, carbonate rock overlain with evaporite rock, and carbonate rock sandwiched with evaporite rock. 2) the geochemical features of the symbiotic system, including δ18O, mainly concentrated in -8‰～-9‰, and δ13C, mainly concentrated in -8‰～9‰; δ13C is mainly concentrated in -1‰～3‰; the results of using carbon and oxygen isotope values to calculate palaeosalinity and palaeotemperature show that the vast majority of Z values>120‰ and δ13C values>-2‰, and the palaeoseawater temperatures are in the range of 23.10～40.64℃; Fe is mainly concentrated in the range of 0～2000×10-5; and Mn is mainly concentrated in the range of 10-5～30×10-5, which indicates that in the Gautai Group During the Gaotai Formation, the depositional environment was a warm or hot paleoclimate and saline seawater environment, with a high degree of oxidation of the water body, and the rock-forming action in a relatively open system related to atmospheric water was experienced.3) Deposition of marine carbonate rocks and evaporites in a symbiotic system against the background of an arid and hot climate and a Ca-rich and low-SO4 calcite sea with a high degree of salinity The period of time can be divided into the sea level falling evaporite - dolomite deposition stage and sea level rising dolomite - evaporite - greywacke deposition stage; "tidal ping Sabuha mode" and "underwater condensed deposition mode" are two kinds of carbonate rock - evaporite symbiosis system development mode.【Conclusion】The carbonatite-evaporite symbiotic system carries the information of paleoenvironment, paleoclimate and paleohaline water chemistry during the depositional period, and also records the depositional process and evolutionary pattern of the symbiotic system. This study provides new ideas and understanding of the depositional environments and depositional patterns of the carbonatite-evaporite symbiotic system of the evaporitic environment of the marine phase of the Cambrian Gautai Formation.
【Objective】The carbonate-evaporite syngenetic system is widely distributed in the Cambrian of the Sichuan Basin, however, the developmental characteristics, depositional environments, depositional processes, and evolutionary modes of this syngenetic system are weakly studied, and the research methods need to be clarified urgently. 【Methods】The petrological characteristics of the symbiotic system, the state of the evaporites, the depositional environment of the symbiotic system, the depositional process and the evolutionary pattern of the symbiotic system are investigated based on the data of drilling cores, field sections, rock sheets and carbon and oxygen isotopes, and the isotopic signatures of Fe, Mn and S. 【Results】The results show that 1) the developmental characteristics of the symbiotic system, 2) the depositional environment, and 3) the evolutionary pattern of the symbiotic system are not well understood, but are not well understood. The results show that: 1) the carbonate-evaporite symbiotic system in the study area has developed five kinds of carbonate-evaporite symbiotic system rock assemblage sequences, including: carbonate rock and evaporite interbedded, evaporite sandwiched with carbonate rock, evaporite overlain carbonate rock, carbonate rock overlain with evaporite rock, and carbonate rock sandwiched with evaporite rock. 2) the geochemical features of the symbiotic system, including δ18O, mainly concentrated in -8‰～-9‰, and δ13C, mainly concentrated in -8‰～9‰; δ13C is mainly concentrated in -1‰～3‰; the results of using carbon and oxygen isotope values to calculate palaeosalinity and palaeotemperature show that the vast majority of Z values>120‰ and δ13C values>-2‰, and the palaeoseawater temperatures are in the range of 23.10～40.64℃; Fe is mainly concentrated in the range of 0～2000×10-5; and Mn is mainly concentrated in the range of 10-5～30×10-5, which indicates that in the Gautai Group During the Gaotai Formation, the depositional environment was a warm or hot paleoclimate and saline seawater environment, with a high degree of oxidation of the water body, and the rock-forming action in a relatively open system related to atmospheric water was experienced.3) Deposition of marine carbonate rocks and evaporites in a symbiotic system against the background of an arid and hot climate and a Ca-rich and low-SO4 calcite sea with a high degree of salinity The period of time can be divided into the sea level falling evaporite - dolomite deposition stage and sea level rising dolomite - evaporite - greywacke deposition stage; "tidal ping Sabuha mode" and "underwater condensed deposition mode" are two kinds of carbonate rock - evaporite symbiosis system development mode.【Conclusion】The carbonatite-evaporite symbiotic system carries the information of paleoenvironment, paleoclimate and paleohaline water chemistry during the depositional period, and also records the depositional process and evolutionary pattern of the symbiotic system. This study provides new ideas and understanding of the depositional environments and depositional patterns of the carbonatite-evaporite symbiotic system of the evaporitic environment of the marine phase of the Cambrian Gautai Formation.
Abstract:
【Objective】Determining the depositional age, sedimentary sources and tectonic background of Neoproterozoic sedimentary strata in Hunan Province is one of the keys to understanding the tectonic evolution process in the middle segment of the Jiangnan orogenic belt.【Method】Six clastic rock samples of the Lengjiaxi and Banxi groups in the middle segment of the Jiangnan Orogenic Belt were collected. Provenance and tectonic setting of the sedimentary basin were constrained by studying the morphology, trace elements, and U-Pb-Lu-Hf isotopic composition of the detrital zircon, in combination with published data of detrital zircon and zircon from source area.【Result】The results showed that the Lengjiaxi Group was formed at about 852-825 Ma, and the Banxi Group was formed at about 820-720 Ma. The most important detrital zircon age peaks in the Lengjiaxi and Banxi groups are 920-790 Ma, while the Lengjiaxi Group also shows 1750-1620 Ma and 2500-2450 Ma age peaks, and the Banxi Group shows 1950-1790 Ma and 2420-2330 Ma. The Hf isotopeic characteristics show that the Lengjiaxi Group mainly receives sediments from the Yangtze Block, while the Banxi Group received detritus from both the Yangtze and Cathaysia blocks. 【Conclusion】The U-Pb and Lu-Hf isotopic compositions of detrital zircons indicates that the Lengjiaxi and Banxi groups were deposited before and after the amalgamation between the Yangtze and Cathaysia blocks. The Lengjiaxi Group was deposited in the backarc basin under the trencharc basin system, while the backarc basin was closed at 825 Ma, accompanied by strong folds developed in the sedimentary strata and a large number of S-type granites intrusions. The Banxi Group was deposited in intraplate rifting setting related to post-collision extension. Rifting related magma was flared up at 780-760 Ma and gradually weakened along the sedimentation of the Banxi Group.
【Objective】Determining the depositional age, sedimentary sources and tectonic background of Neoproterozoic sedimentary strata in Hunan Province is one of the keys to understanding the tectonic evolution process in the middle segment of the Jiangnan orogenic belt.【Method】Six clastic rock samples of the Lengjiaxi and Banxi groups in the middle segment of the Jiangnan Orogenic Belt were collected. Provenance and tectonic setting of the sedimentary basin were constrained by studying the morphology, trace elements, and U-Pb-Lu-Hf isotopic composition of the detrital zircon, in combination with published data of detrital zircon and zircon from source area.【Result】The results showed that the Lengjiaxi Group was formed at about 852-825 Ma, and the Banxi Group was formed at about 820-720 Ma. The most important detrital zircon age peaks in the Lengjiaxi and Banxi groups are 920-790 Ma, while the Lengjiaxi Group also shows 1750-1620 Ma and 2500-2450 Ma age peaks, and the Banxi Group shows 1950-1790 Ma and 2420-2330 Ma. The Hf isotopeic characteristics show that the Lengjiaxi Group mainly receives sediments from the Yangtze Block, while the Banxi Group received detritus from both the Yangtze and Cathaysia blocks. 【Conclusion】The U-Pb and Lu-Hf isotopic compositions of detrital zircons indicates that the Lengjiaxi and Banxi groups were deposited before and after the amalgamation between the Yangtze and Cathaysia blocks. The Lengjiaxi Group was deposited in the backarc basin under the trencharc basin system, while the backarc basin was closed at 825 Ma, accompanied by strong folds developed in the sedimentary strata and a large number of S-type granites intrusions. The Banxi Group was deposited in intraplate rifting setting related to post-collision extension. Rifting related magma was flared up at 780-760 Ma and gradually weakened along the sedimentation of the Banxi Group.
Abstract:
Paleo-geomorphology has a strong control on the sedimentary system and reservoir distribution. This paper analyzes the single-well facies in Gaoquan Structural Zone of Sikeshu Sag, identifies the characteristics of the sedimentary and reservoirs of the Qingshuihe Formation, and then combines with the paleo-geomorphology of the pre-Qingshuihe Formation in Gaoquan Structural Zone, establishes model of the sedimentary evolution under the control of paleo-geomorphology, and clarifies the influence of micro-geomorphology on the distribution of high-quality reservoirs. The study shows that three slope breaks are developed in the Gaoquan Structural zone of Sikeshu Sag, with a near-northwestern-south-eastern trend. Each slope-folding zone can be divided into two types of paleo-geomorphology units, namely, grooves and platforms. Three phases of regressive fan deltas were formed on the three slope breaks during the lake transgression, and each phase of fan deltas can form 9-15m thick conglomerate reservoirs. On each slope breaks, the groove and platform control the mud content of the conglomerate reservoirs, and the brown conglomerate with high mud content is easily developed in the groove area, while the gray or gray-green conglomerate with low mud content is easily developed in the platform area. The conglomerate with low mud content is ready to form high-quality reservoir in the platform area.
Paleo-geomorphology has a strong control on the sedimentary system and reservoir distribution. This paper analyzes the single-well facies in Gaoquan Structural Zone of Sikeshu Sag, identifies the characteristics of the sedimentary and reservoirs of the Qingshuihe Formation, and then combines with the paleo-geomorphology of the pre-Qingshuihe Formation in Gaoquan Structural Zone, establishes model of the sedimentary evolution under the control of paleo-geomorphology, and clarifies the influence of micro-geomorphology on the distribution of high-quality reservoirs. The study shows that three slope breaks are developed in the Gaoquan Structural zone of Sikeshu Sag, with a near-northwestern-south-eastern trend. Each slope-folding zone can be divided into two types of paleo-geomorphology units, namely, grooves and platforms. Three phases of regressive fan deltas were formed on the three slope breaks during the lake transgression, and each phase of fan deltas can form 9-15m thick conglomerate reservoirs. On each slope breaks, the groove and platform control the mud content of the conglomerate reservoirs, and the brown conglomerate with high mud content is easily developed in the groove area, while the gray or gray-green conglomerate with low mud content is easily developed in the platform area. The conglomerate with low mud content is ready to form high-quality reservoir in the platform area.
Abstract:
[Significance] The sedimentary simulation experiment in a circular flume is one of the effective means to simulate the sedimentary process and reveal its formation mechanism. It holds significant importance in research areas such as hydraulic engineering, environmental science, sedimentology, and oil and gas exploration. The circular flume can sustain continuous fluid flow under the influence of inertial and shear forces and is approximated as transporting and depositing over an infinite distance. Therefore, it can approximately replicate environmental fluid conditions in terms of transport distance and fluid velocity, overcoming the limitations of traditional flume simulations. [Progress] To meet the research needs of different application scenarios, circular flume have gradually evolved into four types, including conventional circular flume, in-situ circular flume, Mini circular tanks, and racetrack flume. Physical simulations in circular flume have achieved fruitful results in studying sediment characteristics, bedform morphology, and settling mechanisms, among other aspects. However, there is still a relative scarcity of research in sedimentary physical simulation. With the development of technology and equipment, research in sedimentary physical simulation has also made significant progress. Gravity flow sedimentation, the dynamics of fine-grained sediment transport, and tidal and wave sedimentation have become important research areas in physical simulation of circular flume. Nevertheless, there are several limitations in circular flume physical simulation, such as the influence of secondary circulation and the tracking of lateral sediment evolution, which will be optimized in subsequent research through improved experimental designs and enhanced measurement accuracy. [Conclusions and Prospects] In summary, expanding the application scope of circular flume physical simulation based on sedimentology principles will make significant contributions to the innovative development of fundamental sedimentology theory, as well as various aspects such as fine-grained sedimentology and unconventional petroleum sedimentology.
[Significance] The sedimentary simulation experiment in a circular flume is one of the effective means to simulate the sedimentary process and reveal its formation mechanism. It holds significant importance in research areas such as hydraulic engineering, environmental science, sedimentology, and oil and gas exploration. The circular flume can sustain continuous fluid flow under the influence of inertial and shear forces and is approximated as transporting and depositing over an infinite distance. Therefore, it can approximately replicate environmental fluid conditions in terms of transport distance and fluid velocity, overcoming the limitations of traditional flume simulations. [Progress] To meet the research needs of different application scenarios, circular flume have gradually evolved into four types, including conventional circular flume, in-situ circular flume, Mini circular tanks, and racetrack flume. Physical simulations in circular flume have achieved fruitful results in studying sediment characteristics, bedform morphology, and settling mechanisms, among other aspects. However, there is still a relative scarcity of research in sedimentary physical simulation. With the development of technology and equipment, research in sedimentary physical simulation has also made significant progress. Gravity flow sedimentation, the dynamics of fine-grained sediment transport, and tidal and wave sedimentation have become important research areas in physical simulation of circular flume. Nevertheless, there are several limitations in circular flume physical simulation, such as the influence of secondary circulation and the tracking of lateral sediment evolution, which will be optimized in subsequent research through improved experimental designs and enhanced measurement accuracy. [Conclusions and Prospects] In summary, expanding the application scope of circular flume physical simulation based on sedimentology principles will make significant contributions to the innovative development of fundamental sedimentology theory, as well as various aspects such as fine-grained sedimentology and unconventional petroleum sedimentology.
Abstract:
In order to further study the difference of sedimentary environment and sedimentary tectonic evolution of Wufeng-Longmaxi Formations on the east and west sides of the kangdian old land, support regional shale gas exploration and development, the systematic petromineralogy and geochemistry of Wufeng-Longmaxi Formation black mudstone in Zhaotong area in the east side of the ancient land and Yanyuan Basin in the west side are studied. The results show that there are obvious differences in sedimentary environment between the two sides. The eastern side is confined to Marine shelf deposits, and the organic-rich shale is dominated by siliceous calcareous rocks. The west side is an open Marine shelf, and the organic-rich shale is a siliceous rock series. Well XD2 on the east side is closer to the land, the parent rock is mainly felsic igneous rock, the chemical weathering is stronger, the climate is warmer, the sedimentary water is shallow and dominated by weak oxidation environment. The sedimentary water in the west side is deep and dominated by anoxic reduction environment, with strong tectonic movement and more complex tectonic background, with active continental margin and island arc environmental properties, which may be related to the collision between the western ocean crust and the Yangtze continental crust. Different sedimentary tectonic evolution models have contributed to the difference in genesis and distribution of organic-rich shales in Wufeng -Longmaxi Formation on both sides, which is of great significance to guide regional shale gas exploration and development.
In order to further study the difference of sedimentary environment and sedimentary tectonic evolution of Wufeng-Longmaxi Formations on the east and west sides of the kangdian old land, support regional shale gas exploration and development, the systematic petromineralogy and geochemistry of Wufeng-Longmaxi Formation black mudstone in Zhaotong area in the east side of the ancient land and Yanyuan Basin in the west side are studied. The results show that there are obvious differences in sedimentary environment between the two sides. The eastern side is confined to Marine shelf deposits, and the organic-rich shale is dominated by siliceous calcareous rocks. The west side is an open Marine shelf, and the organic-rich shale is a siliceous rock series. Well XD2 on the east side is closer to the land, the parent rock is mainly felsic igneous rock, the chemical weathering is stronger, the climate is warmer, the sedimentary water is shallow and dominated by weak oxidation environment. The sedimentary water in the west side is deep and dominated by anoxic reduction environment, with strong tectonic movement and more complex tectonic background, with active continental margin and island arc environmental properties, which may be related to the collision between the western ocean crust and the Yangtze continental crust. Different sedimentary tectonic evolution models have contributed to the difference in genesis and distribution of organic-rich shales in Wufeng -Longmaxi Formation on both sides, which is of great significance to guide regional shale gas exploration and development.
Abstract:
Abstract: [Objective] In order to discuss the high-frequency sequence division and effect of high-frequency sedimentary cycle control reservoir in carbonate strata. [Methods] Used the Th/U ratio curve in natural gamma ray spectrum logging as the indicator curve, combined with Fisher diagram and lithological assemblage sequence, high frequency sequence of Ma51-5 sub-member of Majiagou Formation were divided quantitatively in Tao 7 block of Ordos basin. [Results] The research shows that the Th/U ratio in natural gamma-ray spectroscopy logging can be used as an indirect alternative index of astronomical orbit in carbonate strata. The high-frequency sequence can be effectively divided by quantitative and qualitative analysis methods such as spectrum analysis combined with Fisher diagram and lithologic lithofacies analysis. Ma51-5 sub-member in Block 7 can be divided into one third-order sequence, six fourth-order sequences and twenty fifth-order sequences. It is estimated that the average deposition rate of Ma51-5 sub-member is 5.03 cm/kyr and the deposition time is about 2.43 Ma. The gypsum pseudocrystal dolomite , which developed in the upper part of the intertidal zone and in the top of the high-frequency sedimentary cycle with upward shallowing, is the dominant facies belt for reservoir development. Near the high-frequency sequence boundary of the fourth-order sequence is the favorable interval for reservoir development. The frequent changes of sea level cause early karstification, which is the main driving force for the formation of karst model pores in the Ma51-4 sub-member and the basis of later supergene gas karstification. [Conclusion] Quantitative identification and division of high-frequency sequence is impotant for reservoir prediction in carbonate strata.
Abstract: [Objective] In order to discuss the high-frequency sequence division and effect of high-frequency sedimentary cycle control reservoir in carbonate strata. [Methods] Used the Th/U ratio curve in natural gamma ray spectrum logging as the indicator curve, combined with Fisher diagram and lithological assemblage sequence, high frequency sequence of Ma51-5 sub-member of Majiagou Formation were divided quantitatively in Tao 7 block of Ordos basin. [Results] The research shows that the Th/U ratio in natural gamma-ray spectroscopy logging can be used as an indirect alternative index of astronomical orbit in carbonate strata. The high-frequency sequence can be effectively divided by quantitative and qualitative analysis methods such as spectrum analysis combined with Fisher diagram and lithologic lithofacies analysis. Ma51-5 sub-member in Block 7 can be divided into one third-order sequence, six fourth-order sequences and twenty fifth-order sequences. It is estimated that the average deposition rate of Ma51-5 sub-member is 5.03 cm/kyr and the deposition time is about 2.43 Ma. The gypsum pseudocrystal dolomite , which developed in the upper part of the intertidal zone and in the top of the high-frequency sedimentary cycle with upward shallowing, is the dominant facies belt for reservoir development. Near the high-frequency sequence boundary of the fourth-order sequence is the favorable interval for reservoir development. The frequent changes of sea level cause early karstification, which is the main driving force for the formation of karst model pores in the Ma51-4 sub-member and the basis of later supergene gas karstification. [Conclusion] Quantitative identification and division of high-frequency sequence is impotant for reservoir prediction in carbonate strata.
Abstract:
Recently, through a comprehensive and systematic investigation of paleontological fossils, the author and his team discovered abundant macrobiotic fossils in the dark gray silty shale of the lower black layer of the Qiongzhusi Formation in the Daxinshan Pude area of Luquan, Yunnan, which is very similar to the "Chengjiang Biota". The main types of fossils in Daxinshan include Kunmingella douvillei,pest ichnology,Conotheca sp.,Maotianshania sp.,Kutorgina chengjiangensis,Branchiocaris sp.,Vetulicola sp,Eoredlichia sp.and fossil leguminosae (unknown species); However, suspected skeletal fossils have been discovered at the same level in the Pude area. XRF scanning shows that the fossils contain relatively high concentrations of Ca, Cr, and P, which are significantly different from the composition of the surrounding rock (Fe, S, Ti). Ca and P are the main constituent elements of skeletal fossils. The fossils in the Daxinshan Pude area are not only well preserved and relatively abundant, but also have a large number of individuals and diverse species. The occurrence of the fossils is good, and the transportation is convenient. Moreover, the occurrence of the fossils is relatively low compared to the "Chengjiang Biota", which has good research significance and scientific popularization value. It is also expected to establish a second Early Cambrian Biota in the Luquan area of Yunnan: the Daxinshan Pude Biota; This discovery provides important paleontological fossil research materials for studying the patterns, scope, recovery, paleoenvironmental evolution, and stratigraphic regional correlation of early Cambrian biological activities.
Recently, through a comprehensive and systematic investigation of paleontological fossils, the author and his team discovered abundant macrobiotic fossils in the dark gray silty shale of the lower black layer of the Qiongzhusi Formation in the Daxinshan Pude area of Luquan, Yunnan, which is very similar to the "Chengjiang Biota". The main types of fossils in Daxinshan include Kunmingella douvillei,pest ichnology,Conotheca sp.,Maotianshania sp.,Kutorgina chengjiangensis,Branchiocaris sp.,Vetulicola sp,Eoredlichia sp.and fossil leguminosae (unknown species); However, suspected skeletal fossils have been discovered at the same level in the Pude area. XRF scanning shows that the fossils contain relatively high concentrations of Ca, Cr, and P, which are significantly different from the composition of the surrounding rock (Fe, S, Ti). Ca and P are the main constituent elements of skeletal fossils. The fossils in the Daxinshan Pude area are not only well preserved and relatively abundant, but also have a large number of individuals and diverse species. The occurrence of the fossils is good, and the transportation is convenient. Moreover, the occurrence of the fossils is relatively low compared to the "Chengjiang Biota", which has good research significance and scientific popularization value. It is also expected to establish a second Early Cambrian Biota in the Luquan area of Yunnan: the Daxinshan Pude Biota; This discovery provides important paleontological fossil research materials for studying the patterns, scope, recovery, paleoenvironmental evolution, and stratigraphic regional correlation of early Cambrian biological activities.
Abstract:
The Yuzhou area is the most highly studied on Upper Paleozoic coal-bearing strata of the southern part of North China Block, which has abundant sedimentological and stratigraphic paleontological research basis, however, due to the lack of absolute chronology data, the division of chronostratigraphy and regional large-scale stratigraphic correlation are affected. In this paper, a laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was applied to date the U-Pb age of detrital zircon of two mudstone samples near the stratigraphic boundary of Taiyuan-Shanxi Formations and Xiashihezi-Shangshihezi Formations in Yuzhou area, and to determine their maximum depositional ages. The results show that: ①The 40 detrital zircons from the bottom sample of Shanxi Formation (ZK1006-5) constitute a continuous young component spectrum ranging from 283Ma to 343Ma, with the youngest single zircon age (YSG age) is 283±9.4Ma. The 18 detrital zircons from the bottom sample of Shangshihezi Formation (ZK2387-3) constitute a continuous young component spectrum ranging from 257Ma to 299Ma, with the youngest single zircon age (YSG age) is 257±6.8Ma.They can represent the maximum deposition age of the sample strata, indicating that the deposition time is no earlier than 283±9.4Ma and 257±6.8Ma. ②The deposition of Shanxi Formation and Shangshihezi Formation in Yuzhou area began in Kungurian and Wuchiapingian, respectively, which is consistent with regionnal biostratigraphic data. ③The Upper Paleozoic coal-bearing strata of the North China Block have obvious and large span of rock strata penetrating, and the overall performance is characterized by inward penetrating of the plate edge gradually through the new characteristics. Based on the previous studies on volcanic events in the Late Paleozoic basin of North China Block, it is concluded that the eastern margin of the basin was strongly active in which the same sedimentary period, and the induced magmatism, tectonic activity, biological succession and transgression may be related to the formation of the ancient continent of Laurisia land at the same time.
The Yuzhou area is the most highly studied on Upper Paleozoic coal-bearing strata of the southern part of North China Block, which has abundant sedimentological and stratigraphic paleontological research basis, however, due to the lack of absolute chronology data, the division of chronostratigraphy and regional large-scale stratigraphic correlation are affected. In this paper, a laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was applied to date the U-Pb age of detrital zircon of two mudstone samples near the stratigraphic boundary of Taiyuan-Shanxi Formations and Xiashihezi-Shangshihezi Formations in Yuzhou area, and to determine their maximum depositional ages. The results show that: ①The 40 detrital zircons from the bottom sample of Shanxi Formation (ZK1006-5) constitute a continuous young component spectrum ranging from 283Ma to 343Ma, with the youngest single zircon age (YSG age) is 283±9.4Ma. The 18 detrital zircons from the bottom sample of Shangshihezi Formation (ZK2387-3) constitute a continuous young component spectrum ranging from 257Ma to 299Ma, with the youngest single zircon age (YSG age) is 257±6.8Ma.They can represent the maximum deposition age of the sample strata, indicating that the deposition time is no earlier than 283±9.4Ma and 257±6.8Ma. ②The deposition of Shanxi Formation and Shangshihezi Formation in Yuzhou area began in Kungurian and Wuchiapingian, respectively, which is consistent with regionnal biostratigraphic data. ③The Upper Paleozoic coal-bearing strata of the North China Block have obvious and large span of rock strata penetrating, and the overall performance is characterized by inward penetrating of the plate edge gradually through the new characteristics. Based on the previous studies on volcanic events in the Late Paleozoic basin of North China Block, it is concluded that the eastern margin of the basin was strongly active in which the same sedimentary period, and the induced magmatism, tectonic activity, biological succession and transgression may be related to the formation of the ancient continent of Laurisia land at the same time.
Abstract:
[Objective]In order to explore the composition characteristics and main controlling factors of high-abnormally high abundant rearranged hopanes in coal-measure source rocks. [Methods]The saturated hydrocarbons and aromatic hydrocarbons of 29 coal measure source rocks in the Upper Paleozoic of Ordos Basin were analyzed in detail by gas chromatography-mass spectrometry (GC-MS). [Results] According to the peak order and retention time of compounds, GC-MS analysis of coal measure source rocks in Ordos Basin was carried out. Four types of rearranged hopanes with different abundances were systematically identified, which were 17α (H) -rearranged hopane series (C30*), 18α(H) -neohopane series (Ts and C29Ts), early-eluting rearranged hopane series (C30E) and 21-methyl-28-norhopane series (29Nsp). And the peak order of the four types of rearranged hopane compounds is: early-eluting rearranged hopane series > 17α (H) -rearranged hopane series > 18α (H) -neohopane series > 21-methyl-28-norhopane series. The study of the internal composition of four types of rearranged hopane compounds in the coal-bearing source rocks in the study area shows that there is a good correlation between the same series of rearranged hopane compounds; the correlation between different series of rearranged hopanes is different. The correlation between C30*/C30 hopane, C30E/C30 hopane and 29Nsp/C29 hopane is good, but the correlation between C29Ts/C29 hopane and the above three is poor, indicating that the formation mechanism of different series of rearranged hopanes may be different. In addition, The Pr/Ph values of the high-abnormally high abundant of 17α (H)- rearranged hopane series, early-eluting rearranged hopane series, 21-methyl-28-norhopane series in the coal source rock samples in the study area are distributed between 1.0 and 2.0, the Gammacerane（G）/C30 hopane values are mostly distributed around 0.13, and the relative content of dibenzofuran is between 11.67 % and 55.26 %, with an average value of 35.51%. To put it another way, the sedimentary environment of coal-measure source rocks has a great influence on the relative abundance of these rearranged hopane compounds. With the increase of thermal evolution degree of organic matter, the relative abundance of these rearranged hopane compounds shows an approximate normal distribution. In the peak stage of oil generation (Ro is between 0.8%and 0.9%), the relative abundance of high-abnormally high abundant rearranged hopanes compounds reaches the peak. What’s more, The ratios of (C28+C29) tricyclic terpanes（TT）/C30 hopanes and ∑regular steranes / ∑C30-35 hopanes in high-abnormally high abundant rearranged hopanes samples were significantly positively correlated with the ratios of four types of rearranged hopanes, revealing that the biogenic parent materials of high-abnormally high-abundant rearranged hopanes are mainly lower aquatic organisms and algae compounds.[Conclusion] Through the same evolution path and formation mechanism of the same series of rearranged hopanes, it is speculated that the main controlling factors for the formation of 17α (H) -rearranged hopane series, early-eluting rearranged hopane series, 21-methyl-28-norhopane series high-abnormally high rearranged hopanes in coal-measure source rocks are mainly weak oxidation-weak reduction, bacterial hopane precursors developed in brackish water sedimentary environment ; the formation of high-abnormal high-abundance 18α (H)-neohopanes is mainly controlled by the biogenic parent material of organic matter, which may be limonene or C29 hopane compounds.
[Objective]In order to explore the composition characteristics and main controlling factors of high-abnormally high abundant rearranged hopanes in coal-measure source rocks. [Methods]The saturated hydrocarbons and aromatic hydrocarbons of 29 coal measure source rocks in the Upper Paleozoic of Ordos Basin were analyzed in detail by gas chromatography-mass spectrometry (GC-MS). [Results] According to the peak order and retention time of compounds, GC-MS analysis of coal measure source rocks in Ordos Basin was carried out. Four types of rearranged hopanes with different abundances were systematically identified, which were 17α (H) -rearranged hopane series (C30*), 18α(H) -neohopane series (Ts and C29Ts), early-eluting rearranged hopane series (C30E) and 21-methyl-28-norhopane series (29Nsp). And the peak order of the four types of rearranged hopane compounds is: early-eluting rearranged hopane series > 17α (H) -rearranged hopane series > 18α (H) -neohopane series > 21-methyl-28-norhopane series. The study of the internal composition of four types of rearranged hopane compounds in the coal-bearing source rocks in the study area shows that there is a good correlation between the same series of rearranged hopane compounds; the correlation between different series of rearranged hopanes is different. The correlation between C30*/C30 hopane, C30E/C30 hopane and 29Nsp/C29 hopane is good, but the correlation between C29Ts/C29 hopane and the above three is poor, indicating that the formation mechanism of different series of rearranged hopanes may be different. In addition, The Pr/Ph values of the high-abnormally high abundant of 17α (H)- rearranged hopane series, early-eluting rearranged hopane series, 21-methyl-28-norhopane series in the coal source rock samples in the study area are distributed between 1.0 and 2.0, the Gammacerane（G）/C30 hopane values are mostly distributed around 0.13, and the relative content of dibenzofuran is between 11.67 % and 55.26 %, with an average value of 35.51%. To put it another way, the sedimentary environment of coal-measure source rocks has a great influence on the relative abundance of these rearranged hopane compounds. With the increase of thermal evolution degree of organic matter, the relative abundance of these rearranged hopane compounds shows an approximate normal distribution. In the peak stage of oil generation (Ro is between 0.8%and 0.9%), the relative abundance of high-abnormally high abundant rearranged hopanes compounds reaches the peak. What’s more, The ratios of (C28+C29) tricyclic terpanes（TT）/C30 hopanes and ∑regular steranes / ∑C30-35 hopanes in high-abnormally high abundant rearranged hopanes samples were significantly positively correlated with the ratios of four types of rearranged hopanes, revealing that the biogenic parent materials of high-abnormally high-abundant rearranged hopanes are mainly lower aquatic organisms and algae compounds.[Conclusion] Through the same evolution path and formation mechanism of the same series of rearranged hopanes, it is speculated that the main controlling factors for the formation of 17α (H) -rearranged hopane series, early-eluting rearranged hopane series, 21-methyl-28-norhopane series high-abnormally high rearranged hopanes in coal-measure source rocks are mainly weak oxidation-weak reduction, bacterial hopane precursors developed in brackish water sedimentary environment ; the formation of high-abnormal high-abundance 18α (H)-neohopanes is mainly controlled by the biogenic parent material of organic matter, which may be limonene or C29 hopane compounds.
Abstract:
[Significance] Carbonate factories provide the foundation for forming carbonate depositional systems. It closely relates to marine evolution, elemental cycling, and earth surface processes. It is also an essential part reflecting the evolution of the Earth system. The transition of carbonate factories often coincides with biological and environmental changes. Over geological time, the change of carbonate factories frequently occurs along with biological or environmental crises such as mass extinction or the initiation of new life forms such as life explosion or biological recovery after crisis. Therefore, quantifying the characterization of carbonate factories and their controlling factors is a crucial entry point for a deep understanding of the geological information recorded in carbonate rocks. [Progress] However, most studies about carbonate factories, especially those from geological record, are qualitative description based on the lithological and microfacies analysis. This study summarizes the research progress in the semi-quantitative to quantitative characterization of carbonate factories, combining the introduction of the methods for forward modelling of sedimentary processes to provide a perspective for detailed characterization of carbonate factories from a quantitative reconstruction perspective. In addition to providing information about rock components that indicate the ingredients of carbonate factories, statistical analysis of carbonate grains can offer insights into the sedimentary environment by examining parameters such as size, roundness, and sorting. These shape characteristics can serve as quantitative indicators of water energy and grain transportation processes. Elemental geochemical proxies enable the assessment of environmental parameters such as redox conditions, nutrient levels, and climatic conditions. Isotopic geochemical proxies play a crucial role in reconstructing the evolution of environmental factors like temperature and seawater carbonate saturation. By utilizing a combination of multiproxies and sedimentary process modelling, a comprehensive analysis can elucidate the production process, controlling factors, and evolution of carbonate factories. [Prospects] On the basis of traditional carbonate sedimentology, studying carbonate factories should deepen the understanding of their controlling factors, especially the quantitative assessment of factors directly impacting ecosystems. When applying quantitative analysis methods such as sediment transport patterns or hydrodynamic analysis that are established in siliciclastic sedimentary systems, the differences caused by the biogenic nature of some carbonate sediments compared to siliciclastic sediments need to be considered. Furthermore, models and analysis methods applicable to carbonate grains should be further refined. In addition, when quantitatively evaluating the development of carbonate factories in ancient marine using quantitative carbonate grains statistics or geochemical indicators, there should be a strengthened focus on developing quantitative assessment methods for the level of diagenetic alteration. Further efforts should be directed toward developing and applying in-situ elemental and isotopic testing methods to reduce the impact of diagenesis on geochemical signals. While developing and applying new geochemical proxies for carbonate rocks, the mechanisms of occurrence or fractionation need to be clearly understood to interpret the paleoclimate and paleoenvironmental information they have reflected accurately. Simultaneously, when studying deep-time carbonate factories' evolution and production mechanisms through sedimentary process forward modelling, it is essential to consider the applicability of parameters obtained from modern environments in deep-time records and account for the impact of syndepositional-early diagenesis and chemical evolution of seawater geochemical evolution in the modelling process.
[Significance] Carbonate factories provide the foundation for forming carbonate depositional systems. It closely relates to marine evolution, elemental cycling, and earth surface processes. It is also an essential part reflecting the evolution of the Earth system. The transition of carbonate factories often coincides with biological and environmental changes. Over geological time, the change of carbonate factories frequently occurs along with biological or environmental crises such as mass extinction or the initiation of new life forms such as life explosion or biological recovery after crisis. Therefore, quantifying the characterization of carbonate factories and their controlling factors is a crucial entry point for a deep understanding of the geological information recorded in carbonate rocks. [Progress] However, most studies about carbonate factories, especially those from geological record, are qualitative description based on the lithological and microfacies analysis. This study summarizes the research progress in the semi-quantitative to quantitative characterization of carbonate factories, combining the introduction of the methods for forward modelling of sedimentary processes to provide a perspective for detailed characterization of carbonate factories from a quantitative reconstruction perspective. In addition to providing information about rock components that indicate the ingredients of carbonate factories, statistical analysis of carbonate grains can offer insights into the sedimentary environment by examining parameters such as size, roundness, and sorting. These shape characteristics can serve as quantitative indicators of water energy and grain transportation processes. Elemental geochemical proxies enable the assessment of environmental parameters such as redox conditions, nutrient levels, and climatic conditions. Isotopic geochemical proxies play a crucial role in reconstructing the evolution of environmental factors like temperature and seawater carbonate saturation. By utilizing a combination of multiproxies and sedimentary process modelling, a comprehensive analysis can elucidate the production process, controlling factors, and evolution of carbonate factories. [Prospects] On the basis of traditional carbonate sedimentology, studying carbonate factories should deepen the understanding of their controlling factors, especially the quantitative assessment of factors directly impacting ecosystems. When applying quantitative analysis methods such as sediment transport patterns or hydrodynamic analysis that are established in siliciclastic sedimentary systems, the differences caused by the biogenic nature of some carbonate sediments compared to siliciclastic sediments need to be considered. Furthermore, models and analysis methods applicable to carbonate grains should be further refined. In addition, when quantitatively evaluating the development of carbonate factories in ancient marine using quantitative carbonate grains statistics or geochemical indicators, there should be a strengthened focus on developing quantitative assessment methods for the level of diagenetic alteration. Further efforts should be directed toward developing and applying in-situ elemental and isotopic testing methods to reduce the impact of diagenesis on geochemical signals. While developing and applying new geochemical proxies for carbonate rocks, the mechanisms of occurrence or fractionation need to be clearly understood to interpret the paleoclimate and paleoenvironmental information they have reflected accurately. Simultaneously, when studying deep-time carbonate factories' evolution and production mechanisms through sedimentary process forward modelling, it is essential to consider the applicability of parameters obtained from modern environments in deep-time records and account for the impact of syndepositional-early diagenesis and chemical evolution of seawater geochemical evolution in the modelling process.
Abstract:
Aiming at the problem of unclear fan formation mechanism and sedimentary filling process under the condition of no well, from the perspective of source and sink system, three-dimensional seismic data were used to comprehensively analyze the denudation capacity and transport path of the source area of Bonan low bulge, and the sedimentary response characteristics and coupling mechanism of the large sublacustrine fan in the southeastern slope of Bozhong depression were clarified. The developmental model was summarized. The results show that: (1) The second of Dongying sequence is in the transition stage of lake basin fault depression, which can be divided into two tertiary sequences, in which the sublacustrine fan mainly developed in the highstand systems tract of the lower second of Dongying sequence; (2) The denudation flux in the eastern part of the Bonan low bulge is greater than deposition flux of the subestrustrine fan. The large subestrustrine fan is mainly influenced by the near province-source, and the braided river delta formed in the high part of the uplift does not pass through, and is discharged under the depositional break of the slope along five transport channels spreading from south to north. The coupling of near source supply, transport channel and sedimentary slope break formed a large contiguous lacustrine fan deposit. (3) The sedimentary filling of sublacustrine fan is controlled by the location of restricted palaeogeomorphology and the size of the space that can be accommodated. Among them, the early restricted paleogeomorphology formed A large space, and the fan sand body of the lake bottom was preferentially unloaded, forming three fan sediments, A, B and C. Due to the spatial limitations on both sides and the strong hydrodynamic conditions, the seismic facies showed the characteristics of chaotic downcut waterways and lateral migration superimposed, and the sand body formed was relatively rich in sand. With the inflow of gravity flow, the space that can be contained gradually decreases, and the gravity flow overflows along the recharge channel to the center of the open lake basin, forming two fan bodies D and E. Due to the dilution of water concentration and the change of paleo-geomorphology, the energy gradually weakens. The seismic phase has the characteristics of laminary-strong amplitude reflection, the channel features are not obvious, and the sand richness is moderate. The late source supply and energy are weak, and compared with the local erosion and reconstruction of the early fan, the earthquake shows weak amplitude reflection, which is dominated by the muddy channel. The large-scale sublustrine fan has the spatial and temporal distribution and evolution law of early restricted filling, mid-stage overflow adjustment and late erosion transformation, which constitute the development characteristics of transverse continuous and vertical multi-stage superposition. The three fan bodies A, B and C below the first stage have good sand-rich, shallow burial, good oil and gas migration and accumulation conditions, and good reservoer-cap combination. The sweet area of C-fan is large, which is an important target for lithologic reservoir exploration in Bohai sea in recent years.
Aiming at the problem of unclear fan formation mechanism and sedimentary filling process under the condition of no well, from the perspective of source and sink system, three-dimensional seismic data were used to comprehensively analyze the denudation capacity and transport path of the source area of Bonan low bulge, and the sedimentary response characteristics and coupling mechanism of the large sublacustrine fan in the southeastern slope of Bozhong depression were clarified. The developmental model was summarized. The results show that: (1) The second of Dongying sequence is in the transition stage of lake basin fault depression, which can be divided into two tertiary sequences, in which the sublacustrine fan mainly developed in the highstand systems tract of the lower second of Dongying sequence; (2) The denudation flux in the eastern part of the Bonan low bulge is greater than deposition flux of the subestrustrine fan. The large subestrustrine fan is mainly influenced by the near province-source, and the braided river delta formed in the high part of the uplift does not pass through, and is discharged under the depositional break of the slope along five transport channels spreading from south to north. The coupling of near source supply, transport channel and sedimentary slope break formed a large contiguous lacustrine fan deposit. (3) The sedimentary filling of sublacustrine fan is controlled by the location of restricted palaeogeomorphology and the size of the space that can be accommodated. Among them, the early restricted paleogeomorphology formed A large space, and the fan sand body of the lake bottom was preferentially unloaded, forming three fan sediments, A, B and C. Due to the spatial limitations on both sides and the strong hydrodynamic conditions, the seismic facies showed the characteristics of chaotic downcut waterways and lateral migration superimposed, and the sand body formed was relatively rich in sand. With the inflow of gravity flow, the space that can be contained gradually decreases, and the gravity flow overflows along the recharge channel to the center of the open lake basin, forming two fan bodies D and E. Due to the dilution of water concentration and the change of paleo-geomorphology, the energy gradually weakens. The seismic phase has the characteristics of laminary-strong amplitude reflection, the channel features are not obvious, and the sand richness is moderate. The late source supply and energy are weak, and compared with the local erosion and reconstruction of the early fan, the earthquake shows weak amplitude reflection, which is dominated by the muddy channel. The large-scale sublustrine fan has the spatial and temporal distribution and evolution law of early restricted filling, mid-stage overflow adjustment and late erosion transformation, which constitute the development characteristics of transverse continuous and vertical multi-stage superposition. The three fan bodies A, B and C below the first stage have good sand-rich, shallow burial, good oil and gas migration and accumulation conditions, and good reservoer-cap combination. The sweet area of C-fan is large, which is an important target for lithologic reservoir exploration in Bohai sea in recent years.
Abstract:
[Objective] The previous regional geological data regarded the Sanjie sand and gravel section in Juren Town, Binxian County, Heilongjiang Province as the Luojiaweng Formation, but the study of its stratigraphic properties and sedimentary environment is weak, which greatly limits the in-depth understanding of regional surface processes recorded in this stratum. [Methods] From the perspectives of sedimentology, mineralogy, elemental geochemistry and U-Pb chronology of detrital zircon, the chemical weathering characteristics, source rock properties, sedimentary environment and tectonic setting of the provenance area of this section are discussed. [Results]The results show that the weathered degree of sand and gravel in the three sections is low, the sorting is poor, the roundness is good, and there is no obvious directional arrangement. The gravel lithology is mainly granitic rock (46.31%) and quartzy rock (28.19%), followed by syenite (0.2%), tuff (0.01%) and schist (0.01%). The heavy minerals were mainly sphenite (65.18%) and epidote (11.87%), but the other heavy minerals were less active. Elemental geochemistry reveals that the sediments in the three sections have a weak to moderate degree of chemical weathering, most of the sediments have undergone a primary cycle, and their parent rock type is felsic. According to the geochemical migration and enrichment of elements and the paleoclimate discrimination diagram, the sediment is revealed to be braided river sediment in arid oxidation environment. The zircon U-Pb ages are distributed in a narrow range (134.2~220.3 Ma, with a peak age of ~168 Ma), indicating the detrite contribution from Zhangguangcai Mountain, and indicating the subduction movement of Mudanjiang Ocean and the transformation of extensional environment after the subduction of the eastern oceanic plate to orogeny. Compared with the standard strata of Luojiawang Formation, there are significant differences in sedimentological characteristics, genetic types and geomorphologic characteristics of the three-section section. It is speculated that the formation time of the three-section section is later than that of the standard strata of Luojiawang Formation, and the time is roughly the same as that of Baitushan Formation. [Conclusion]This provides important indications for the division of the Quaternary strata in Harbin and the coupling relationship between regional structure geomorphology climate water system evolution.
[Objective] The previous regional geological data regarded the Sanjie sand and gravel section in Juren Town, Binxian County, Heilongjiang Province as the Luojiaweng Formation, but the study of its stratigraphic properties and sedimentary environment is weak, which greatly limits the in-depth understanding of regional surface processes recorded in this stratum. [Methods] From the perspectives of sedimentology, mineralogy, elemental geochemistry and U-Pb chronology of detrital zircon, the chemical weathering characteristics, source rock properties, sedimentary environment and tectonic setting of the provenance area of this section are discussed. [Results]The results show that the weathered degree of sand and gravel in the three sections is low, the sorting is poor, the roundness is good, and there is no obvious directional arrangement. The gravel lithology is mainly granitic rock (46.31%) and quartzy rock (28.19%), followed by syenite (0.2%), tuff (0.01%) and schist (0.01%). The heavy minerals were mainly sphenite (65.18%) and epidote (11.87%), but the other heavy minerals were less active. Elemental geochemistry reveals that the sediments in the three sections have a weak to moderate degree of chemical weathering, most of the sediments have undergone a primary cycle, and their parent rock type is felsic. According to the geochemical migration and enrichment of elements and the paleoclimate discrimination diagram, the sediment is revealed to be braided river sediment in arid oxidation environment. The zircon U-Pb ages are distributed in a narrow range (134.2~220.3 Ma, with a peak age of ~168 Ma), indicating the detrite contribution from Zhangguangcai Mountain, and indicating the subduction movement of Mudanjiang Ocean and the transformation of extensional environment after the subduction of the eastern oceanic plate to orogeny. Compared with the standard strata of Luojiawang Formation, there are significant differences in sedimentological characteristics, genetic types and geomorphologic characteristics of the three-section section. It is speculated that the formation time of the three-section section is later than that of the standard strata of Luojiawang Formation, and the time is roughly the same as that of Baitushan Formation. [Conclusion]This provides important indications for the division of the Quaternary strata in Harbin and the coupling relationship between regional structure geomorphology climate water system evolution.
Abstract:
C-6 Oilfield is one of the most principal oilfields of Caofeidian oil province with hundred million cubic metre of reserves. The third oil group of Guantao Formation (N1gⅢ), the main production zone of C-6 Oilfield, is sand-rich braided river deposit. The architecture and connectivity of braided river sandstone are the key geological factors affecting the development effect of the Oilfield. Calibrated with limited log data, intelligent fusion technology of multiple seismic attributes was introduced to finely characterizes the spatial distribution of the level-4 architecture units of the braided river reservoir. According to log interpretation, N1gⅢ of C-6 oilfield mainly develops two types of level-4 architecture units, namely, channel bar and braided channel, and braided bar is the best reservoir with high sandstone thickness and excellent physical properties. Based on seismic attribute extraction and correlation analysis with lithological and physical parameters, reflection intensity, relative impedance, sweet point, original amplitude, envelop were chosen as intelligent fusion seismic attributes with Deep Feed-Forward Neural Network (DFNN) algorithm under the supervision of porosity. The 3D attribute of DFNN fusion, representative of lithology and petrophysical property, largely improves the detecting ability of braided river sandstone unit and its boundary, and can be used to finely characterize the plan and section distribution of braided river level-4 architecture units effectively. A NE-SW braided flow zone was developed in N1gⅢ of C-6 oilfield, which could be internally sub-divided into 15 rhombic level-4 architecture units of the braided bar. Distributary channels, another level-4 architecture units, surrounded braided bar in a narrow strip. The level-4 architecture interface between the two units played as seepage barriers for fluid migration. The braided bars cut and overlapped one another vertically, forming “big bar and small channel” plan reservoir architecture pattern. The fine characterization results deepened the understanding of the reservoir connectivity of the braided river reservoir with sparse well pattern, which provided direct geological bases for the making of optimized adjustment plan of C-6 Oilfield.
C-6 Oilfield is one of the most principal oilfields of Caofeidian oil province with hundred million cubic metre of reserves. The third oil group of Guantao Formation (N1gⅢ), the main production zone of C-6 Oilfield, is sand-rich braided river deposit. The architecture and connectivity of braided river sandstone are the key geological factors affecting the development effect of the Oilfield. Calibrated with limited log data, intelligent fusion technology of multiple seismic attributes was introduced to finely characterizes the spatial distribution of the level-4 architecture units of the braided river reservoir. According to log interpretation, N1gⅢ of C-6 oilfield mainly develops two types of level-4 architecture units, namely, channel bar and braided channel, and braided bar is the best reservoir with high sandstone thickness and excellent physical properties. Based on seismic attribute extraction and correlation analysis with lithological and physical parameters, reflection intensity, relative impedance, sweet point, original amplitude, envelop were chosen as intelligent fusion seismic attributes with Deep Feed-Forward Neural Network (DFNN) algorithm under the supervision of porosity. The 3D attribute of DFNN fusion, representative of lithology and petrophysical property, largely improves the detecting ability of braided river sandstone unit and its boundary, and can be used to finely characterize the plan and section distribution of braided river level-4 architecture units effectively. A NE-SW braided flow zone was developed in N1gⅢ of C-6 oilfield, which could be internally sub-divided into 15 rhombic level-4 architecture units of the braided bar. Distributary channels, another level-4 architecture units, surrounded braided bar in a narrow strip. The level-4 architecture interface between the two units played as seepage barriers for fluid migration. The braided bars cut and overlapped one another vertically, forming “big bar and small channel” plan reservoir architecture pattern. The fine characterization results deepened the understanding of the reservoir connectivity of the braided river reservoir with sparse well pattern, which provided direct geological bases for the making of optimized adjustment plan of C-6 Oilfield.
Abstract:
The Middle Permian to Lower Triassic stratigraphy in the Fars region of the southern Persian Gulf harbors substantial oil and gas resources, making it a focal area for China's overseas exploration efforts. The current research lacks a comprehensive sequence stratigraphy and a macroscopic understanding of the sedimentary evolution of the entire oil-bearing succession. This study focuses on the Middle to Upper Permian Dalan Formation and the Lower Triassic Kangan Formation in the region. Utilizing data from individual wells, cross-sections, core samples, thin sections, well logging, and IHS and C&C databases, along with consideration of regional geological context and existing knowledge, the Middle Permian to Early Triassic in the study area is stratigraphically divided into sequences. The study provides an in-depth analysis of the characteristics and evolution of each sequence's sedimentary systems. The results indicate that the Dalan-Kangan formations in the study area exhibit six sequence boundaries and five maximum flooding surfaces. Based on the types of sequence boundaries and the development of maximum flooding surfaces, the Dalan and Kangan formations in the southern Persian Gulf are subdivided into five third-order sequences. The targeted lithology is predominantly carbonate ramp deposits, further categorized into inner ramp, mid-ramp, and outer ramp subfacies. The inner ramp can be subdivided into six depositional microfacies: Sabkha in the intertidal zone, tidal flat, lagoon, back-barrier, shoal middle, and shoal front. The study area represents an arid and hot shallow-water carbonate deposition environment. During the SQ1-SQ2 deposition period, a predominantly progradational sedimentary model is observed, while the SQ3 deposition period follows a retrogradational and aggradational model. The SQ4 deposition period returns to a progradational model, and the SQ5 deposition period is characterized by a retrogradational model.
The Middle Permian to Lower Triassic stratigraphy in the Fars region of the southern Persian Gulf harbors substantial oil and gas resources, making it a focal area for China's overseas exploration efforts. The current research lacks a comprehensive sequence stratigraphy and a macroscopic understanding of the sedimentary evolution of the entire oil-bearing succession. This study focuses on the Middle to Upper Permian Dalan Formation and the Lower Triassic Kangan Formation in the region. Utilizing data from individual wells, cross-sections, core samples, thin sections, well logging, and IHS and C&C databases, along with consideration of regional geological context and existing knowledge, the Middle Permian to Early Triassic in the study area is stratigraphically divided into sequences. The study provides an in-depth analysis of the characteristics and evolution of each sequence's sedimentary systems. The results indicate that the Dalan-Kangan formations in the study area exhibit six sequence boundaries and five maximum flooding surfaces. Based on the types of sequence boundaries and the development of maximum flooding surfaces, the Dalan and Kangan formations in the southern Persian Gulf are subdivided into five third-order sequences. The targeted lithology is predominantly carbonate ramp deposits, further categorized into inner ramp, mid-ramp, and outer ramp subfacies. The inner ramp can be subdivided into six depositional microfacies: Sabkha in the intertidal zone, tidal flat, lagoon, back-barrier, shoal middle, and shoal front. The study area represents an arid and hot shallow-water carbonate deposition environment. During the SQ1-SQ2 deposition period, a predominantly progradational sedimentary model is observed, while the SQ3 deposition period follows a retrogradational and aggradational model. The SQ4 deposition period returns to a progradational model, and the SQ5 deposition period is characterized by a retrogradational model.
Abstract:
[Objective] By analyzing the influence of the periodic change of earth orbit on the periodic change of climate, this paper discusses the climate change characteristics of the Lower Oil Sand Mountain Formation in Qaidam Basin, and establishes a high-resolution astronomical scale for the Lower Oil Sand Mountain in Qaidam Basin based on Milankovitch theory to identify and divide high-frequency sequences. [Methods] Firstly, Laskar algorithm is used to calculate the variation period of the earth's orbital parameters during the summer solstice at 35 north latitude from 14.5~23.8 Ma, and the Miocene cycle theory and Miocene cycle ratio in this sedimentary period are determined. Then, taking wells Xianzhong 39, Xianzhong 8-9 and Xianzhong 8-12 in Nanbaxian oil and gas field as examples, the natural gamma data are analyzed by frequency spectrum and continuous wavelet transform. Finally, according to the orbital period, the average sedimentation rate of the Lower Youshashan Formation is calculated, and the "floating" astronomical scale of well Xianzhong 39 is established. [Results] Through the analysis of frequency spectrum and continuous wavelet transform, the Neogene Lower Youshashan Formation is mainly controlled by eccentricity periods of 400 ka and 95 ka. The average sedimentation rate of the Lower Youshashan Formation is 0.094 41 m/ka, and the sedimentation duration is 7.2 Ma. Based on the 400 ka long eccentric period curve and 95 ka short eccentric period curve as benchmark curves, 18 fourth-order quasi-sequence groups and 72 fifth-order quasi-sequence groups were identified. [Conclusion] The results show that the climate change recorded in the Lower Youshashan Formation is obviously controlled and driven by cycles. Identification and division based on Milankovitch theory can reduce the influence of subjective factors, improve the accuracy of division results, and more accurately describe the climate change characteristics in sediments. These research results are helpful to deeply understand the evolution law of the earth's climate and provide important reference for oil and gas exploration and resource evaluation.
[Objective] By analyzing the influence of the periodic change of earth orbit on the periodic change of climate, this paper discusses the climate change characteristics of the Lower Oil Sand Mountain Formation in Qaidam Basin, and establishes a high-resolution astronomical scale for the Lower Oil Sand Mountain in Qaidam Basin based on Milankovitch theory to identify and divide high-frequency sequences. [Methods] Firstly, Laskar algorithm is used to calculate the variation period of the earth's orbital parameters during the summer solstice at 35 north latitude from 14.5~23.8 Ma, and the Miocene cycle theory and Miocene cycle ratio in this sedimentary period are determined. Then, taking wells Xianzhong 39, Xianzhong 8-9 and Xianzhong 8-12 in Nanbaxian oil and gas field as examples, the natural gamma data are analyzed by frequency spectrum and continuous wavelet transform. Finally, according to the orbital period, the average sedimentation rate of the Lower Youshashan Formation is calculated, and the "floating" astronomical scale of well Xianzhong 39 is established. [Results] Through the analysis of frequency spectrum and continuous wavelet transform, the Neogene Lower Youshashan Formation is mainly controlled by eccentricity periods of 400 ka and 95 ka. The average sedimentation rate of the Lower Youshashan Formation is 0.094 41 m/ka, and the sedimentation duration is 7.2 Ma. Based on the 400 ka long eccentric period curve and 95 ka short eccentric period curve as benchmark curves, 18 fourth-order quasi-sequence groups and 72 fifth-order quasi-sequence groups were identified. [Conclusion] The results show that the climate change recorded in the Lower Youshashan Formation is obviously controlled and driven by cycles. Identification and division based on Milankovitch theory can reduce the influence of subjective factors, improve the accuracy of division results, and more accurately describe the climate change characteristics in sediments. These research results are helpful to deeply understand the evolution law of the earth's climate and provide important reference for oil and gas exploration and resource evaluation.
Abstract:
The East Mariana Basin of the West Pacific Ocean, which is located in the east of the Mariana Trench, south of the Magellan Seamounts, and north of the Caroline Seamounts, is an ideal area for the study of Asina aeolian dust deposits, but the sediment research of the East Mariana Basin is still weak. In order to reveal the sediment geochemical characteristics and provide background data for further sediment provenance, seabed mineral resources evaluation and climate-environment evolution research, element geochemistry of surface sediments in the East Mariana Basin was studied. Based on 28 pelagic clay surface sediment (0-10 cm) samples collected in the east part of the East Mariana Basin using the box sampler and gravity sampler, contents of major elements, trace elements and rare earth elements of the sediments were analyzed by ICP-OES and ICP-MS methods. Then, elements geochemical characteristics were analyzed, and their influencing factors and indicative significance were discussed. The results show that the contents of major elements in the pelagic clay sediments in the study area are roughly the same as those in the neighboring sea areas of the West Pacific Ocean. The distribution pattern of major elements (oxides) in sediments follows SiO2 > Al2O3 > Fe2O3 > Na2O > MgO > K2O > CaO > MnO > TiO2 > P2O5. The highest content of major elements is SiO2, with an average of 49.14%, followed by Al2O3, with an average of 15.85%. The highest content of trace elements was Ba, with an average of 770×10-6, followed by Cu, with an average of 289×10-6. The average of total rare earth elements ∑REE is 284×10-6, which is light rare earth-rich type, with the highest Ce, Nd and La contents. Principal component analysis of elements shows that the composition of chemical elements can be divided into four categories: The first type is closely related to rare earth elements, including rare earth elements ( except Ce ), P2O5, TiO2, etc., the second type is related to Fe-Mn micronodules, including Fe2O3, MnO, Cr, Co, Ni, Cu, Ba and other metal elements, the third type is related to terrigenous debris, including Al2O3, MgO, SiO2, U, Hf, Th, Ce, etc., the fourth type is related to biological sources, including Cd, CaO, Mo. The elements combination indices (Si/Al, Fe/Al, La/Tb, Th/Sc, etc.) and the elements combination projection diagrams (La-Th-Sc triangle diagram, La/Th-Hf bivariate diagram) further reveal that the sediments sources are dominated by terrigenous materials, especially terrestrial aeolian dust materials. Meanwhile, the Chemical Index of Alteration (CIA) indicates that the parent rocks in the sediment source area are in low-medium chemical weathering conditions. The redox sensitive elements (Cr, Ni, V, U, Th, etc.) combination reveals that the bottom sedimentary environment in the study area is oxidation-weak oxidation environment. This study has some reference significance for understanding the influence of the Asian aeolian dust on the sediment provenance of the East Mariana Basin, revealing the sedimentary environment characteristics of the basin and the distribution of seabed mineral resources.
The East Mariana Basin of the West Pacific Ocean, which is located in the east of the Mariana Trench, south of the Magellan Seamounts, and north of the Caroline Seamounts, is an ideal area for the study of Asina aeolian dust deposits, but the sediment research of the East Mariana Basin is still weak. In order to reveal the sediment geochemical characteristics and provide background data for further sediment provenance, seabed mineral resources evaluation and climate-environment evolution research, element geochemistry of surface sediments in the East Mariana Basin was studied. Based on 28 pelagic clay surface sediment (0-10 cm) samples collected in the east part of the East Mariana Basin using the box sampler and gravity sampler, contents of major elements, trace elements and rare earth elements of the sediments were analyzed by ICP-OES and ICP-MS methods. Then, elements geochemical characteristics were analyzed, and their influencing factors and indicative significance were discussed. The results show that the contents of major elements in the pelagic clay sediments in the study area are roughly the same as those in the neighboring sea areas of the West Pacific Ocean. The distribution pattern of major elements (oxides) in sediments follows SiO2 > Al2O3 > Fe2O3 > Na2O > MgO > K2O > CaO > MnO > TiO2 > P2O5. The highest content of major elements is SiO2, with an average of 49.14%, followed by Al2O3, with an average of 15.85%. The highest content of trace elements was Ba, with an average of 770×10-6, followed by Cu, with an average of 289×10-6. The average of total rare earth elements ∑REE is 284×10-6, which is light rare earth-rich type, with the highest Ce, Nd and La contents. Principal component analysis of elements shows that the composition of chemical elements can be divided into four categories: The first type is closely related to rare earth elements, including rare earth elements ( except Ce ), P2O5, TiO2, etc., the second type is related to Fe-Mn micronodules, including Fe2O3, MnO, Cr, Co, Ni, Cu, Ba and other metal elements, the third type is related to terrigenous debris, including Al2O3, MgO, SiO2, U, Hf, Th, Ce, etc., the fourth type is related to biological sources, including Cd, CaO, Mo. The elements combination indices (Si/Al, Fe/Al, La/Tb, Th/Sc, etc.) and the elements combination projection diagrams (La-Th-Sc triangle diagram, La/Th-Hf bivariate diagram) further reveal that the sediments sources are dominated by terrigenous materials, especially terrestrial aeolian dust materials. Meanwhile, the Chemical Index of Alteration (CIA) indicates that the parent rocks in the sediment source area are in low-medium chemical weathering conditions. The redox sensitive elements (Cr, Ni, V, U, Th, etc.) combination reveals that the bottom sedimentary environment in the study area is oxidation-weak oxidation environment. This study has some reference significance for understanding the influence of the Asian aeolian dust on the sediment provenance of the East Mariana Basin, revealing the sedimentary environment characteristics of the basin and the distribution of seabed mineral resources.
Abstract:
[Objective]Dengying Formation in Penglai area of central Sichuan has achieved a major breakthrough in exploration, but the fourth member of Dengying Formation has experienced multiple stages of oil and gas charging due to its large burial depth, old age and complex diagenetic evolution, the relationship between reservoir pore evolution and oil and gas charging is still unclear. [Methods]Based on drilling coring data, the petrological types, reservoir space characteristics, diagenetic types, diagenetic evolution sequence, pore evolution and hydrocarbon charging of the fourth member of Dengying Formation were studied by means of thin section observation, cathodoluminescence, in situ microelement analysis and fluid inclusion. [Results and Discussions]The reservoir rock types of the fourth member of Dengying Formation in Penglai area are mainly crystalline dolomite, granular dolomite and microbial dolomite. The types of reservoir space can be divided into three types: cave, pore and fracture. The main diagenetic types are syngene-parsyngenetic dissolution, epigenetic dissolution, buried dissolution, cementation and filling, tectonic rupture, silicification and compaction. Diagenesis in different stages jointly controls the evolution of reservoir pores in the four members of Dengying Formation, among which the constructive diagenesis is dissolution and rupture in different periods, and the destructive diagenesis is compaction and pressure-dissolution and cementation and filling.[Conclusion] The filling sequence of cement in the solution holes of the fourth member of the reservoir can be divided into: the first generation blade-like dolomite → the second generation powdery dolomite → the first stage bitumen → the third generation fine crystalline dolomite → the fourth generation medium crystalline dolomite → the fifth generation coarse crystalline dolomite → the sixth generation giant crystalline saddle-shaped dolomite → the second stage bitumen → the seventh generation quartz, fluorite and other minerals. According to the information of fluid inclusions captured by cemented fill of each stage, such as type, phase state and homogenization temperature, the oil-gas charging process of Dthe fourth member of Dengying Formation in Penglai Area was reconstructed: Paleo-oil reservoir charging in the middle and late Silurian period (the first phase of fossil oil reservoir) → late Caledonian paleo-oil reservoir destruction → Middle Triassic paleo-oil reservoir charging (the second phase of fossil oil reservoir) → Late Jurassic paleo-gas cracking in paleo-oil reservoir → the adjustment and formation of paleo-gas reservoir from Late Cretaceous to present, in which the late oil cracking gas → gaseous hydrocarbon charging is the main forming period of the fourth member gas reservoir of Dengying Formation.
[Objective]Dengying Formation in Penglai area of central Sichuan has achieved a major breakthrough in exploration, but the fourth member of Dengying Formation has experienced multiple stages of oil and gas charging due to its large burial depth, old age and complex diagenetic evolution, the relationship between reservoir pore evolution and oil and gas charging is still unclear. [Methods]Based on drilling coring data, the petrological types, reservoir space characteristics, diagenetic types, diagenetic evolution sequence, pore evolution and hydrocarbon charging of the fourth member of Dengying Formation were studied by means of thin section observation, cathodoluminescence, in situ microelement analysis and fluid inclusion. [Results and Discussions]The reservoir rock types of the fourth member of Dengying Formation in Penglai area are mainly crystalline dolomite, granular dolomite and microbial dolomite. The types of reservoir space can be divided into three types: cave, pore and fracture. The main diagenetic types are syngene-parsyngenetic dissolution, epigenetic dissolution, buried dissolution, cementation and filling, tectonic rupture, silicification and compaction. Diagenesis in different stages jointly controls the evolution of reservoir pores in the four members of Dengying Formation, among which the constructive diagenesis is dissolution and rupture in different periods, and the destructive diagenesis is compaction and pressure-dissolution and cementation and filling.[Conclusion] The filling sequence of cement in the solution holes of the fourth member of the reservoir can be divided into: the first generation blade-like dolomite → the second generation powdery dolomite → the first stage bitumen → the third generation fine crystalline dolomite → the fourth generation medium crystalline dolomite → the fifth generation coarse crystalline dolomite → the sixth generation giant crystalline saddle-shaped dolomite → the second stage bitumen → the seventh generation quartz, fluorite and other minerals. According to the information of fluid inclusions captured by cemented fill of each stage, such as type, phase state and homogenization temperature, the oil-gas charging process of Dthe fourth member of Dengying Formation in Penglai Area was reconstructed: Paleo-oil reservoir charging in the middle and late Silurian period (the first phase of fossil oil reservoir) → late Caledonian paleo-oil reservoir destruction → Middle Triassic paleo-oil reservoir charging (the second phase of fossil oil reservoir) → Late Jurassic paleo-gas cracking in paleo-oil reservoir → the adjustment and formation of paleo-gas reservoir from Late Cretaceous to present, in which the late oil cracking gas → gaseous hydrocarbon charging is the main forming period of the fourth member gas reservoir of Dengying Formation.
Abstract:
The Huodiya Group is an important Precambrian stratigraphic unit in the northwestern margin of the Yangtze Block and a significant graphite-bearing stratum in China. However, its depositional age, provenance and tectonic attribution have been controversial for a long time. In this paper, LA-ICP-MS zircon U-Pb dating were carried out on the sericite phyllite from the Shangliang Formation of Huodiya Group, Wangcang area. The results show that maximum depositional age of the sample is 837.6 ± 6.0 Ma (MSWD = 0.60, n = 5). Combined existing data, the depositional age of Huodiya Group is constrained between ~910 Ma and ~835 Ma. The age populations of detrital zircons are concentrated in the four peaks of 832-843 Ma, 855-883 Ma, 895-936 Ma and 952-988 Ma, and the detrital provenance is mainly from the southeast and northwest magmatic rocks. A synthesis of depositional age, provenance and regional geological background, the Shangliang Formation of Huodiya Group in Wangcang area probably deposited in the backarc basin receiving provenance from both sides, which is a response to the late-stage convergence of the Rodinia supercontinent in the northwestern margin of the Yangtze Block.
The Huodiya Group is an important Precambrian stratigraphic unit in the northwestern margin of the Yangtze Block and a significant graphite-bearing stratum in China. However, its depositional age, provenance and tectonic attribution have been controversial for a long time. In this paper, LA-ICP-MS zircon U-Pb dating were carried out on the sericite phyllite from the Shangliang Formation of Huodiya Group, Wangcang area. The results show that maximum depositional age of the sample is 837.6 ± 6.0 Ma (MSWD = 0.60, n = 5). Combined existing data, the depositional age of Huodiya Group is constrained between ~910 Ma and ~835 Ma. The age populations of detrital zircons are concentrated in the four peaks of 832-843 Ma, 855-883 Ma, 895-936 Ma and 952-988 Ma, and the detrital provenance is mainly from the southeast and northwest magmatic rocks. A synthesis of depositional age, provenance and regional geological background, the Shangliang Formation of Huodiya Group in Wangcang area probably deposited in the backarc basin receiving provenance from both sides, which is a response to the late-stage convergence of the Rodinia supercontinent in the northwestern margin of the Yangtze Block.
Abstract:
[Objective]The reticulated laterite in southern China is a good geological carrier for Quaternary environmental changes, but there is currently relatively little microscopic research on bedrock reticulation,Not conducive to a comprehensive understanding of reticulation [Methods]Through micro-area analysis techniques, geostatistical and factor analysis, and other methods to conducted elemental geochemical analysis on the white vein micro-area of the bedrock and variegated?horizon in the Langxi profile. [Results]1) Element content in the white vein micro-area of sandstone: The content of iron group elements such as Fe2O3 (1.14-13.29%) and Mn (87×10-6 -3230×10-6) showed a trend of increasing from the center of the white veins to the periphery. However, the spatial distribution of other major elements and stable elements such as Ti (1294×10-6~ 2454×10-6) and Zr (171 ×10-6~ 197×10-6) showed the opposite trend. The spatial distribution of element content in the bedrock layer is basically consistent with that of the variegated?horizon. 2) Ti/Zr (bedrock layer: 7.23-12.89; reticulated layer: 12.82-21.84) and Ti/Al2O3 (bedrock layer: 0.013-0.018; reticulated layer: 0.044-0.062) were divided into two groups through cluster analysis and scatter plot.They have different provenance 3) The weathering of the bedrock layer (CIA: 86.37%~87.49%; SA: 5.23~5.76) is slightly higher than that of the variegated?horizon (CIA: 85.10%~86.07%; SA: 6.91~8.16;). The leaching intensity of elements such as Al2O3 (17.3%)、 Fe2O3 (73.91%) and MnO (76.68%)in the bedrock white vein micro-area and Al2O3 (15.78%) 、Fe2O3 (70.39%) and MnO (74.84%) in the variegated?horizon white vein micro-area showed a trend of decreasing from the center of the white veins to the periphery. 4) The common feature of factors in the white vein micro-area of the bedrock layer and the reticulated layer is that they are mainly factor 1, which reflects the leaching migration of iron and iron group elements in the white vein, as well as the relative enrichment process of constant elements such asAl2O3、SiO2and K2O and stable elements such as Ti and Zr. [Conclusion] The composition of the white reticulated interior of the bedrock layer and the variegated?horizon are non-spatially homogeneous, and the process of reticulation is a spatial process that extends from the center to the periphery.During reticulation, the leaching of iron and iron group elements dominates, but other elements also have a certain degree of migration, mainly characterized by strong leaching at the center of the reticulation and relatively weak leaching at the periphery. The reticulation process of the two is quite similar.
[Objective]The reticulated laterite in southern China is a good geological carrier for Quaternary environmental changes, but there is currently relatively little microscopic research on bedrock reticulation,Not conducive to a comprehensive understanding of reticulation [Methods]Through micro-area analysis techniques, geostatistical and factor analysis, and other methods to conducted elemental geochemical analysis on the white vein micro-area of the bedrock and variegated?horizon in the Langxi profile. [Results]1) Element content in the white vein micro-area of sandstone: The content of iron group elements such as Fe2O3 (1.14-13.29%) and Mn (87×10-6 -3230×10-6) showed a trend of increasing from the center of the white veins to the periphery. However, the spatial distribution of other major elements and stable elements such as Ti (1294×10-6~ 2454×10-6) and Zr (171 ×10-6~ 197×10-6) showed the opposite trend. The spatial distribution of element content in the bedrock layer is basically consistent with that of the variegated?horizon. 2) Ti/Zr (bedrock layer: 7.23-12.89; reticulated layer: 12.82-21.84) and Ti/Al2O3 (bedrock layer: 0.013-0.018; reticulated layer: 0.044-0.062) were divided into two groups through cluster analysis and scatter plot.They have different provenance 3) The weathering of the bedrock layer (CIA: 86.37%~87.49%; SA: 5.23~5.76) is slightly higher than that of the variegated?horizon (CIA: 85.10%~86.07%; SA: 6.91~8.16;). The leaching intensity of elements such as Al2O3 (17.3%)、 Fe2O3 (73.91%) and MnO (76.68%)in the bedrock white vein micro-area and Al2O3 (15.78%) 、Fe2O3 (70.39%) and MnO (74.84%) in the variegated?horizon white vein micro-area showed a trend of decreasing from the center of the white veins to the periphery. 4) The common feature of factors in the white vein micro-area of the bedrock layer and the reticulated layer is that they are mainly factor 1, which reflects the leaching migration of iron and iron group elements in the white vein, as well as the relative enrichment process of constant elements such asAl2O3、SiO2and K2O and stable elements such as Ti and Zr. [Conclusion] The composition of the white reticulated interior of the bedrock layer and the variegated?horizon are non-spatially homogeneous, and the process of reticulation is a spatial process that extends from the center to the periphery.During reticulation, the leaching of iron and iron group elements dominates, but other elements also have a certain degree of migration, mainly characterized by strong leaching at the center of the reticulation and relatively weak leaching at the periphery. The reticulation process of the two is quite similar.
Abstract:
Carbon and oxygen isotopes are one of the important geochemical indexes of carbonate rocks, which play an important role in revealing the characteristics of sedimentary and diagenetic fluids, and have become one of the basic means to study carbonate rocks. At present, a large number of studies on carbon and oxygen isotopes in limestone-dolomite stage have been carried out at home and abroad, but there are few studies on carbon and oxygen isotopes in sulfate-carbonate stage. In the sulfate-carbonate stage of the sedimentary sequence, there are still carbon and oxygen elements that can be tested, which makes this study possible. There are currently studies on carbon and oxygen isotopes of the plaster salt section of Huaying Mountain and the plaster bearing strata of Reichenhall Formation, Australia, but these studies are few and concentrated on surface samples. It is of great theoretical significance to study carbon and oxygen isotopes of underground paste and salt samples. In the process of drilling geothermal well in Doubei Village, Gaotan Town, Linshui County, Sichuan, to the east of Huaying Mountain in Sichuan Basin, carbon and oxygen isotopes of paste salt interbedding cuttings of Jialingjiang-Lekoupo Formation were studied. It is concluded that :(1) The δ18OPDB (‰) of paste salt interbedding cuttings in the Daobei geothermal well ranges from -2.56 to -15.47, with an average value of -9.13. The δ13CPDB (‰) ranges from -4.68 to -0.12, with an average value of -2.86, and the Z-value is generally lower than 120, which is inconsistent with the characteristics of Marine sediments. The δ18OPDB and δ13CPDB values were significantly lower than that of the Triassic paste salt formation in Huaying Mountain and the Reichenhall paste salt formation in Australia; (2) The carbon and oxygen isotopes of the underground salt-salt strata in this area are mainly affected by the thermochemical sulfate reduction (TSR) in the deep-buried stage before the formation fold and the massive inflow of surface fresh water after the formation fold, fracture and uplift. The high formation temperature and the exchange of organic carbon before fold and the desalting of surface water after fold and fracture are the fundamental reasons for the low δ18O and δ13C of the gypsum salt layer in this area. At the same time, there may be a little freshwater action and influence during the depositional stage; (3) The formation temperature calculated by Craig'sequation (1965) was 28.18~111.71℃, with an average of 68.45℃. The formation temperature calculated by Vasconcelos'sequation (2005) ranges from 4.54℃ to 78.21℃, with an average value of 38.91℃. The former is close to the present temperature of the formation, while the latter is lower, which may be related to the selection of δ18O in the calculation formula.
Carbon and oxygen isotopes are one of the important geochemical indexes of carbonate rocks, which play an important role in revealing the characteristics of sedimentary and diagenetic fluids, and have become one of the basic means to study carbonate rocks. At present, a large number of studies on carbon and oxygen isotopes in limestone-dolomite stage have been carried out at home and abroad, but there are few studies on carbon and oxygen isotopes in sulfate-carbonate stage. In the sulfate-carbonate stage of the sedimentary sequence, there are still carbon and oxygen elements that can be tested, which makes this study possible. There are currently studies on carbon and oxygen isotopes of the plaster salt section of Huaying Mountain and the plaster bearing strata of Reichenhall Formation, Australia, but these studies are few and concentrated on surface samples. It is of great theoretical significance to study carbon and oxygen isotopes of underground paste and salt samples. In the process of drilling geothermal well in Doubei Village, Gaotan Town, Linshui County, Sichuan, to the east of Huaying Mountain in Sichuan Basin, carbon and oxygen isotopes of paste salt interbedding cuttings of Jialingjiang-Lekoupo Formation were studied. It is concluded that :(1) The δ18OPDB (‰) of paste salt interbedding cuttings in the Daobei geothermal well ranges from -2.56 to -15.47, with an average value of -9.13. The δ13CPDB (‰) ranges from -4.68 to -0.12, with an average value of -2.86, and the Z-value is generally lower than 120, which is inconsistent with the characteristics of Marine sediments. The δ18OPDB and δ13CPDB values were significantly lower than that of the Triassic paste salt formation in Huaying Mountain and the Reichenhall paste salt formation in Australia; (2) The carbon and oxygen isotopes of the underground salt-salt strata in this area are mainly affected by the thermochemical sulfate reduction (TSR) in the deep-buried stage before the formation fold and the massive inflow of surface fresh water after the formation fold, fracture and uplift. The high formation temperature and the exchange of organic carbon before fold and the desalting of surface water after fold and fracture are the fundamental reasons for the low δ18O and δ13C of the gypsum salt layer in this area. At the same time, there may be a little freshwater action and influence during the depositional stage; (3) The formation temperature calculated by Craig'sequation (1965) was 28.18~111.71℃, with an average of 68.45℃. The formation temperature calculated by Vasconcelos'sequation (2005) ranges from 4.54℃ to 78.21℃, with an average value of 38.91℃. The former is close to the present temperature of the formation, while the latter is lower, which may be related to the selection of δ18O in the calculation formula.
Abstract:
[Objective] The Paleogene Shahejie Formation around the Bozhong sag, Bohai Bay Basin is characterized by the development of mixed rock reservoirs, which are rich in abundant oil and gas resources. The unique biomass cavity pores in mixed rock reservoirs make the initial porosity of mixed rocks cannot be obtained by the conventional initial porosity recovery formula for mixed rocks, and there is a lack of initial porosity recovery methods for mixed rocks both at home and abroad, and the accurate recovery of the initial porosity is a key element in the study of the evolution of the reservoir. [Methods] In this study, modern snail samples were selected to determine the volume of snail body cavities using experimental and formulaic methods. Subsequently, a physical simulation experiment was conducted to simulate the filling conditions of snail body cavities in a real depositional environment. Finally, based on commonly used formulas for calculating initial porosity in reservoirs, a formula suitable for mixed rock reservoirs was derived. [Results and Discussions] Studies have shown that the species of snails is the biggest factor affecting the percentage of body cavity pore volume of snails. Different species of snails have different sizes of effective storage space, i.e., body cavity pore porosity; the main reason affecting the difference in body cavity pore porosity of the same species of snails is the size of the body cavity pore lumen. Taking the mixed rock sample from well QHD36-3-A at a depth of 3765.03m with a high content of fragmented bioclasts as an example, the initial porosity of this mixed rock at that depth was determined to be 51.68%. [Conclusions] In this paper, a new calculation method to find the initial porosity of mixed rocks is established based on the previous formula and combined with physical simulation experiments, which is of great significance for the study of mixed reservoir evolution.
[Objective] The Paleogene Shahejie Formation around the Bozhong sag, Bohai Bay Basin is characterized by the development of mixed rock reservoirs, which are rich in abundant oil and gas resources. The unique biomass cavity pores in mixed rock reservoirs make the initial porosity of mixed rocks cannot be obtained by the conventional initial porosity recovery formula for mixed rocks, and there is a lack of initial porosity recovery methods for mixed rocks both at home and abroad, and the accurate recovery of the initial porosity is a key element in the study of the evolution of the reservoir. [Methods] In this study, modern snail samples were selected to determine the volume of snail body cavities using experimental and formulaic methods. Subsequently, a physical simulation experiment was conducted to simulate the filling conditions of snail body cavities in a real depositional environment. Finally, based on commonly used formulas for calculating initial porosity in reservoirs, a formula suitable for mixed rock reservoirs was derived. [Results and Discussions] Studies have shown that the species of snails is the biggest factor affecting the percentage of body cavity pore volume of snails. Different species of snails have different sizes of effective storage space, i.e., body cavity pore porosity; the main reason affecting the difference in body cavity pore porosity of the same species of snails is the size of the body cavity pore lumen. Taking the mixed rock sample from well QHD36-3-A at a depth of 3765.03m with a high content of fragmented bioclasts as an example, the initial porosity of this mixed rock at that depth was determined to be 51.68%. [Conclusions] In this paper, a new calculation method to find the initial porosity of mixed rocks is established based on the previous formula and combined with physical simulation experiments, which is of great significance for the study of mixed reservoir evolution.
Abstract:
[Objective]Typical beach bar sedimentary sand bodies are developed in the middle and upper member of Shibei Formation in Tainan area of Qinghai Oilfield, which is an important high-quality reservoir. The formation mechanism and distribution pattern of beach bar sand body have always been controversial, which has limited the further exploration and development of the gas field. Therefore, it is urgent to carry out fine characterization of the interior of the beach bar in Tainan area, identify the sedimentary microfacies types, and establish the corresponding deposit model of the beach bar. [Methods] Under the guidance of sedimentology, considering the hydrodynamic difference during the formation of beach bars, the sand body particle size and permeability are different. Using the median grain size as the link, the corresponding logging template of rock facies is summarized, and the interpretation standard and dynamic response of sedimentary microfacies are determined, so as to classify the sedimentary microfacies more scientifically. Based on the modern sedimentary analogy, sand body anatomy in the dense well pattern area of the study area, and the reasonableness of microfacies classification by horizontal Wells and production data, a beach bar sedimentation model suitable for the inland depression lake basin is proposed considering that the formation of beach bars is controlled by multiple factors. [Results] The beach and bar subfacies in the middle and upper part of Sebei Formation in Tainan area can be further divided into the main body of the bar, the edge of the bar, the main body of the beach and the edge of the beach. The bar sand is distributed above the beach sand, extending vertically to the northwest wind, in the form of lenticular and mencrescent, with a length of about 1,250 m and a width of about 250 m. The linear positive correlation between the length and width of the bar sand is about 5, which is consistent with modern sedimentation. As a whole, the beach sand is distributed in the direction of vertical provenance, and broadly and gently covers the edge of the bar sand. By comparing the deposit of beach bar in Tiangiz Lake, the extension direction, distribution rule and length-width ratio of bar sand are well corresponding. Sufficient source supply provides a good material basis for the formation of beach bars. The coupling of palaeostructure, palaeogeomorphology, palaeomonsoon and palaeowater depth controls the development scale of beach bars, the development form and geographical location of bar sands. [Conclusions] The sedimentary model of "open shallow lake, wind wave transformation, oblique row type" is proposed, which can provide theoretical guidance for the identification of beach bar system in inland depression lake basins.
[Objective]Typical beach bar sedimentary sand bodies are developed in the middle and upper member of Shibei Formation in Tainan area of Qinghai Oilfield, which is an important high-quality reservoir. The formation mechanism and distribution pattern of beach bar sand body have always been controversial, which has limited the further exploration and development of the gas field. Therefore, it is urgent to carry out fine characterization of the interior of the beach bar in Tainan area, identify the sedimentary microfacies types, and establish the corresponding deposit model of the beach bar. [Methods] Under the guidance of sedimentology, considering the hydrodynamic difference during the formation of beach bars, the sand body particle size and permeability are different. Using the median grain size as the link, the corresponding logging template of rock facies is summarized, and the interpretation standard and dynamic response of sedimentary microfacies are determined, so as to classify the sedimentary microfacies more scientifically. Based on the modern sedimentary analogy, sand body anatomy in the dense well pattern area of the study area, and the reasonableness of microfacies classification by horizontal Wells and production data, a beach bar sedimentation model suitable for the inland depression lake basin is proposed considering that the formation of beach bars is controlled by multiple factors. [Results] The beach and bar subfacies in the middle and upper part of Sebei Formation in Tainan area can be further divided into the main body of the bar, the edge of the bar, the main body of the beach and the edge of the beach. The bar sand is distributed above the beach sand, extending vertically to the northwest wind, in the form of lenticular and mencrescent, with a length of about 1,250 m and a width of about 250 m. The linear positive correlation between the length and width of the bar sand is about 5, which is consistent with modern sedimentation. As a whole, the beach sand is distributed in the direction of vertical provenance, and broadly and gently covers the edge of the bar sand. By comparing the deposit of beach bar in Tiangiz Lake, the extension direction, distribution rule and length-width ratio of bar sand are well corresponding. Sufficient source supply provides a good material basis for the formation of beach bars. The coupling of palaeostructure, palaeogeomorphology, palaeomonsoon and palaeowater depth controls the development scale of beach bars, the development form and geographical location of bar sands. [Conclusions] The sedimentary model of "open shallow lake, wind wave transformation, oblique row type" is proposed, which can provide theoretical guidance for the identification of beach bar system in inland depression lake basins.
Abstract:
During the Cretaceous period, extreme greenhouse climate, global oceanic anoxia and oxygen enrichment events occurred in geological history, and the Cretaceous red beds that appeared during this period contain important information related to paleoclimate and paleoenvironment, which is of great significance to the understanding of stratigraphic surface systems. In this paper, a set of red mudstones developed during the Cretaceous in the North Yellow Sea Basin of eastern China is taken as the object of study. X-ray diffraction and diffuse reflectance spectroscopy (XRD) analyses were carried out on the red and grey mudstones of the Middle Jurassic to Lower Cretaceous, in order to semi-quantitatively and quantitatively analyze the composition and content of chromogenic minerals in the rocks. We also combined the ordinary thin section and scanning electron microscope methods to observe the characteristics of chromogenic minerals in the red mudstone, such as the morphology, distribution and crystalline size of iron-containing minerals. XRD results show that the chromogenic minerals in the red mudstone are hematite and goethite, of which hematite is the main content, the highest content can reach 14%, while the grey mudstone does not contain hematite; hematite aggregates can be seen under the microscope in the red mudstone, and alteration phenomena can be seen in the local area; hematite can be seen in the form of granular or plate aggregates under the scanning electron microscope; Using the characteristics of changes in hematite and goethite content, it is concluded that the climate was relatively humid from the Late Jurassic to the Early Cretaceous, and hot and dry from the Middle to Late Early Cretaceous. Combined with the paleoclimate data of the North Yellow Sea Basin and its adjacent areas, the method of using iron oxides to reflect the paleoclimate changes in the study area is found to be feasible, and a comprehensive analysis suggests that the chromogenic minerals in the red mudstone of the Lower Cretaceous have indicative significance for the paleoenvironment and paleoclimate.
During the Cretaceous period, extreme greenhouse climate, global oceanic anoxia and oxygen enrichment events occurred in geological history, and the Cretaceous red beds that appeared during this period contain important information related to paleoclimate and paleoenvironment, which is of great significance to the understanding of stratigraphic surface systems. In this paper, a set of red mudstones developed during the Cretaceous in the North Yellow Sea Basin of eastern China is taken as the object of study. X-ray diffraction and diffuse reflectance spectroscopy (XRD) analyses were carried out on the red and grey mudstones of the Middle Jurassic to Lower Cretaceous, in order to semi-quantitatively and quantitatively analyze the composition and content of chromogenic minerals in the rocks. We also combined the ordinary thin section and scanning electron microscope methods to observe the characteristics of chromogenic minerals in the red mudstone, such as the morphology, distribution and crystalline size of iron-containing minerals. XRD results show that the chromogenic minerals in the red mudstone are hematite and goethite, of which hematite is the main content, the highest content can reach 14%, while the grey mudstone does not contain hematite; hematite aggregates can be seen under the microscope in the red mudstone, and alteration phenomena can be seen in the local area; hematite can be seen in the form of granular or plate aggregates under the scanning electron microscope; Using the characteristics of changes in hematite and goethite content, it is concluded that the climate was relatively humid from the Late Jurassic to the Early Cretaceous, and hot and dry from the Middle to Late Early Cretaceous. Combined with the paleoclimate data of the North Yellow Sea Basin and its adjacent areas, the method of using iron oxides to reflect the paleoclimate changes in the study area is found to be feasible, and a comprehensive analysis suggests that the chromogenic minerals in the red mudstone of the Lower Cretaceous have indicative significance for the paleoenvironment and paleoclimate.
Abstract:
There are many curvilinear ridges that have similar planforms as rivers in the northern and southwestern Qaidam Basin. To investigate their formation mechanism, [Methods] we carry out statistical and morphological analyses on 21 sets of gravel samples from the curvilinear ridges, and compared them with the gravel layer sediments in the middle reaches of the Han River. [Results and Discussions] Analysis shows that the ratio of round and sub-round gravels in the curvilinear ridges area ranges from 71.6% to 74.0%, and the ratio of discoid gravels ranges from 44.9% to 54.0%. The correlation coefficients of the gravel morphological features between the distribution area of curvilinear ridges in the Qaidam Basin and the sampling area of the middle reaches of the Han River range from 0.688 to 0.7258. [Conclusions] Studies indicate the following: the gravels in the curvilinear ridges are fluvial deposits. And if based on the modern hydrologic parameters of the Qaidam Basin, the paleocurrent velocity, paleo-discharge and paleo-annual runoff rate of the river indicated by the curvilinear ridges are 0.217 m/s, 1.39 m3/s, and 0.1440 × 108 m3, respectively. The paleo-discharge range of the curvilinear ridges of the Qaidam Basin reconstructed based on the channel width is 14-16 m3/s. Accordingly, the paleo-runoff depth is estimated to be about 2 m, and the corresponding paleocurrent velocity, paleo-discharge and paleo-annual runoff rate are about 0.357 m/s, 14.28 m3/s, and 1.4801×108 m3, respectively. Keywords: Qaidam Basin; Curvilinear ridge; Gravel morphology; Han River; Paleohydrology
There are many curvilinear ridges that have similar planforms as rivers in the northern and southwestern Qaidam Basin. To investigate their formation mechanism, [Methods] we carry out statistical and morphological analyses on 21 sets of gravel samples from the curvilinear ridges, and compared them with the gravel layer sediments in the middle reaches of the Han River. [Results and Discussions] Analysis shows that the ratio of round and sub-round gravels in the curvilinear ridges area ranges from 71.6% to 74.0%, and the ratio of discoid gravels ranges from 44.9% to 54.0%. The correlation coefficients of the gravel morphological features between the distribution area of curvilinear ridges in the Qaidam Basin and the sampling area of the middle reaches of the Han River range from 0.688 to 0.7258. [Conclusions] Studies indicate the following: the gravels in the curvilinear ridges are fluvial deposits. And if based on the modern hydrologic parameters of the Qaidam Basin, the paleocurrent velocity, paleo-discharge and paleo-annual runoff rate of the river indicated by the curvilinear ridges are 0.217 m/s, 1.39 m3/s, and 0.1440 × 108 m3, respectively. The paleo-discharge range of the curvilinear ridges of the Qaidam Basin reconstructed based on the channel width is 14-16 m3/s. Accordingly, the paleo-runoff depth is estimated to be about 2 m, and the corresponding paleocurrent velocity, paleo-discharge and paleo-annual runoff rate are about 0.357 m/s, 14.28 m3/s, and 1.4801×108 m3, respectively. Keywords: Qaidam Basin; Curvilinear ridge; Gravel morphology; Han River; Paleohydrology
Abstract:
[Objective] Chemical weathering is a key geological process that regulates the long-term climate of the earth and participates in the global carbon cycle, and is particularly important for the evolution of terrestrial climate and reconstruction of paleoclimate. [Methods] In this study, 89 aeolian sand and river sand fine-grained components (<63 μm), revealed the spatial distribution characteristics of chemical weathering in the four major sandy lands in Northeast China (Hunshandake Sandy Land, Horqin Sandy Land, Songnen Sandy Land and Hulunbuir Sandy Land), and evaluated the response relationship and influencing factors of 13 weathering indexes to climatic factors (average annual temperature and average annual precipitation). [Results and Discussions] The results showed that the same chemical weathering index (e.g. CIA) showed a poor spatial distribution law and poor correlation with climatic factors in different sandy lands (the correlation coefficient was generally less than 0.4), but different weathering indicators showed a good correlation with climatic factors in different sandy lands (the correlation coefficient was generally greater than 0.6). This indicates that the sensitivity of different weathering indicators to climate varies significantly in different regions. The chemical weathering of the sandy land in Northeast China measured by different indexes is generally controlled by climatic factors, but there are still obvious differences in the correlation between the chemical weathering intensity and temperature and precipitation in different sandy lands. The chemical weathering intensity of Hunshandake Sandy Land was generally low under the influence of precipitation, which was mainly controlled by river action and monsoon precipitation. The weathering intensity in the eastern part of the sandy land is affected by precipitation, which is mainly controlled by topography, monsoon, water vapor transport and element migration, while the weathering intensity in the western part of the sandy land is affected by temperature, which is mainly affected by the topography and high annual average temperature. The southern part of the Songnen Sandy Land has a high weathering rate and is most significantly affected by precipitation, which is mainly affected by geographical location, monsoon precipitation, distribution of multiple rivers and vegetation coverage. The weathering intensity of Hulunbuir Sandy Land is at a moderate level and is relatively weak due to temperature and precipitation, which is mainly controlled by topography, wind erosion, physical weathering and vegetation coverage. [Conclusions] In general, in addition to climatic conditions, the effects of river development, topography and regional geographical characteristics on chemical weathering intensity are equally important, so it is necessary to choose carefully when using sediment chemical weathering indicators for paleoclimate reconstruction, and this study also provides a new idea for sandy land environmental management in Northeast China.
[Objective] Chemical weathering is a key geological process that regulates the long-term climate of the earth and participates in the global carbon cycle, and is particularly important for the evolution of terrestrial climate and reconstruction of paleoclimate. [Methods] In this study, 89 aeolian sand and river sand fine-grained components (<63 μm), revealed the spatial distribution characteristics of chemical weathering in the four major sandy lands in Northeast China (Hunshandake Sandy Land, Horqin Sandy Land, Songnen Sandy Land and Hulunbuir Sandy Land), and evaluated the response relationship and influencing factors of 13 weathering indexes to climatic factors (average annual temperature and average annual precipitation). [Results and Discussions] The results showed that the same chemical weathering index (e.g. CIA) showed a poor spatial distribution law and poor correlation with climatic factors in different sandy lands (the correlation coefficient was generally less than 0.4), but different weathering indicators showed a good correlation with climatic factors in different sandy lands (the correlation coefficient was generally greater than 0.6). This indicates that the sensitivity of different weathering indicators to climate varies significantly in different regions. The chemical weathering of the sandy land in Northeast China measured by different indexes is generally controlled by climatic factors, but there are still obvious differences in the correlation between the chemical weathering intensity and temperature and precipitation in different sandy lands. The chemical weathering intensity of Hunshandake Sandy Land was generally low under the influence of precipitation, which was mainly controlled by river action and monsoon precipitation. The weathering intensity in the eastern part of the sandy land is affected by precipitation, which is mainly controlled by topography, monsoon, water vapor transport and element migration, while the weathering intensity in the western part of the sandy land is affected by temperature, which is mainly affected by the topography and high annual average temperature. The southern part of the Songnen Sandy Land has a high weathering rate and is most significantly affected by precipitation, which is mainly affected by geographical location, monsoon precipitation, distribution of multiple rivers and vegetation coverage. The weathering intensity of Hulunbuir Sandy Land is at a moderate level and is relatively weak due to temperature and precipitation, which is mainly controlled by topography, wind erosion, physical weathering and vegetation coverage. [Conclusions] In general, in addition to climatic conditions, the effects of river development, topography and regional geographical characteristics on chemical weathering intensity are equally important, so it is necessary to choose carefully when using sediment chemical weathering indicators for paleoclimate reconstruction, and this study also provides a new idea for sandy land environmental management in Northeast China.
Abstract:
The coal-bearing strata of Carboniferous Taiyuan Formation are widely distributed in Dawangzhuang area, Jiyang depression, and the high-quality reservoir control factors in the coal strata have an important influence on oil and gas exploration. Combined with burial history, tectonic evolution history, multi-proxy analyses, including the cast thin sections, fluorescence, cathodoluminescence, scanning electron microscopy and energy spectrum, petrography of fluid inclusion and homogeneous temperature, and laser ablation inductively coupled plasma mass spectrometry, were conducted to systematically study the characteristics and genetic model of high-quality reservoir of clastic rocks in coal-bearing strata of Carboniferous Taiyuan Formation(C3t) in Dawangzhuang area, Jiyang depression. The clastic reservoir of Taiyuan Formation coal measure strata in Dawangzhuang area is mainly composed of lithic quartz sandstone. The high-quality reservoir space is dominated by secondary pores, and the dissolution is strong. It is mainly filled with interstitial materials dissolution pores and feldspar dissolution pores formed by atmospheric fresh water and organic acid dissolution. The porosity of high-quality reservoir is more than 8 %, and the permeability is more than 5 × 10-3 μm2. And two phases of quartz overgrowth and two phases of carbonate cements are developed, dominated by carbonate cements. The first phase of quartz overgrowth mainly derives from pressure dissolution of quartz particles, the second phase of quartz overgrowth is mainly sourced from the feldspar dissolution. Siderite is mainly formed by pore water precipitation in syndepositional-early diagenesis stage, and ankerite is mainly related to organic acid decarboxylation. The evolution process of the diagenesis-accumulation system of the clastic rock reservoir of coal measures strata of C3t in Dawangzhuang area of Jiyang depression is: siderite → first phase of quartz overgrowth → first-stage oil in yellow fluorescence emplacement → kaolinite → ankerite →second-stage oil emplacement in blue fluorescence → second phase of quartz overgrowth and kaolinite →pyrite. High-quality reservoirs are mainly controlled by sedimentary facies, tectonic action and diagenesis. The sedimentary facies controls the distribution of lithofacies of high-quality reservoirs. The high-quality reservoirs are mainly controlled by the late uplift of atmospheric fresh water leaching dissolution and buried organic acid dissolution. At the same time, they are controlled by tectonic activities. The distance between the high-quality reservoirs and the docking faults should be far away, so as to avoid the further precipitation of CO2 produced by decarboxylation to form ankerite to destroy the reservoir properties.
The coal-bearing strata of Carboniferous Taiyuan Formation are widely distributed in Dawangzhuang area, Jiyang depression, and the high-quality reservoir control factors in the coal strata have an important influence on oil and gas exploration. Combined with burial history, tectonic evolution history, multi-proxy analyses, including the cast thin sections, fluorescence, cathodoluminescence, scanning electron microscopy and energy spectrum, petrography of fluid inclusion and homogeneous temperature, and laser ablation inductively coupled plasma mass spectrometry, were conducted to systematically study the characteristics and genetic model of high-quality reservoir of clastic rocks in coal-bearing strata of Carboniferous Taiyuan Formation(C3t) in Dawangzhuang area, Jiyang depression. The clastic reservoir of Taiyuan Formation coal measure strata in Dawangzhuang area is mainly composed of lithic quartz sandstone. The high-quality reservoir space is dominated by secondary pores, and the dissolution is strong. It is mainly filled with interstitial materials dissolution pores and feldspar dissolution pores formed by atmospheric fresh water and organic acid dissolution. The porosity of high-quality reservoir is more than 8 %, and the permeability is more than 5 × 10-3 μm2. And two phases of quartz overgrowth and two phases of carbonate cements are developed, dominated by carbonate cements. The first phase of quartz overgrowth mainly derives from pressure dissolution of quartz particles, the second phase of quartz overgrowth is mainly sourced from the feldspar dissolution. Siderite is mainly formed by pore water precipitation in syndepositional-early diagenesis stage, and ankerite is mainly related to organic acid decarboxylation. The evolution process of the diagenesis-accumulation system of the clastic rock reservoir of coal measures strata of C3t in Dawangzhuang area of Jiyang depression is: siderite → first phase of quartz overgrowth → first-stage oil in yellow fluorescence emplacement → kaolinite → ankerite →second-stage oil emplacement in blue fluorescence → second phase of quartz overgrowth and kaolinite →pyrite. High-quality reservoirs are mainly controlled by sedimentary facies, tectonic action and diagenesis. The sedimentary facies controls the distribution of lithofacies of high-quality reservoirs. The high-quality reservoirs are mainly controlled by the late uplift of atmospheric fresh water leaching dissolution and buried organic acid dissolution. At the same time, they are controlled by tectonic activities. The distance between the high-quality reservoirs and the docking faults should be far away, so as to avoid the further precipitation of CO2 produced by decarboxylation to form ankerite to destroy the reservoir properties.
Abstract:
The formation and preservation of large-scale oolites in marine environments serve as valuable tools for the study of climate and environmental evolution, as well as being important petroleum reservoirs. However, traditional studies on oolites have focused primarily on their depositional sequences, patterns, and spatial distribution, almost overlooking the dynamic processes of ooid shoal accumulation, burial, and diagenesis, as well as the influence of environmental controls on their development. This paper aims to address this gap by comprehensively analyzing two typical ooid factories (rimmed shelves and ramps) in modern marine environments, examining their producers, composition, distribution, construction process, relationship with other factories, and the potential physical, chemical, and biological factors influencing their development. The study reveals that the key factors limiting the development of ooid factories include strong turbulent water conditions, extremely high carbonate mineral saturation, and effective active-stationary stage transitions. Additionally, the developmental environment and scale of these factories are determined by their specific depositional backgrounds. From a carbonate factory perspective, this paper not only provides important insights into the initiation, flourishing, and decline processes of large-scale ooid factories, but also offers multidimensional evidence for evaluating their value in oil and gas exploration. Furthermore, it offers new insights for exploring the deep time and predicting the future environmental changes on carbonate depositional systems.
The formation and preservation of large-scale oolites in marine environments serve as valuable tools for the study of climate and environmental evolution, as well as being important petroleum reservoirs. However, traditional studies on oolites have focused primarily on their depositional sequences, patterns, and spatial distribution, almost overlooking the dynamic processes of ooid shoal accumulation, burial, and diagenesis, as well as the influence of environmental controls on their development. This paper aims to address this gap by comprehensively analyzing two typical ooid factories (rimmed shelves and ramps) in modern marine environments, examining their producers, composition, distribution, construction process, relationship with other factories, and the potential physical, chemical, and biological factors influencing their development. The study reveals that the key factors limiting the development of ooid factories include strong turbulent water conditions, extremely high carbonate mineral saturation, and effective active-stationary stage transitions. Additionally, the developmental environment and scale of these factories are determined by their specific depositional backgrounds. From a carbonate factory perspective, this paper not only provides important insights into the initiation, flourishing, and decline processes of large-scale ooid factories, but also offers multidimensional evidence for evaluating their value in oil and gas exploration. Furthermore, it offers new insights for exploring the deep time and predicting the future environmental changes on carbonate depositional systems.
Abstract:
Abstract: [Objective] In this study, to redetermine the stratigraphic boundary of the Carboniferous-Permian Taiyuan and Shanxi formations in the Yichuan area of the Ordos Basin and to solve the problem of sedimentary age attribution of coal-bearing natural gas mudstone section near the boundary of the Shanxi-Taiyuan Formation. [Methods] We selected the sandstone and mudstone samples of the transition layer between the Taiyuan and Shanxi formations in the typical coring well Yi 120 to conduct the U-Pb dating of detrital zircon and we used the maximum depositional age ( MDA ) estimation method, combined with previous research results on the zircon age of the Taiyuan Formation-Shanxi Formation in the North China Plate, to constrain the depositional age of the study interval. [Results] Experiments show that the maximum likelihood age (MLA) of the sandstone and mudstone at the top of the Taiyuan Formation is 298±2 Ma, which represents the age of the latest deposition of the strata. This interval was deposited in the Early Asselian period, which is consistent with the Asselian sedimentary age determined by conodont. The MLAs of the sandstone samples at the bottom of the Shanxi Formation is 295±1 Ma, representing the age of the earliest deposition, which was deposited in the Middle-Late Arthurian period. [Conclusions] Based on the above research and analysis, we conclude that the boundary between the Taiyuan and Shanxi formations in the study area should be above the sand-mudstone section at the top of the Taiyuan Formation; that is, the marine sand-mudstone layer at the top of the Taiyuan Formation in the study area should belong to the Taiyuan Formation, which provides a certain basis for the stratigraphic division of the Shanxi-Taiyuan Formation in the study area.
Abstract: [Objective] In this study, to redetermine the stratigraphic boundary of the Carboniferous-Permian Taiyuan and Shanxi formations in the Yichuan area of the Ordos Basin and to solve the problem of sedimentary age attribution of coal-bearing natural gas mudstone section near the boundary of the Shanxi-Taiyuan Formation. [Methods] We selected the sandstone and mudstone samples of the transition layer between the Taiyuan and Shanxi formations in the typical coring well Yi 120 to conduct the U-Pb dating of detrital zircon and we used the maximum depositional age ( MDA ) estimation method, combined with previous research results on the zircon age of the Taiyuan Formation-Shanxi Formation in the North China Plate, to constrain the depositional age of the study interval. [Results] Experiments show that the maximum likelihood age (MLA) of the sandstone and mudstone at the top of the Taiyuan Formation is 298±2 Ma, which represents the age of the latest deposition of the strata. This interval was deposited in the Early Asselian period, which is consistent with the Asselian sedimentary age determined by conodont. The MLAs of the sandstone samples at the bottom of the Shanxi Formation is 295±1 Ma, representing the age of the earliest deposition, which was deposited in the Middle-Late Arthurian period. [Conclusions] Based on the above research and analysis, we conclude that the boundary between the Taiyuan and Shanxi formations in the study area should be above the sand-mudstone section at the top of the Taiyuan Formation; that is, the marine sand-mudstone layer at the top of the Taiyuan Formation in the study area should belong to the Taiyuan Formation, which provides a certain basis for the stratigraphic division of the Shanxi-Taiyuan Formation in the study area.
Abstract:
China possesses abundant terrestrial shale oil resources, exhibiting significant potential for exploration. This study focuses on the black shale of the Shahejie Formation(Es1) in the Jiyang Depression as its research subject. By employing XRD, rock pyrolysis, fluorescence analysis, scanning electron microscopy, and low-temperature gas adsorption techniques, we comprehensively investigate the laminated black shale layer types, layer combinations, pore types, and structural characteristics of the Es1 section within the southern Bohai Bay Basin. The findings reveal a diverse range of mineral types present in the Es1 section black shales. Based on their distinct compositions, these black shale layers are classified into five categories: feldspar-quartz lamina, clay mineral-rich lamina, aragonite lamina, micritic calcite-dominated lamina, and organic-rich lamina. Furthermore, the vertical stacking relationships allow for their division into three binary layer combinations: "organic-rich + micritic calcite", "organic-rich + aragonite" and "organic-rich + feldspar-quartz". Among these, the porosity of "organic-rich + aragonite" and "organic-rich + feldspar-quartz" binary lamina combination shales was relatively higher, with superior pore structure and connectivity compared to those of "organic-rich + micritic calcite" binary lamina combination shales. In the "organic-rich + aragonite" binary lamina combination shale, shale oil is present in a free state within interlayer crevices, exhibiting optimal mobility. The coexistence of free oil and adsorbed oil in the "organic-rich + aragonite" binary lamina combination shale results in relatively poor mobility. However, in the "organic-rich + feldspar-quartz" binary lamina combination shale, shale oil primarily exists in an adsorbed form, demonstrating the lowest mobility. The research findings elucidate the heterogeneity in reservoir properties among different lamina combinations and establish the pivotal influence of diverse lamina combinations on balck shale's porosity, pore structure, and hydrocarbon storage capacity. Based on these discoveries, we have developed shale oil storage models tailored to various lamina combination black shales, thereby providing a scientific foundation for future extraction of shale oil and gas.
China possesses abundant terrestrial shale oil resources, exhibiting significant potential for exploration. This study focuses on the black shale of the Shahejie Formation(Es1) in the Jiyang Depression as its research subject. By employing XRD, rock pyrolysis, fluorescence analysis, scanning electron microscopy, and low-temperature gas adsorption techniques, we comprehensively investigate the laminated black shale layer types, layer combinations, pore types, and structural characteristics of the Es1 section within the southern Bohai Bay Basin. The findings reveal a diverse range of mineral types present in the Es1 section black shales. Based on their distinct compositions, these black shale layers are classified into five categories: feldspar-quartz lamina, clay mineral-rich lamina, aragonite lamina, micritic calcite-dominated lamina, and organic-rich lamina. Furthermore, the vertical stacking relationships allow for their division into three binary layer combinations: "organic-rich + micritic calcite", "organic-rich + aragonite" and "organic-rich + feldspar-quartz". Among these, the porosity of "organic-rich + aragonite" and "organic-rich + feldspar-quartz" binary lamina combination shales was relatively higher, with superior pore structure and connectivity compared to those of "organic-rich + micritic calcite" binary lamina combination shales. In the "organic-rich + aragonite" binary lamina combination shale, shale oil is present in a free state within interlayer crevices, exhibiting optimal mobility. The coexistence of free oil and adsorbed oil in the "organic-rich + aragonite" binary lamina combination shale results in relatively poor mobility. However, in the "organic-rich + feldspar-quartz" binary lamina combination shale, shale oil primarily exists in an adsorbed form, demonstrating the lowest mobility. The research findings elucidate the heterogeneity in reservoir properties among different lamina combinations and establish the pivotal influence of diverse lamina combinations on balck shale's porosity, pore structure, and hydrocarbon storage capacity. Based on these discoveries, we have developed shale oil storage models tailored to various lamina combination black shales, thereby providing a scientific foundation for future extraction of shale oil and gas.
Abstract:
[Objective] To investigate the sorting movement and sedimentary characteristics of fine-grained sediments under the action of turbidity currents, and analyze the controlled factors of their transport distance and sediments spatial distribution. [Methods] Based on circular flume simulation experiment, the transport and deposition process of fine-grained sediments carried by turbidity currents were simulated and analyzed by controlling three conditions: initial fluid velocity, sediment concentration, and sand mud ratio, and the sedimentary dynamic mechanism was explored. [Results] The simulation results indicate that: (1) Fine-grained sediments transported by turbidity currents will experience experimental phenomena such as “water jump”, “double flow segmentation”, “lofting”, “head lifting”, and “new head” during the flow process. (2) During fluid transportation, the movement speed and distance of fine-grained sediments are influenced by the concentration difference between the fluid and the environmental fluid. (3) Factors such as initial flow velocity, water jump, and lofting control fluid flow velocity, fine-grained sediments transport distance, and spatial distribution. (4) The “new head” phenomenon causes the sand bodies carried by the fluid to become discontinuous, isolated or dispersed. [Conclusions] According to the “new head” phenomenon in the simulation process, the causes of dispersed sand bodies are speculated, which has certain guiding significance for the study of the formation and distribution of dispersed sand bodies.
[Objective] To investigate the sorting movement and sedimentary characteristics of fine-grained sediments under the action of turbidity currents, and analyze the controlled factors of their transport distance and sediments spatial distribution. [Methods] Based on circular flume simulation experiment, the transport and deposition process of fine-grained sediments carried by turbidity currents were simulated and analyzed by controlling three conditions: initial fluid velocity, sediment concentration, and sand mud ratio, and the sedimentary dynamic mechanism was explored. [Results] The simulation results indicate that: (1) Fine-grained sediments transported by turbidity currents will experience experimental phenomena such as “water jump”, “double flow segmentation”, “lofting”, “head lifting”, and “new head” during the flow process. (2) During fluid transportation, the movement speed and distance of fine-grained sediments are influenced by the concentration difference between the fluid and the environmental fluid. (3) Factors such as initial flow velocity, water jump, and lofting control fluid flow velocity, fine-grained sediments transport distance, and spatial distribution. (4) The “new head” phenomenon causes the sand bodies carried by the fluid to become discontinuous, isolated or dispersed. [Conclusions] According to the “new head” phenomenon in the simulation process, the causes of dispersed sand bodies are speculated, which has certain guiding significance for the study of the formation and distribution of dispersed sand bodies.
Abstract:
[Objective] The Late Paleozoic Ice Age is the most remarkable icehouse period since the flourish of the terrestrial ecosystem and is characterized by multiple discrete glacial and interglacial periods. Several global warming events occurred during the Late Paleozoic Ice Age, which have received wide attention in recent years. An abrupt negative excursion in carbon isotopes (δ13C) has been recorded near the Late Pennsylvanian Kasimovian-Gzhelian Boundary (KGB), accompanied by significant global warming. The KGB warming event under the background of interglacial period of the icehouse climate is of great interest, but the study on this warming event is still in the initial stage. [Methods] In this paper, detailed sedimentological and comprehensive stratigraphic studies were carried out on the ~20-m-thick strata across the KGB in the Naqing, Shanglong, and Narao sections of the Luodian Basin, South China. [Results and Discussions] Four sedimentary lithofacies are identified, including the lime mudstone facies, bioclastic wacke- to packstone facies, normal-graded packstone facies, and dark calcareous mudstone facies, indicating a deep-water slope environment with frequent sea-level fluctuations. The newly obtained carbonate δ13C record from the Shanglong section can be well compared with the previously published records from the Naqing and Narao sections, and the negative excursion in δ13C across the KGB is recorded around the world. Three cycles of paleo-water-depth variation at the Heckelina eudoraensis zone, Idiognathodus naraoensis zone, and the bottom of H. simulator zone in the study interval show a similar pace with the astronomical cycles and can be correlated to those of the Mid-continent from North America. [Conclusions] The studied successions of the Luodian Basin provide an important reference for the study of the KGB warming event.
[Objective] The Late Paleozoic Ice Age is the most remarkable icehouse period since the flourish of the terrestrial ecosystem and is characterized by multiple discrete glacial and interglacial periods. Several global warming events occurred during the Late Paleozoic Ice Age, which have received wide attention in recent years. An abrupt negative excursion in carbon isotopes (δ13C) has been recorded near the Late Pennsylvanian Kasimovian-Gzhelian Boundary (KGB), accompanied by significant global warming. The KGB warming event under the background of interglacial period of the icehouse climate is of great interest, but the study on this warming event is still in the initial stage. [Methods] In this paper, detailed sedimentological and comprehensive stratigraphic studies were carried out on the ~20-m-thick strata across the KGB in the Naqing, Shanglong, and Narao sections of the Luodian Basin, South China. [Results and Discussions] Four sedimentary lithofacies are identified, including the lime mudstone facies, bioclastic wacke- to packstone facies, normal-graded packstone facies, and dark calcareous mudstone facies, indicating a deep-water slope environment with frequent sea-level fluctuations. The newly obtained carbonate δ13C record from the Shanglong section can be well compared with the previously published records from the Naqing and Narao sections, and the negative excursion in δ13C across the KGB is recorded around the world. Three cycles of paleo-water-depth variation at the Heckelina eudoraensis zone, Idiognathodus naraoensis zone, and the bottom of H. simulator zone in the study interval show a similar pace with the astronomical cycles and can be correlated to those of the Mid-continent from North America. [Conclusions] The studied successions of the Luodian Basin provide an important reference for the study of the KGB warming event.
Abstract:
[Objective] Selective dissolution is common in marine carbonates, and its origin is typically to be related to meteoric fluids, while other possible origins lack further discussion. [Methods] In this study, the origin of marine selective dissolution of Buqu Formation of well GK-1 in the south Qiangtang basin is investigated based on petrography, stable carbon and oxygen isotopes, and elemental geochemistry. [Results] Eight sedimentary cycles (C1 to C8 from bottom to top) were recognized from the studied Buqu Formation, each cycle with limestone in the lower part and dolostone in the upper part. These eight cycles can be divided into two types (A and B) according to different carbonate components. The limestone in type A cycle (C1-C4) is dominated by bioclastic packstone and grainstone, while in type B cycle (C5-C8) is dominated by bioclastic wackstone and packstone. Upwards from type A cycle to the type B cycle, the content of aragonite fossils (gastropods and bivalves) in the limestone decreases and the content of peloids increases. The dolomite in both type A and type B cycles is crystalline dolomite with ooid ghosts showing selective dissolution pores. Importantly, the overall percentage of selective dissolution pores in type A cycles is significantly higher than that in type B cycles. In terms of geochemistry, the carbon and oxygen isotopes of dolostone are relatively higher compared to those of limestone in each cycle. Rare earth elements and yttrium concentrations (0.44~7.25 ppm) of dolostone and limestone are extremely low and the Y/Ho ratios (35.63~75.55) are basically within the range of modern seawater. Dolostone exhibits a seawater-like PAAS-normalized REE+ Y pattern, showing a left leaning style with relatively LREE depletion and HREE enrichment, while PAAS-normalized REE+ Y pattern of limestone is relatively flat; δCe values of dolostone range from 0.55 to 0.78 (average 0.63) and δCe value of limestoe range from 0.80 to 0.88 (average 0.84); The concentrations of redox sensitive elements (U, Mo, V) are very low, and the V/V+Ni ratios range from 0.04 to 0.45. In each cycle, both Cu and Zn content in dolostone is higher than limestone, and type A cycles overall have higher Cu and Zn contents than type B cycles. [Conclusions] Based on lithology and sedimentary components, the Buqu Formation of GK-1 Well was likely deposited in shallow marine grain-shoal settings, with sea water becoming restricted from type A cycles to type B cycles. Based on the comprehensive petrological and geochemical analysis, selective dissolution herein is interpreted to be produced by early marine diagenesis rather than meteoric diagenesis or deep burial. During early marine diagenesis, aragonite may be selectively dissolved by undersaturated pore fluids via organic matter decomposition. In the sedimentary cycles, the differential development of selective dissolution is probably controlled by aragonite content, paleoproductivity, and early marine diagenetic redox boundary: i) in a single cycle, dolostone interval is characterized by higher paleoproductivity and more oxic pore water than limestone interval during early diagenesis, favoring the production of undersaturated fluids and the formation of selective dissolution pores; ii) in comparison between type A and type B cycles, type A cycles have higher content of aragonite, higher paleoproductivity and lower early marine diagenetic redox boundary, therefore resulting in a better development of selective dissolution.
[Objective] Selective dissolution is common in marine carbonates, and its origin is typically to be related to meteoric fluids, while other possible origins lack further discussion. [Methods] In this study, the origin of marine selective dissolution of Buqu Formation of well GK-1 in the south Qiangtang basin is investigated based on petrography, stable carbon and oxygen isotopes, and elemental geochemistry. [Results] Eight sedimentary cycles (C1 to C8 from bottom to top) were recognized from the studied Buqu Formation, each cycle with limestone in the lower part and dolostone in the upper part. These eight cycles can be divided into two types (A and B) according to different carbonate components. The limestone in type A cycle (C1-C4) is dominated by bioclastic packstone and grainstone, while in type B cycle (C5-C8) is dominated by bioclastic wackstone and packstone. Upwards from type A cycle to the type B cycle, the content of aragonite fossils (gastropods and bivalves) in the limestone decreases and the content of peloids increases. The dolomite in both type A and type B cycles is crystalline dolomite with ooid ghosts showing selective dissolution pores. Importantly, the overall percentage of selective dissolution pores in type A cycles is significantly higher than that in type B cycles. In terms of geochemistry, the carbon and oxygen isotopes of dolostone are relatively higher compared to those of limestone in each cycle. Rare earth elements and yttrium concentrations (0.44~7.25 ppm) of dolostone and limestone are extremely low and the Y/Ho ratios (35.63~75.55) are basically within the range of modern seawater. Dolostone exhibits a seawater-like PAAS-normalized REE+ Y pattern, showing a left leaning style with relatively LREE depletion and HREE enrichment, while PAAS-normalized REE+ Y pattern of limestone is relatively flat; δCe values of dolostone range from 0.55 to 0.78 (average 0.63) and δCe value of limestoe range from 0.80 to 0.88 (average 0.84); The concentrations of redox sensitive elements (U, Mo, V) are very low, and the V/V+Ni ratios range from 0.04 to 0.45. In each cycle, both Cu and Zn content in dolostone is higher than limestone, and type A cycles overall have higher Cu and Zn contents than type B cycles. [Conclusions] Based on lithology and sedimentary components, the Buqu Formation of GK-1 Well was likely deposited in shallow marine grain-shoal settings, with sea water becoming restricted from type A cycles to type B cycles. Based on the comprehensive petrological and geochemical analysis, selective dissolution herein is interpreted to be produced by early marine diagenesis rather than meteoric diagenesis or deep burial. During early marine diagenesis, aragonite may be selectively dissolved by undersaturated pore fluids via organic matter decomposition. In the sedimentary cycles, the differential development of selective dissolution is probably controlled by aragonite content, paleoproductivity, and early marine diagenetic redox boundary: i) in a single cycle, dolostone interval is characterized by higher paleoproductivity and more oxic pore water than limestone interval during early diagenesis, favoring the production of undersaturated fluids and the formation of selective dissolution pores; ii) in comparison between type A and type B cycles, type A cycles have higher content of aragonite, higher paleoproductivity and lower early marine diagenetic redox boundary, therefore resulting in a better development of selective dissolution.
Abstract:
The Neoproterozoic Sturtian Glaciation (~ 717-660 Ma) developed widely across the world, which was well recorded in the Nanhua Basin, but the study of carbon cycling during this period has been lacked. To understand the characteristics of carbon isotope compositions during the Sturtian Glaciation, this study focuses on the drillcore ZK2115 in Songtao area, eastern Guizhou Province, and the high-resolution organic and inorganic carbon isotopes (δ13Corg and δ13Ccarb) of the syn-Sturtian Tiesi’ao Formation are analyzed. The results show that the values of δ13Ccarb vary between -9.29 ‰ and -3.37 ‰ (mean -7.24 ‰), indicating the genitive excursion. The values of δ13Corg vary between -33.63 ‰ and -23.35 ‰ (mean -29.29 ‰). Besides, there is a positive correlation for the δ13Corg and δ13Ccarb values, indicating that the inorganic carbon isotope compositions are not affected by the diagenesis, and can be used to reflect the original carbon isotope signals of the syn-Sturtian Nanhua Basin. Combined with the low TOC contents (mean 0.25%) of the Tiesi’ao Formation, showing that the photosynthesis still existed under the extreme glacial conditions with low rates, and the contents of the generated organic matter are low. The research of carbon isotope compositions in this study can be used to explore the carbon cycling during the Sturtian Glaciation and can also provide evidence for the paleo-marine environment and biogeochemical cycle under extreme climatic condition.
The Neoproterozoic Sturtian Glaciation (~ 717-660 Ma) developed widely across the world, which was well recorded in the Nanhua Basin, but the study of carbon cycling during this period has been lacked. To understand the characteristics of carbon isotope compositions during the Sturtian Glaciation, this study focuses on the drillcore ZK2115 in Songtao area, eastern Guizhou Province, and the high-resolution organic and inorganic carbon isotopes (δ13Corg and δ13Ccarb) of the syn-Sturtian Tiesi’ao Formation are analyzed. The results show that the values of δ13Ccarb vary between -9.29 ‰ and -3.37 ‰ (mean -7.24 ‰), indicating the genitive excursion. The values of δ13Corg vary between -33.63 ‰ and -23.35 ‰ (mean -29.29 ‰). Besides, there is a positive correlation for the δ13Corg and δ13Ccarb values, indicating that the inorganic carbon isotope compositions are not affected by the diagenesis, and can be used to reflect the original carbon isotope signals of the syn-Sturtian Nanhua Basin. Combined with the low TOC contents (mean 0.25%) of the Tiesi’ao Formation, showing that the photosynthesis still existed under the extreme glacial conditions with low rates, and the contents of the generated organic matter are low. The research of carbon isotope compositions in this study can be used to explore the carbon cycling during the Sturtian Glaciation and can also provide evidence for the paleo-marine environment and biogeochemical cycle under extreme climatic condition.
Abstract:
The Early Cambrian Biological Explosion is usually thought to be related to the increase of nitrate concentration, however, the latest study shows that there is no significant change of nitrate concentration in this period. In order to find out the influence of nitrate on the biological explosion, this paper takes the borehole core (well ZK0202) in the slope area of the southeastern margin of Yangtze as an anatomical research object, and reconstructs the characteristics of the paleomarine environment and nitrogen cycle in this period through the analyses of the indexes of major elements, trace elements, rare earth elements, and carbon-nitrogen isotopes. The results show that the Early Cambrian Fortunian-Middle Age 2 (>526 Ma) paleomarine was characterized by a stratified ocean with deepened chemocline, and both nitrogen fixation and denitrification developed during the same period, with nitrogen fixation being the dominant effect. Late Age 2-Early Age 3 (~526-518 Ma), the paleomarine was characterized by strong anoxic-euxinic, and nitrogen fixation was the most important nitrogen cycle pathway. Middle and late Age 3 (<518 Ma) paleomarine was dominated by suboxic-oxic, and nitrogen fixation was still developed. The evolution of the redox state may be related to the high primary productivity induced by hydrothermal action, while the persistent nitrogen fixation shows that the nitrate concentration was maintained at a low level during the same period. Therefore, the abundance of nitrate may not be the main controlling factor for the biological explosion, and ocean oxidation and the abundance of organisms at the base of the food chain may be important triggers for the biological explosion. The above research results further enhance the degree of research on the paleomarine environment and nitrogen cycle in the Early Cambrian slope area of the southeast margin of the South China Yangtze, and provide new references for the correct understanding of the mechanism of the environment-biological co-evolution in this period.
The Early Cambrian Biological Explosion is usually thought to be related to the increase of nitrate concentration, however, the latest study shows that there is no significant change of nitrate concentration in this period. In order to find out the influence of nitrate on the biological explosion, this paper takes the borehole core (well ZK0202) in the slope area of the southeastern margin of Yangtze as an anatomical research object, and reconstructs the characteristics of the paleomarine environment and nitrogen cycle in this period through the analyses of the indexes of major elements, trace elements, rare earth elements, and carbon-nitrogen isotopes. The results show that the Early Cambrian Fortunian-Middle Age 2 (>526 Ma) paleomarine was characterized by a stratified ocean with deepened chemocline, and both nitrogen fixation and denitrification developed during the same period, with nitrogen fixation being the dominant effect. Late Age 2-Early Age 3 (~526-518 Ma), the paleomarine was characterized by strong anoxic-euxinic, and nitrogen fixation was the most important nitrogen cycle pathway. Middle and late Age 3 (<518 Ma) paleomarine was dominated by suboxic-oxic, and nitrogen fixation was still developed. The evolution of the redox state may be related to the high primary productivity induced by hydrothermal action, while the persistent nitrogen fixation shows that the nitrate concentration was maintained at a low level during the same period. Therefore, the abundance of nitrate may not be the main controlling factor for the biological explosion, and ocean oxidation and the abundance of organisms at the base of the food chain may be important triggers for the biological explosion. The above research results further enhance the degree of research on the paleomarine environment and nitrogen cycle in the Early Cambrian slope area of the southeast margin of the South China Yangtze, and provide new references for the correct understanding of the mechanism of the environment-biological co-evolution in this period.
Abstract:
[Objective]The lacustrine fine-grained sedimentary rock mixed with clay, felsic, carbonate and analcite minerals develops in the fourth member of Shahejie Formation of the Leijia area in the western sag of the Liaohe Depression in the Bohai Bay Basin and is the main carrier for the occurrence of oil and gas. Due to the complex composition and rapid lateral changes of mixed fine-grained rocks, the reservoirs are highly heterogeneous, which brings certain difficulties to the prediction of high-quality reservoirs. [Methods] Taking the Lei 15 well, the Lei 14 well and the Lei 61 well in the Leijia area of the western sag as an example, based on time series analysis method, high-precision carbonate U-Pb dating and natural gamma logging data, the mixed fine-grained rocks of the the fourth member of Shahejie Formation were analyzed by cyclostratigraphy. [Results and Discussions] 1) The optimal sedimentation rates of Well Lei 15, Well Lei 14 and Well Lei 61 were estimated by COCO, and it was found that the optimal sedimentation rates increased sequentially and were 10.57 cm/kyr, 11.4 cm/kyr and 13.93 cm respectively; Perform spectrum analysis on the paleoclimate proxy indicator (GR) and compare it with the data spectrum analysis results of the standard ETP curve (eccentricity, slope, precession composite curve), identify astronomical cycle signals in mixed fine-grained rocks in Wells Lei 15, Lei 14 and Lei 61, And use the 405 kyr long eccentricity for astronomical tuning, and use the age 43.4±1.7 Ma at 2766.61m in Well Lei 14 as the anchor point to establish an absolute astronomical time scale. [Conclusions] By conducting cyclostratigraphic research on the fourth member of Shahejie Formation in the Leijia area, astronomical cycle signals in the fourth member of Shahejie Formation can be effectively identified. This method quantitatively establishes a fine stratigraphic division and comparison framework with time attributes, which plays an important role in guiding further oil and gas exploration in the area and broadens the applicability of cycle stratigraphy in the Bohai Bay Basin.
[Objective]The lacustrine fine-grained sedimentary rock mixed with clay, felsic, carbonate and analcite minerals develops in the fourth member of Shahejie Formation of the Leijia area in the western sag of the Liaohe Depression in the Bohai Bay Basin and is the main carrier for the occurrence of oil and gas. Due to the complex composition and rapid lateral changes of mixed fine-grained rocks, the reservoirs are highly heterogeneous, which brings certain difficulties to the prediction of high-quality reservoirs. [Methods] Taking the Lei 15 well, the Lei 14 well and the Lei 61 well in the Leijia area of the western sag as an example, based on time series analysis method, high-precision carbonate U-Pb dating and natural gamma logging data, the mixed fine-grained rocks of the the fourth member of Shahejie Formation were analyzed by cyclostratigraphy. [Results and Discussions] 1) The optimal sedimentation rates of Well Lei 15, Well Lei 14 and Well Lei 61 were estimated by COCO, and it was found that the optimal sedimentation rates increased sequentially and were 10.57 cm/kyr, 11.4 cm/kyr and 13.93 cm respectively; Perform spectrum analysis on the paleoclimate proxy indicator (GR) and compare it with the data spectrum analysis results of the standard ETP curve (eccentricity, slope, precession composite curve), identify astronomical cycle signals in mixed fine-grained rocks in Wells Lei 15, Lei 14 and Lei 61, And use the 405 kyr long eccentricity for astronomical tuning, and use the age 43.4±1.7 Ma at 2766.61m in Well Lei 14 as the anchor point to establish an absolute astronomical time scale. [Conclusions] By conducting cyclostratigraphic research on the fourth member of Shahejie Formation in the Leijia area, astronomical cycle signals in the fourth member of Shahejie Formation can be effectively identified. This method quantitatively establishes a fine stratigraphic division and comparison framework with time attributes, which plays an important role in guiding further oil and gas exploration in the area and broadens the applicability of cycle stratigraphy in the Bohai Bay Basin.
Abstract:
[Objective] The Shaximiao Formation in the Middle Jurassic of the Sichuan Basin is an important tight gas exploration and development layer. Currently, the exploration and development of the first Member of the Shaximiao Formation is in its initial stage, with weak overall research, especially the lack of a systematic understanding of the controlling role of high-quality sand body distribution. [Methods] Based on petrological analysis and geochemistry, this article studied the provenance of the first Member of the Shaximiao Formation in the central Sichuan region. [Results] The results are as follows: (1) The sandstone of the first Member of the Shaximiao Formation in the central Sichuan region is mainly composed of feldspar litharenite and lithic feldspar sandstone. Igneous rock lithic fragments are mainly andesite, which are less developed in the underlying formations but commonly found in the northeast and southeast of Sichuan. (2) The dominant heavy minerals in the first Member of the Shaximiao Formation in the central Sichuan region are garnet and epidote. Epidote is rare in the underlying formations but highly developed in the northeast and southeast of Sichuan. (3) Discrimination diagrams, including F1-F2, SiO2-TiO2, La/Th-Hf, and Co/Th-La/Sc, show that the provenance of the first Member of the Shaximiao Formation in the central Sichuan region is mainly felsic volcanic rocks with a small amount of sedimentary rocks. [Conclusion] Based on the above analysis, the sandstone provenance of the first Member of the Shaximiao Formation in the central Sichuan Basin is mainly derived from the Daba Mountains in the northeast of Sichuan Basin, while other directions around the basin have limited provenance supply. These results provide strong support for the division of favorable areas for tight gas in the Sichuan Basin and for exploration and development activities.
[Objective] The Shaximiao Formation in the Middle Jurassic of the Sichuan Basin is an important tight gas exploration and development layer. Currently, the exploration and development of the first Member of the Shaximiao Formation is in its initial stage, with weak overall research, especially the lack of a systematic understanding of the controlling role of high-quality sand body distribution. [Methods] Based on petrological analysis and geochemistry, this article studied the provenance of the first Member of the Shaximiao Formation in the central Sichuan region. [Results] The results are as follows: (1) The sandstone of the first Member of the Shaximiao Formation in the central Sichuan region is mainly composed of feldspar litharenite and lithic feldspar sandstone. Igneous rock lithic fragments are mainly andesite, which are less developed in the underlying formations but commonly found in the northeast and southeast of Sichuan. (2) The dominant heavy minerals in the first Member of the Shaximiao Formation in the central Sichuan region are garnet and epidote. Epidote is rare in the underlying formations but highly developed in the northeast and southeast of Sichuan. (3) Discrimination diagrams, including F1-F2, SiO2-TiO2, La/Th-Hf, and Co/Th-La/Sc, show that the provenance of the first Member of the Shaximiao Formation in the central Sichuan region is mainly felsic volcanic rocks with a small amount of sedimentary rocks. [Conclusion] Based on the above analysis, the sandstone provenance of the first Member of the Shaximiao Formation in the central Sichuan Basin is mainly derived from the Daba Mountains in the northeast of Sichuan Basin, while other directions around the basin have limited provenance supply. These results provide strong support for the division of favorable areas for tight gas in the Sichuan Basin and for exploration and development activities.
Multigenic formation models of the Cambrian-Ordovician Dolomites in Northern and Central Tarim Basin
Abstract:
[Objective] The genesis of dolomites is still controversy, and dolomite reservoirs play an important role in carbonate oil and gas exploration, whose reservoir properties (porosity and permeability) are largely influenced by the genesis types and texture characteristics of dolomites. In recent years, many large oil and gas fields have also been found in domestic dolomite reservoirs, such as Tarim Basin, Sulige Gas Field in Ordos Basin, Puguang Yuanba Gas Field in Sichuan Basin and Anyue Large or Super Large Gas Field. Therefore, systematic researches on the texture types and genetic mechanisms of dolomites yield profound theoretical significance, and also will promote greater advance in carbonate oil and gas exploration in China. The Tarim Basin shares large volume of oil and gas reserves, the exploration target horizons within which has gradually shifted from medium shallow- to medium-buried layers to ultra-deep layers. Especially, industrial oil and gas flows were encountered in the deeply-buried dolomites of the Lower Paleozoic in the Tarim Basin, making the formation, evolution and reservoir characteristics of deep dolomite reservoirs in this basin become the focus of scholars' research. However, the formation mechanism of the Cambrian-Ordovician dolomites in the Tarim Basin has not yet reached a consensus due to deep burial depth, complex genesis and difficult exploration of these dolomites. This study increases the understanding of their origin and provides theoretical support for oil and gas exploration in the region. [Methods] Detailed petrographic and geochemical (trace-rare earth elements, stable carbon and oxygen isotopes and 87Sr/86Sr ratios) studies of these dolomites were conducted on the Cambrian-Ordovician dolomites of northern and central Tarim Basin based on the classification scheme of dolomite proposed in [25, 26]. [Results and Conclusions] According to the occurrence of dolomites, the Cambrian-Ordovician dolomites were divided into matrix and cement dolomites. Based on grain sizes, contact relationship between crystal planes (plane or curved surface), and crystal shape (euhedral, subhedral or anhedral), six types of dolomite structures were further identified for the matrix dolomite: (1) very fine to fine crystalline dolomite (Md1); (2) relict mimetic dolomite (Md2); (3) very fine to fine crystalline, planar-e(s) floating dolomite (Md3); (4) fine crystalline, planar-e(s) dolomite (Md4); (5) fine to coarse crystalline, nonplanar-a dolomite (Md5); and (6) coarse crystalline, nonplanar saddle dolomite (Md6). The distribution patterns of rare earth elements (REE) (slight enrichment or depletion of light REE, weak Eu negative or positive anomalies, and weak Ce negative or positive anomalies), δ13C values (-2.83–1.72‰; average -1.64‰) and 87Sr/86Sr values (0.7087–0.7116; average 0.7095) in the six matrices are similar to Cambrian-Ordovician micritic limestone and contemporaneous seawater. The diagenetic fluids of Md1 and Md2 are coeval seawater with varying degrees of evaporation and concentration, and were formed by (pene)contemporaneous dolomitization (including sabkha and reflux infiltration dolomitization) in relatively restricted depositional environments. The parent fluids of Md3 and Md4 are residual seawater, created by shallow- to medium-burial dolomitization. The diagenetic fluids of Md5 are variants of contemporaneous water, formed by deep-burial dolomitization or recrystallization of earlier dolomites. Deep hydrothermal fluids were responsible for the Md6 formation as a result of strong water–rock interactions with the host dolomites, influenced by mutual regulation and re-equilibration between the deep hydrothermal fluids and surrounding rocks.
[Objective] The genesis of dolomites is still controversy, and dolomite reservoirs play an important role in carbonate oil and gas exploration, whose reservoir properties (porosity and permeability) are largely influenced by the genesis types and texture characteristics of dolomites. In recent years, many large oil and gas fields have also been found in domestic dolomite reservoirs, such as Tarim Basin, Sulige Gas Field in Ordos Basin, Puguang Yuanba Gas Field in Sichuan Basin and Anyue Large or Super Large Gas Field. Therefore, systematic researches on the texture types and genetic mechanisms of dolomites yield profound theoretical significance, and also will promote greater advance in carbonate oil and gas exploration in China. The Tarim Basin shares large volume of oil and gas reserves, the exploration target horizons within which has gradually shifted from medium shallow- to medium-buried layers to ultra-deep layers. Especially, industrial oil and gas flows were encountered in the deeply-buried dolomites of the Lower Paleozoic in the Tarim Basin, making the formation, evolution and reservoir characteristics of deep dolomite reservoirs in this basin become the focus of scholars' research. However, the formation mechanism of the Cambrian-Ordovician dolomites in the Tarim Basin has not yet reached a consensus due to deep burial depth, complex genesis and difficult exploration of these dolomites. This study increases the understanding of their origin and provides theoretical support for oil and gas exploration in the region. [Methods] Detailed petrographic and geochemical (trace-rare earth elements, stable carbon and oxygen isotopes and 87Sr/86Sr ratios) studies of these dolomites were conducted on the Cambrian-Ordovician dolomites of northern and central Tarim Basin based on the classification scheme of dolomite proposed in [25, 26]. [Results and Conclusions] According to the occurrence of dolomites, the Cambrian-Ordovician dolomites were divided into matrix and cement dolomites. Based on grain sizes, contact relationship between crystal planes (plane or curved surface), and crystal shape (euhedral, subhedral or anhedral), six types of dolomite structures were further identified for the matrix dolomite: (1) very fine to fine crystalline dolomite (Md1); (2) relict mimetic dolomite (Md2); (3) very fine to fine crystalline, planar-e(s) floating dolomite (Md3); (4) fine crystalline, planar-e(s) dolomite (Md4); (5) fine to coarse crystalline, nonplanar-a dolomite (Md5); and (6) coarse crystalline, nonplanar saddle dolomite (Md6). The distribution patterns of rare earth elements (REE) (slight enrichment or depletion of light REE, weak Eu negative or positive anomalies, and weak Ce negative or positive anomalies), δ13C values (-2.83–1.72‰; average -1.64‰) and 87Sr/86Sr values (0.7087–0.7116; average 0.7095) in the six matrices are similar to Cambrian-Ordovician micritic limestone and contemporaneous seawater. The diagenetic fluids of Md1 and Md2 are coeval seawater with varying degrees of evaporation and concentration, and were formed by (pene)contemporaneous dolomitization (including sabkha and reflux infiltration dolomitization) in relatively restricted depositional environments. The parent fluids of Md3 and Md4 are residual seawater, created by shallow- to medium-burial dolomitization. The diagenetic fluids of Md5 are variants of contemporaneous water, formed by deep-burial dolomitization or recrystallization of earlier dolomites. Deep hydrothermal fluids were responsible for the Md6 formation as a result of strong water–rock interactions with the host dolomites, influenced by mutual regulation and re-equilibration between the deep hydrothermal fluids and surrounding rocks.
Abstract:
Calcite containing hydrocarbon and asphalt inclusions developed in the Ordovician Majiagou Formation subsalt carbonate reservoir in the Wusenqi area of Ordos Basin, which is an important indicator of fluid source and hydrocarbon accumulation process. Through core observation, rock thin section identification, cathodoluminescence, rare earth elements, C, O, Sr isotopes, microfluorescence and laser Raman spectroscopy analysis, the calcite development stages were divided, the sources of vein forming fluids were analyzed, and the fluid inclusions were studied to reveal the significance of hydrocarbon activities on oil and gas accumulation. The results show that 4 stages of calcite veins (C1, C2, C3, C4) are developed in the carbonate reservoir in Wushenqi area, and the characteristics and sources of secondary calcite veins are different in different periods. The C1 and C4 vein forming fluids are derived from the dissolved surrounding rocks, and their isotopes of strontium carbon and oxygen are consistent with those of Ordovician seawater, and the rare earth partition pattern is characteristic of seawater. C2 calcite veins have very negative carbon isotope values, high 87Sr/86Sr values and total rare earth content, positive Eu anomaly, and the fluid comes from external fluids. C3 calcite has a negative carbon isotope value, higher 87Sr/86Sr values and total rare earth content, and the fluid is derived from external fluids. In addition, asphaltic inclusions developed on calcite veins of C2 and C3 stages, indicating that oil and gas cracked after entering the reservoir. The geochemical characteristics of calcite veins in the study area provide a theoretical basis for oil and gas accumulation in Ordos Basin and other basins.
Calcite containing hydrocarbon and asphalt inclusions developed in the Ordovician Majiagou Formation subsalt carbonate reservoir in the Wusenqi area of Ordos Basin, which is an important indicator of fluid source and hydrocarbon accumulation process. Through core observation, rock thin section identification, cathodoluminescence, rare earth elements, C, O, Sr isotopes, microfluorescence and laser Raman spectroscopy analysis, the calcite development stages were divided, the sources of vein forming fluids were analyzed, and the fluid inclusions were studied to reveal the significance of hydrocarbon activities on oil and gas accumulation. The results show that 4 stages of calcite veins (C1, C2, C3, C4) are developed in the carbonate reservoir in Wushenqi area, and the characteristics and sources of secondary calcite veins are different in different periods. The C1 and C4 vein forming fluids are derived from the dissolved surrounding rocks, and their isotopes of strontium carbon and oxygen are consistent with those of Ordovician seawater, and the rare earth partition pattern is characteristic of seawater. C2 calcite veins have very negative carbon isotope values, high 87Sr/86Sr values and total rare earth content, positive Eu anomaly, and the fluid comes from external fluids. C3 calcite has a negative carbon isotope value, higher 87Sr/86Sr values and total rare earth content, and the fluid is derived from external fluids. In addition, asphaltic inclusions developed on calcite veins of C2 and C3 stages, indicating that oil and gas cracked after entering the reservoir. The geochemical characteristics of calcite veins in the study area provide a theoretical basis for oil and gas accumulation in Ordos Basin and other basins.
Abstract:
[Objective] The Dianqianbei depression is not only an important exploration area and producing area of marine shale gas in southern China, but also an important large-super large MVT type lead-zinc metallogenic area associated with key metals in China. Asphalt or ancient reservoirs with high Pb and Zn contents are constantly found in lead-zinc deposits. In this paper, the geochemical characteristics of elements in black shale kerogen are used to study the material contribution of mineralization and the sedimentary environment of black shale. [Methods] The black shale outcrops in the Dashiban, Liangfengao and Maoba areas in the eastern part of the northern Yunnan-Guizhou depression were taken as the research objects. The kerogen in the black shale was pretreated by microwave digestion, and the trace elements in the black shale and kerogen were analyzed and tested in combination with petrographic and mineralographic observations. [Results] Three redox indicators (U/Th ,Ni/Co, V/Cr) in the kerogen of the Wufeng Formation-Longmaxi Formation showed that the Wufeng Formation was oxygen-poor-anoxic, and the Longmaxi Formation was oxygen-rich-oxygen-poor environment ; the ancient water depth calculated by Co element in the study area is 6.85 ~ 54.37 m. The paleoclimate reflected by Sr / Cu is warm and humid, and the Sr / Ba value is less than 0.5, which represents the sedimentary environment of brackish water. At the same time, in terms of paleoproductivity, Wufeng Formation is greater than Longmaxi Formation. The contents of Pb, Zn, Cd, Bi, Sb, V, Cr and Ni in the whole rock are higher than those in the kerogen, and the contents of Ag, Ge, U and Th in the kerogen are higher than those in the whole rock. The light rare earth elements in the whole rock and kerogen are relatively enriched, and the heavy rare earth elements are relatively depleted. The Eu and some Ce in the whole rock show weak negative anomalies. The kerogen shows strong Ce negative anomaly, Eu weak negative anomaly, and the total rare earth content is higher than that of the whole rock. [Conclusions] The Wufeng Formation-Longmaxi Formation in the study area is generally an oxygen-rich-oxygen-poor turbulent sedimentary environment, with a warm and humid climate of brackish water shallow shelf facies sedimentary characteristics. Trace elements in black shale kerogen can provide a basis for regional prospecting.
[Objective] The Dianqianbei depression is not only an important exploration area and producing area of marine shale gas in southern China, but also an important large-super large MVT type lead-zinc metallogenic area associated with key metals in China. Asphalt or ancient reservoirs with high Pb and Zn contents are constantly found in lead-zinc deposits. In this paper, the geochemical characteristics of elements in black shale kerogen are used to study the material contribution of mineralization and the sedimentary environment of black shale. [Methods] The black shale outcrops in the Dashiban, Liangfengao and Maoba areas in the eastern part of the northern Yunnan-Guizhou depression were taken as the research objects. The kerogen in the black shale was pretreated by microwave digestion, and the trace elements in the black shale and kerogen were analyzed and tested in combination with petrographic and mineralographic observations. [Results] Three redox indicators (U/Th ,Ni/Co, V/Cr) in the kerogen of the Wufeng Formation-Longmaxi Formation showed that the Wufeng Formation was oxygen-poor-anoxic, and the Longmaxi Formation was oxygen-rich-oxygen-poor environment ; the ancient water depth calculated by Co element in the study area is 6.85 ~ 54.37 m. The paleoclimate reflected by Sr / Cu is warm and humid, and the Sr / Ba value is less than 0.5, which represents the sedimentary environment of brackish water. At the same time, in terms of paleoproductivity, Wufeng Formation is greater than Longmaxi Formation. The contents of Pb, Zn, Cd, Bi, Sb, V, Cr and Ni in the whole rock are higher than those in the kerogen, and the contents of Ag, Ge, U and Th in the kerogen are higher than those in the whole rock. The light rare earth elements in the whole rock and kerogen are relatively enriched, and the heavy rare earth elements are relatively depleted. The Eu and some Ce in the whole rock show weak negative anomalies. The kerogen shows strong Ce negative anomaly, Eu weak negative anomaly, and the total rare earth content is higher than that of the whole rock. [Conclusions] The Wufeng Formation-Longmaxi Formation in the study area is generally an oxygen-rich-oxygen-poor turbulent sedimentary environment, with a warm and humid climate of brackish water shallow shelf facies sedimentary characteristics. Trace elements in black shale kerogen can provide a basis for regional prospecting.
Abstract:
Peperite is a kind of transitional rock formed by the syngenetic mixing of hot magmatic materials and wet and cold unconsolidated sediments, which has important paleoenvironmental implications. Based on outcrop geological survey and microscopic petrological analysis, typical peperites are recognized in Ediacaran basic volcanic rocks in northwestern Tarim Basin, which mainly include blocky peperites and fluidal peperites, and their host is purplish red sandy sediments. The peperites were mainly formed by the intrusion of magma into the water-rich unconsolidated sandy sediments or by the flow of magma on the surface of the water-rich unconsolidated sandy sediments. Among them, the thinner magmatic intrusion and the bottom of the surface flood basalt mainly formed the fluidal peperites, while the thicker magmatic intrusion developed the blocky peperites. It is concluded that the northern margin of the Tarim craton was still in an intraplate rift setting related to the breakup of the Rodinia supercontinent during the early Ediacaran, and the development of peperite indicates that the basalt eruption in this area was mainly in a littoral subaqueous sedimentary environment; with the end of volcanic eruption, the northern Tarim Craton transformed into a relatively stable passive continental margin basin or a cratonic basin during the late Ediacaran.
Peperite is a kind of transitional rock formed by the syngenetic mixing of hot magmatic materials and wet and cold unconsolidated sediments, which has important paleoenvironmental implications. Based on outcrop geological survey and microscopic petrological analysis, typical peperites are recognized in Ediacaran basic volcanic rocks in northwestern Tarim Basin, which mainly include blocky peperites and fluidal peperites, and their host is purplish red sandy sediments. The peperites were mainly formed by the intrusion of magma into the water-rich unconsolidated sandy sediments or by the flow of magma on the surface of the water-rich unconsolidated sandy sediments. Among them, the thinner magmatic intrusion and the bottom of the surface flood basalt mainly formed the fluidal peperites, while the thicker magmatic intrusion developed the blocky peperites. It is concluded that the northern margin of the Tarim craton was still in an intraplate rift setting related to the breakup of the Rodinia supercontinent during the early Ediacaran, and the development of peperite indicates that the basalt eruption in this area was mainly in a littoral subaqueous sedimentary environment; with the end of volcanic eruption, the northern Tarim Craton transformed into a relatively stable passive continental margin basin or a cratonic basin during the late Ediacaran.
Abstract:
Mangrove is an important coastal blue carbon ecosystem, which has a strong carbon sink function and has a profound impact on the global carbon cycle. In this study, the mangrove forest in Yanpu Bay was selected as the study area, and the high-resolution chronostratigraphic framework was established by 210Pb dating technology to analyze the sediment mass burial rate (SAR), organic carbon burial flux (OCAR) and potential sources of organic carbon. The results showed that the sedimentation rate of mangrove was about 2.2cm/a. The sediment was mainly silt and clay, the particle size was basically stable with depth, and there was a good correlation between excess 210Pb activity and depth, indicating that the sedimentary environment was relatively stable. The SAR values of the sediments ranged from 52.03~233.61g/(cm2·a), and the OCAR values ranged from 112.78~473.97g/(m2·a). The higher SAR values were conducive to the deposition and burial of organic carbon. Based on the distribution characteristics of stable carbon and nitrogen isotopes and the Bayesian mixing model (MixSIAR), it can be seen that sediment organic carbon was a mixed source of sea and land, in which the average contribution rate of mangrove litter was 59.70% and suspended organic particles (POM) is 25.68%. Therefore, mangrove areas and POM delivery are important factors affecting sediment organic carbon burial. At the same time, extreme weather, natural disasters, water conservancy projects and other factors further promote the complexity of organic carbon source changes.
Mangrove is an important coastal blue carbon ecosystem, which has a strong carbon sink function and has a profound impact on the global carbon cycle. In this study, the mangrove forest in Yanpu Bay was selected as the study area, and the high-resolution chronostratigraphic framework was established by 210Pb dating technology to analyze the sediment mass burial rate (SAR), organic carbon burial flux (OCAR) and potential sources of organic carbon. The results showed that the sedimentation rate of mangrove was about 2.2cm/a. The sediment was mainly silt and clay, the particle size was basically stable with depth, and there was a good correlation between excess 210Pb activity and depth, indicating that the sedimentary environment was relatively stable. The SAR values of the sediments ranged from 52.03~233.61g/(cm2·a), and the OCAR values ranged from 112.78~473.97g/(m2·a). The higher SAR values were conducive to the deposition and burial of organic carbon. Based on the distribution characteristics of stable carbon and nitrogen isotopes and the Bayesian mixing model (MixSIAR), it can be seen that sediment organic carbon was a mixed source of sea and land, in which the average contribution rate of mangrove litter was 59.70% and suspended organic particles (POM) is 25.68%. Therefore, mangrove areas and POM delivery are important factors affecting sediment organic carbon burial. At the same time, extreme weather, natural disasters, water conservancy projects and other factors further promote the complexity of organic carbon source changes.
Abstract:
【Objective】Significant breakthroughs have been made in the exploration of Jurassic continental shale oil in the Sichuan Basin. Due to the small grain size, rapid facies changes, and strong heterogeneity of continental mud shale facies, it is urgently necessary to systematically study the development rules of lithofacies and organic matter enrichment characteristics in the Dongyuemiao Member.【Methods】Taking the Dongyuemiao Member of the Ziliujing Formation in the eastern part of the Sichuan Basin as an example, based on core data, non-core lithofacies interpretation from well logging, and utilizing theory of cyclic stratigraphy, various methods such as data preprocessing, power spectral analysis, evolutionary spectral analysis, filtering of data, correlation coefficient analysis, and astronomical tuning were employed to establish a 4-5 level high frequency stratigraphic framework for the study area. Through the sedimentary response of astronomical cycles, the development rules of mud shale lithofacies and organic matter enrichment characteristics under high-frequency chronostratigraphic framework were discussed. 【Results】 ① The Dongyuemiao Member shows a good astronomical cycle signal,and the long eccentricity (405 ka), short eccentricity (128 ka), obliquity (43 ka), and precession (21 ka) astronomical cycles were extracted. There are four long eccentricity cycles and fifteen short eccentricity cycles. The long eccentricity cycle and short eccentricity cycle were used as the subdivision units of the fourth-order and fifth-order sequences, establishing a 4-5 level chronostratigraphic framework for the Dongyuemiao Member in the study area. ②Taking into account rock thins, X-ray diffraction whole-rock analysis, and well logging data, the principle of structure first is employed, with components and sedimentary structures as the primary basis The Dongyuemiao Member is divided into two lithofacies types: indigenous organic-rich laminated mudstone facies , exogenous shell-bearing mudstone and shell limestone facies and seven lithofacies types including laminated silty-rich argillaceous mudstone facies, massive silt-rich argillaceous mudstone facies, laminated mixed mudstone facies, laminated shell-bearing silty-rich argillaceous mudstone facies, laminated silt-bearing shell-rich argillaceous mudstone facies, laminated silt-bearing clay-rich shelly mudstone facies, massive argillaceous shell limestone facies, massive argillaceous shell limestone facies. ③ The coupling relationship between paleoclimate, mineral composition, and eccentricity revealed that during periods of high eccentricity and its maximum amplitude, the climate was humid and hot with significant seasonal variations. This led to the input of a large amount of fine-grained material and organic matter from the land and the development of indigenous organic-rich laminated mud shale facies. During periods of low eccentricity and its minimum amplitude, the climate was dry and cold with less input of terrigenous materials. The clay mineral and detrital mineral content were lower, and the lithofacies were dominated by exogenous shell-bearing mudstone facies, which affected the development of organic matter. The Dongyuemiao Member is mainly controlled by eccentricity cycles, and the deposition of lacustrine shelly mud shale is controlled by the climate changes driven by the 405 ka and 128 ka eccentricity cycles.【Conclusion】Long eccentricity is a key factor controlling the distribution of lithofacies and organic matter enrichment. It controls the ordered development of indigenous organic-rich laminated mud shale facies and exogenous shell-bearing mudstone and shell-bearing limestone facies. Short eccentricity has a limited impact on organic matter enrichment but significantly controls the lithofacies composition of exogenous shell-bearing mudstone.
【Objective】Significant breakthroughs have been made in the exploration of Jurassic continental shale oil in the Sichuan Basin. Due to the small grain size, rapid facies changes, and strong heterogeneity of continental mud shale facies, it is urgently necessary to systematically study the development rules of lithofacies and organic matter enrichment characteristics in the Dongyuemiao Member.【Methods】Taking the Dongyuemiao Member of the Ziliujing Formation in the eastern part of the Sichuan Basin as an example, based on core data, non-core lithofacies interpretation from well logging, and utilizing theory of cyclic stratigraphy, various methods such as data preprocessing, power spectral analysis, evolutionary spectral analysis, filtering of data, correlation coefficient analysis, and astronomical tuning were employed to establish a 4-5 level high frequency stratigraphic framework for the study area. Through the sedimentary response of astronomical cycles, the development rules of mud shale lithofacies and organic matter enrichment characteristics under high-frequency chronostratigraphic framework were discussed. 【Results】 ① The Dongyuemiao Member shows a good astronomical cycle signal,and the long eccentricity (405 ka), short eccentricity (128 ka), obliquity (43 ka), and precession (21 ka) astronomical cycles were extracted. There are four long eccentricity cycles and fifteen short eccentricity cycles. The long eccentricity cycle and short eccentricity cycle were used as the subdivision units of the fourth-order and fifth-order sequences, establishing a 4-5 level chronostratigraphic framework for the Dongyuemiao Member in the study area. ②Taking into account rock thins, X-ray diffraction whole-rock analysis, and well logging data, the principle of structure first is employed, with components and sedimentary structures as the primary basis The Dongyuemiao Member is divided into two lithofacies types: indigenous organic-rich laminated mudstone facies , exogenous shell-bearing mudstone and shell limestone facies and seven lithofacies types including laminated silty-rich argillaceous mudstone facies, massive silt-rich argillaceous mudstone facies, laminated mixed mudstone facies, laminated shell-bearing silty-rich argillaceous mudstone facies, laminated silt-bearing shell-rich argillaceous mudstone facies, laminated silt-bearing clay-rich shelly mudstone facies, massive argillaceous shell limestone facies, massive argillaceous shell limestone facies. ③ The coupling relationship between paleoclimate, mineral composition, and eccentricity revealed that during periods of high eccentricity and its maximum amplitude, the climate was humid and hot with significant seasonal variations. This led to the input of a large amount of fine-grained material and organic matter from the land and the development of indigenous organic-rich laminated mud shale facies. During periods of low eccentricity and its minimum amplitude, the climate was dry and cold with less input of terrigenous materials. The clay mineral and detrital mineral content were lower, and the lithofacies were dominated by exogenous shell-bearing mudstone facies, which affected the development of organic matter. The Dongyuemiao Member is mainly controlled by eccentricity cycles, and the deposition of lacustrine shelly mud shale is controlled by the climate changes driven by the 405 ka and 128 ka eccentricity cycles.【Conclusion】Long eccentricity is a key factor controlling the distribution of lithofacies and organic matter enrichment. It controls the ordered development of indigenous organic-rich laminated mud shale facies and exogenous shell-bearing mudstone and shell-bearing limestone facies. Short eccentricity has a limited impact on organic matter enrichment but significantly controls the lithofacies composition of exogenous shell-bearing mudstone.
Abstract:
Designing a reliable and rapid method for determining shale facies while scientifically constructing a framework for shale facies combination is of great significance for studying the vertical and horizontal changes of shale facies combinations. Accurately calculate the composition of shale minerals through methods such as X-ray diffraction analysis, lithology logging, multi mineral model, and Ipsom neural network, and use Python software to quickly divide the "three terminal element" lithofacies; Extracting high-frequency stratigraphic cycles as a framework for shale facies combination through time series analysis methods such as spectral analysis, optimal sedimentation rate estimation, and Gaussian filtering. The results show that there are 12 types of lithofacies developed in the strata, and the fifth order cycle is divided into five order sequences with good correspondence with vertical lithofacies changes. The fourth order cycle defines the fourth order sequence and divides the strata into eight 405kyr sedimentary lithofacies combinations (E1-E8). The Guanyinqiao Formation is used as a time anchor to calculate the starting and ending depths of each well lithofacies combination, and to build an isochronous ithofacies combination framework; Through comparative study of lithofacies combinations, it was found that the E1 to E4 lithofacies combinations at the bottom of the Longyi Formation have the characteristics of multiple types of lithofacies, rapid changes in vertical and horizontal lithofacies. The lithofacies types of the E5 to E8 lithofacies combinations are relatively simple, and the lateral changes are small; From the low point to the high point of the ancient structure, the E1 lithofacies combination shows a continuous decreasing trend of siliceous shale content, the E2-E4 lithofacies combination shows a significant decrease in mixed shale content, the E5-E8 lithofacies combination shows a decrease in siliceous shale content, and the siliceous/muddy mixed shale content significantly increases. Overall, the shale lithofacies combination framework built based on time series analysis methods, combined with new lithofacies judgment methods, can accurately describe the differences in stratigraphic lithofacies combinations in different regions during the same period, providing new ideas for quantitative analysis of shale lithofacies heterogeneity and its changing trends.
Designing a reliable and rapid method for determining shale facies while scientifically constructing a framework for shale facies combination is of great significance for studying the vertical and horizontal changes of shale facies combinations. Accurately calculate the composition of shale minerals through methods such as X-ray diffraction analysis, lithology logging, multi mineral model, and Ipsom neural network, and use Python software to quickly divide the "three terminal element" lithofacies; Extracting high-frequency stratigraphic cycles as a framework for shale facies combination through time series analysis methods such as spectral analysis, optimal sedimentation rate estimation, and Gaussian filtering. The results show that there are 12 types of lithofacies developed in the strata, and the fifth order cycle is divided into five order sequences with good correspondence with vertical lithofacies changes. The fourth order cycle defines the fourth order sequence and divides the strata into eight 405kyr sedimentary lithofacies combinations (E1-E8). The Guanyinqiao Formation is used as a time anchor to calculate the starting and ending depths of each well lithofacies combination, and to build an isochronous ithofacies combination framework; Through comparative study of lithofacies combinations, it was found that the E1 to E4 lithofacies combinations at the bottom of the Longyi Formation have the characteristics of multiple types of lithofacies, rapid changes in vertical and horizontal lithofacies. The lithofacies types of the E5 to E8 lithofacies combinations are relatively simple, and the lateral changes are small; From the low point to the high point of the ancient structure, the E1 lithofacies combination shows a continuous decreasing trend of siliceous shale content, the E2-E4 lithofacies combination shows a significant decrease in mixed shale content, the E5-E8 lithofacies combination shows a decrease in siliceous shale content, and the siliceous/muddy mixed shale content significantly increases. Overall, the shale lithofacies combination framework built based on time series analysis methods, combined with new lithofacies judgment methods, can accurately describe the differences in stratigraphic lithofacies combinations in different regions during the same period, providing new ideas for quantitative analysis of shale lithofacies heterogeneity and its changing trends.
Abstract:
[Objective] The southeastern part of South China underwent a strong orogenic process, known as the Kwangsian Orogeny, during the Early Paleozoic period. However, there is still considerable controversy surrounding the nature of Kwangsian Orogeny. In order to understand the geodynamic mechanism of the Early Paleozoic Kwangsian Orogeny of South China, the detrital multi-minerals U-Pb geochronology is used to reconstruct the paleogeographic position of the Yangtze and Cathaysia blocks in Early Paleozoic and restore the relative displacement of terranes. [Methods] U-Pb geochronology and hafnium isotopic data of detrital zircon and detrital monazite from Early Paleozoic strata of South China are revisited to trace the potential sources. [Results and Discussions] The results show that: (1) Detrital zircons from the Cambrian to Ordovician strata in Western Yangtze yield predominantly age peaks at 850-750 Ma and 550-500 Ma, with minor age peaks at 1 000-900 Ma, 1 900-1 800 Ma and 2 550-2 400 Ma. In combination with the regional lithofacies evolution, provenance analysis demonstrates that the abundant 550-500 Ma detrital zircons with positive εHf(t) values reflecting a large involvement of juvenile crustal materials, were probably derived exotically from the Cadomian arc belt along the Iran-Turkey margin. The Neoproterozoic detrital zircons preserved in the Cambrian to Ordovician strata of the western margin of the Yangtze Block are mainly derived from the Neoproterozoic igneous rock in the western Yangtze Block and the recycled materials from the underlying Neoproterozoic strata in Western Yangtze. Provenance analysis reveals that the Yangtze Block and Iran have a close provenance linkage; (2) U-Pb ages of detrital zircon and detrital monazite from the Cambrian to Ordovician strata in the Cathaysia Block exhibit predominantly age peaks at 1 000-900 Ma and 550-500 Ma. The former aligns with the age of the magmatic and metamorphic zircon/monazite in the East Ghats belt of NE India and Rayner belt in East Antarctic, while the latter corresponds to the U-Pb ages of the zircon and monazite from the Kuunga Orogen in NE India, and (3) In the age spectra of the Silurian succession of southeastern Yangtze, the peaks older than 440 Ma are identical to those of the pre-Silurian strata, indicating that the provenance of detritus of those ages is mainly the recycled materials from the underlying Cambrian to Ordovician strata. The age peak of 460-410 Ma coincides with the timing of the massive 440?400 Ma Syn-orogenic granites in Cathaysia Block. [Conclusions] The result of provenance analysis favors an oblique distribution of the Yangtze and Cathaysia blocks along the northeastern margin of East Gondwana. The Yangtze Block, specifically, was situated in the northeast of Iran, while Cathaysia occupied the northeastern margin of India during the Cambrian to Ordovician period, respectively. It is crucial to note that the Kwangsian Orogeny is an oblique strike-slip convergence triggered by the final assembly of the supercontinent Gondwana during the late Neoproterozoic to Cambrian period. The oblique strike-slip convergence of the Yangtze and Cathaysia blocks after the late Ordovician (~460 Ma) resulted in the formation of the Wuyi-Yunkai Orogenic Belt (460-420 Ma).
[Objective] The southeastern part of South China underwent a strong orogenic process, known as the Kwangsian Orogeny, during the Early Paleozoic period. However, there is still considerable controversy surrounding the nature of Kwangsian Orogeny. In order to understand the geodynamic mechanism of the Early Paleozoic Kwangsian Orogeny of South China, the detrital multi-minerals U-Pb geochronology is used to reconstruct the paleogeographic position of the Yangtze and Cathaysia blocks in Early Paleozoic and restore the relative displacement of terranes. [Methods] U-Pb geochronology and hafnium isotopic data of detrital zircon and detrital monazite from Early Paleozoic strata of South China are revisited to trace the potential sources. [Results and Discussions] The results show that: (1) Detrital zircons from the Cambrian to Ordovician strata in Western Yangtze yield predominantly age peaks at 850-750 Ma and 550-500 Ma, with minor age peaks at 1 000-900 Ma, 1 900-1 800 Ma and 2 550-2 400 Ma. In combination with the regional lithofacies evolution, provenance analysis demonstrates that the abundant 550-500 Ma detrital zircons with positive εHf(t) values reflecting a large involvement of juvenile crustal materials, were probably derived exotically from the Cadomian arc belt along the Iran-Turkey margin. The Neoproterozoic detrital zircons preserved in the Cambrian to Ordovician strata of the western margin of the Yangtze Block are mainly derived from the Neoproterozoic igneous rock in the western Yangtze Block and the recycled materials from the underlying Neoproterozoic strata in Western Yangtze. Provenance analysis reveals that the Yangtze Block and Iran have a close provenance linkage; (2) U-Pb ages of detrital zircon and detrital monazite from the Cambrian to Ordovician strata in the Cathaysia Block exhibit predominantly age peaks at 1 000-900 Ma and 550-500 Ma. The former aligns with the age of the magmatic and metamorphic zircon/monazite in the East Ghats belt of NE India and Rayner belt in East Antarctic, while the latter corresponds to the U-Pb ages of the zircon and monazite from the Kuunga Orogen in NE India, and (3) In the age spectra of the Silurian succession of southeastern Yangtze, the peaks older than 440 Ma are identical to those of the pre-Silurian strata, indicating that the provenance of detritus of those ages is mainly the recycled materials from the underlying Cambrian to Ordovician strata. The age peak of 460-410 Ma coincides with the timing of the massive 440?400 Ma Syn-orogenic granites in Cathaysia Block. [Conclusions] The result of provenance analysis favors an oblique distribution of the Yangtze and Cathaysia blocks along the northeastern margin of East Gondwana. The Yangtze Block, specifically, was situated in the northeast of Iran, while Cathaysia occupied the northeastern margin of India during the Cambrian to Ordovician period, respectively. It is crucial to note that the Kwangsian Orogeny is an oblique strike-slip convergence triggered by the final assembly of the supercontinent Gondwana during the late Neoproterozoic to Cambrian period. The oblique strike-slip convergence of the Yangtze and Cathaysia blocks after the late Ordovician (~460 Ma) resulted in the formation of the Wuyi-Yunkai Orogenic Belt (460-420 Ma).
Abstract:
Abstract : In order to deeply evaluate the natural gas exploration prospect and predict the favorable exploration zone and exploration target of the shore-shallow lake beach-bar sand body of the Quaternary Qigequan Formation in Tainan area, eastern Qaidam Basin, based on the comprehensive analysis of core observation and drilling ( logging ) data, combined with the analysis results of rock thin section, grain size-standard deviation, grain size cumulative probability curve, environmental sensitive grain size composition and other parameters, the grain size variation characteristics and hydrodynamic significance of the shore-shallow lake beach-bar sand body of Qigequan Formation in this area were studied in detail. The results show that : First, The sedimentary period of Qigequan in Tainan area is the sedimentary environment of shore-shallow lake. The beach-bar sand body is developed, the rock is loose, the cementation is poor, easy to break, and the grain size is fine. The lithology is mainly lithic feldspar fine sandstone-siltstone, the composition maturity is medium-poor, and the sorting roundness is medium-poor. Second,During the sedimentary period of Qigequan Formation, the cumulative probability curve of grain size of beach-bar sand bodies in the study area showed six patterns, mainly one-hop one-suspension type ( 64.4% ), followed by multi-segment type ( 11.9% ), and the third is two-hop one-suspension type ( 7.5% ). Third, During the sedimentary period of Qigequan Formation, the grain size-standard deviation diagram of beach-bar sand bodies in the study area showed four patterns, mainly multimodal ( 50.0% ), followed by bimodal ( 33.4% ), unimodal and trimodal ( both 8.3% ). Combined with the cumulative curve of grain size probability and paleogeomorphology analysis, four environmentally sensitive grain size components were determined, which represented suspension transport, wave, coastal current and storm wave, respectively. Among them, wave was the main hydrodynamic type controlling the formation and development of beach-bar sand bodies, and the average value of environmentally sensitive grain size components accounted for 27.9% of the total grain size. Followed by the suspension effect ( average of 15.3% ) and the coastal current ( average of 11.4% ), the storm wave effect is the weakest ( average of 2.3% ). Forth, Through the study of the plane distribution characteristics of the percentage content of environmentally sensitive grain size components, it is shown that the influence of waves on the formation and development of beach-bar sand bodies is weakened from the core to the wing of the anticline, while the suspension effect is just the opposite, and the influence of coastal flow on beach-bar sand bodies is concentrated on the east and west wings of the anticline.
Abstract : In order to deeply evaluate the natural gas exploration prospect and predict the favorable exploration zone and exploration target of the shore-shallow lake beach-bar sand body of the Quaternary Qigequan Formation in Tainan area, eastern Qaidam Basin, based on the comprehensive analysis of core observation and drilling ( logging ) data, combined with the analysis results of rock thin section, grain size-standard deviation, grain size cumulative probability curve, environmental sensitive grain size composition and other parameters, the grain size variation characteristics and hydrodynamic significance of the shore-shallow lake beach-bar sand body of Qigequan Formation in this area were studied in detail. The results show that : First, The sedimentary period of Qigequan in Tainan area is the sedimentary environment of shore-shallow lake. The beach-bar sand body is developed, the rock is loose, the cementation is poor, easy to break, and the grain size is fine. The lithology is mainly lithic feldspar fine sandstone-siltstone, the composition maturity is medium-poor, and the sorting roundness is medium-poor. Second,During the sedimentary period of Qigequan Formation, the cumulative probability curve of grain size of beach-bar sand bodies in the study area showed six patterns, mainly one-hop one-suspension type ( 64.4% ), followed by multi-segment type ( 11.9% ), and the third is two-hop one-suspension type ( 7.5% ). Third, During the sedimentary period of Qigequan Formation, the grain size-standard deviation diagram of beach-bar sand bodies in the study area showed four patterns, mainly multimodal ( 50.0% ), followed by bimodal ( 33.4% ), unimodal and trimodal ( both 8.3% ). Combined with the cumulative curve of grain size probability and paleogeomorphology analysis, four environmentally sensitive grain size components were determined, which represented suspension transport, wave, coastal current and storm wave, respectively. Among them, wave was the main hydrodynamic type controlling the formation and development of beach-bar sand bodies, and the average value of environmentally sensitive grain size components accounted for 27.9% of the total grain size. Followed by the suspension effect ( average of 15.3% ) and the coastal current ( average of 11.4% ), the storm wave effect is the weakest ( average of 2.3% ). Forth, Through the study of the plane distribution characteristics of the percentage content of environmentally sensitive grain size components, it is shown that the influence of waves on the formation and development of beach-bar sand bodies is weakened from the core to the wing of the anticline, while the suspension effect is just the opposite, and the influence of coastal flow on beach-bar sand bodies is concentrated on the east and west wings of the anticline.
Abstract:
Abstract: [Objective] "Carbonate factory " is generally used to describe the large-scale formation and preservation of carbonate. The composition of different carbonate factories is related to a series of factors, such as biological species, hydrodynamics, nutrient, climatic and marine chemistry. Accurately identifying the types of carbonate factories and elucidating the factors controlling their development, quantitatively evaluating the environmental conditions in the process of factory development, and clarifying their paleoecological and sedimentological signals during the transformation of sedimentary systems are the key to understanding the evolution of carbonate sedimentary system. [Methods] In this paper, the Ma'antang Formation in the Guanyinya section of Hanwang Town, Mianzhu City, Northwest Sichuan, with systematic carbonate rock microfacies analysis and carbonate grains size statistics, we carry out the quantitative analysis of 20,391 grains. The composition, characteristics, transformation, and driving factors of tropical shallow-water carbonate factories during this period are described in detail. [Results and Discussions]The results showed that carbonate factories in tropical shallow water in the study area could be further divided into subdivided types of factories, such as peloids, ooids, bioclasts and reefs, indicating different carbonate production patterns.[Conclusions] By analyzing the composition, depositional environment, hydrodynamic and nutrient level of tropical shallow water factories in the study area, it is found that each subdivision factory has "habitat" characteristics and the transformation of factory type is more Controlled by regional sea level change. Frequently sea level changes lead to rapid changes in nutrient levels and hydrodynamic and carbonate mineral saturation, which drives the transformation of specific subdivided factories. The rapid changes in the sedimentary sequence of carbonate subdivisions indicate their Symbiosis horizontally. Therefore, the rapid transformation of subdivided production factory types on a shorter time scale is mainly related to the migration of horizontal factory types controlled by regional sea level and may not be related to extreme events and the transformation of sedimentary systems.
Abstract: [Objective] "Carbonate factory " is generally used to describe the large-scale formation and preservation of carbonate. The composition of different carbonate factories is related to a series of factors, such as biological species, hydrodynamics, nutrient, climatic and marine chemistry. Accurately identifying the types of carbonate factories and elucidating the factors controlling their development, quantitatively evaluating the environmental conditions in the process of factory development, and clarifying their paleoecological and sedimentological signals during the transformation of sedimentary systems are the key to understanding the evolution of carbonate sedimentary system. [Methods] In this paper, the Ma'antang Formation in the Guanyinya section of Hanwang Town, Mianzhu City, Northwest Sichuan, with systematic carbonate rock microfacies analysis and carbonate grains size statistics, we carry out the quantitative analysis of 20,391 grains. The composition, characteristics, transformation, and driving factors of tropical shallow-water carbonate factories during this period are described in detail. [Results and Discussions]The results showed that carbonate factories in tropical shallow water in the study area could be further divided into subdivided types of factories, such as peloids, ooids, bioclasts and reefs, indicating different carbonate production patterns.[Conclusions] By analyzing the composition, depositional environment, hydrodynamic and nutrient level of tropical shallow water factories in the study area, it is found that each subdivision factory has "habitat" characteristics and the transformation of factory type is more Controlled by regional sea level change. Frequently sea level changes lead to rapid changes in nutrient levels and hydrodynamic and carbonate mineral saturation, which drives the transformation of specific subdivided factories. The rapid changes in the sedimentary sequence of carbonate subdivisions indicate their Symbiosis horizontally. Therefore, the rapid transformation of subdivided production factory types on a shorter time scale is mainly related to the migration of horizontal factory types controlled by regional sea level and may not be related to extreme events and the transformation of sedimentary systems.
Abstract:
To investigate the degradation characteristics of n-hexadecane, a major component of petroleum, by microorganisms isolated from petroleum-contaminated soil in the Changqing oilfield, we employed petroleum-contaminated soil as the source of bacteria. Our objectives were to isolate strains with highly efficient n-hexadecane degradation capabilities, implement a salinization approach to achieve salinity-resistant degradation, and elucidate the kinetics and associated metabolic mechanisms of degradation. Through morphological characterization, Gram staining, biochemical and physicochemical tests, as well as 16S rDNA sequence analysis, we identified the isolated strain as Acinetobacter sp.5-5. The optimal degradation conditions for Acinetobacter sp.5-5 were determined through a resistance experiment spanning 14 days. It was found that, under specific conditions of n-hexadecane concentration (0.5% (V/V)), salinity (0.5%), and pH (7.0), the petroleum hydrocarbon-degrading bacteria achieved a remarkable total degradation of n-hexadecane, reaching 99.24%, with rapid degradation completed within two days (82.13%). Subsequently, the kinetics of n-hexadecane degradation were explored using zero-, quasi-primary, and quasi-secondary kinetic models. The results revealed that the quasi-primary model provided the best fit for describing the degradation process of n-hexadecane. Furthermore, in conjunction with the analysis of culture solution pH and degradation product profiles, our preliminary assessment of the n-hexadecane degradation pathway within this microbial degradation system indicated a process involving chain cleavage and acid production. This study underscores the high degradation potential of petroleum hydrocarbon-degrading bacteria, including their resilience to saline and alkaline conditions, particularly in the context of n-hexadecane. These findings hold promise for their widespread application in the remediation of salinized petroleum-contaminated soils.
To investigate the degradation characteristics of n-hexadecane, a major component of petroleum, by microorganisms isolated from petroleum-contaminated soil in the Changqing oilfield, we employed petroleum-contaminated soil as the source of bacteria. Our objectives were to isolate strains with highly efficient n-hexadecane degradation capabilities, implement a salinization approach to achieve salinity-resistant degradation, and elucidate the kinetics and associated metabolic mechanisms of degradation. Through morphological characterization, Gram staining, biochemical and physicochemical tests, as well as 16S rDNA sequence analysis, we identified the isolated strain as Acinetobacter sp.5-5. The optimal degradation conditions for Acinetobacter sp.5-5 were determined through a resistance experiment spanning 14 days. It was found that, under specific conditions of n-hexadecane concentration (0.5% (V/V)), salinity (0.5%), and pH (7.0), the petroleum hydrocarbon-degrading bacteria achieved a remarkable total degradation of n-hexadecane, reaching 99.24%, with rapid degradation completed within two days (82.13%). Subsequently, the kinetics of n-hexadecane degradation were explored using zero-, quasi-primary, and quasi-secondary kinetic models. The results revealed that the quasi-primary model provided the best fit for describing the degradation process of n-hexadecane. Furthermore, in conjunction with the analysis of culture solution pH and degradation product profiles, our preliminary assessment of the n-hexadecane degradation pathway within this microbial degradation system indicated a process involving chain cleavage and acid production. This study underscores the high degradation potential of petroleum hydrocarbon-degrading bacteria, including their resilience to saline and alkaline conditions, particularly in the context of n-hexadecane. These findings hold promise for their widespread application in the remediation of salinized petroleum-contaminated soils.
Abstract:
[Objective] Recently, the shale gas exploration of the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin marked a significant breakthrough, which has aroused widespread attention towards the redox conditions and the organic matter enrichment mechanisms of the Qiongzhusi Formation during its sedimentary period. Redox conditions of water column play a pivotal role in controlling the enrichment of organic matter. The particle sizes and distribution of framboidal pyrites serve as powerful redox indicators. These proxies, when combined with relevant geochemical data, enable the reconstruction of the ancient oceanic redox environment. Furthermore, the assemblage of pyrite morphological characteristics may correspond to changes in redox conditions. However, a detailed comparison between the pyrite morphological characteristics and redox conditions revealed by other geochemical markers remains limited. [Methods] This study focuses on the black shale of the Lower Cambrian Qiongzhusi Formation from well W207 in the Sichuan Basin. We conducted a comprehensive analysis of the particle sizes and distribution patterns of the framboidal pyrites. Using this method, in combination with total organic carbon (TOC), total sulfur (TS), pyrite content and previously published iron speciation data, we aimed to reconstruct the paleo-marine redox environment of the Qiongzhusi Formation during its sedimentary period. On this basis, the relationship between the assemblage of pyrite morphological characteristics and redox condition changes is discussed. [Results and Discussions] The redox environmental evolution of the Qiongzhusi Formation can be divided into three stages, i.e., strongly euxinic, intermittently euxinic and dysoxic from the bottom to the top. The morphology of pyrite is abundant, mainly framboidal pyrites, accompanied by a few pyrite microcrystals and euhedral pyrites. The framboidal pyrites are generally small in size, ranging from 2.2 to 18.4 μm with an average value of 6.39 ± 1.7 μm. More than 80% of the particle sizes are in the range of 3 – 8 μm, reflecting that framboidal pyrites were mainly formed in the water bodies of the synsedimentary period. In addition, a robust positive correlation between pyrite content and TOC and TS contents was observed. Pyrite morphology varies in different redox conditions. Specifically, in euxinic environment, the pyrite content is notably high, mainly contributed by abundant framboidal pyrites. In this setting, spherulitic microcrystals are dominant, while pyrite microcrystals and well-defined euhedral pyrites with clear edges are rare. As the redox environment gradually shifted to a more oxidized state, there was a decline in both the pyrite content and the quantity of framboidal pyrites. Concurrently, the microcrystals of the framboidal pyrites evolved from octahedral to cubic forms. At this stage, the pyrite microcrystals and euhedral pyrites both increase in quantity, with the former often exhibiting aggregates and the latter featuring irregular edges. [Conclusions] In conclusion, the observed changes in redox conditions during the sedimentation of the Qiongzhusi Formation reflect a gradual oxidation process in the inner shelf areas of South China. This aligns with the rise in atmospheric oxygen level during the early Cambrian. Notably, sedimentary pyrite displays distinct variations in the distribution of particle sizes and the assemblage of pyrite morphological characteristics as a function of redox conditions. These differences can serve as valuable supplementary indicators for assessing redox conditions in subsequent studies.
[Objective] Recently, the shale gas exploration of the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin marked a significant breakthrough, which has aroused widespread attention towards the redox conditions and the organic matter enrichment mechanisms of the Qiongzhusi Formation during its sedimentary period. Redox conditions of water column play a pivotal role in controlling the enrichment of organic matter. The particle sizes and distribution of framboidal pyrites serve as powerful redox indicators. These proxies, when combined with relevant geochemical data, enable the reconstruction of the ancient oceanic redox environment. Furthermore, the assemblage of pyrite morphological characteristics may correspond to changes in redox conditions. However, a detailed comparison between the pyrite morphological characteristics and redox conditions revealed by other geochemical markers remains limited. [Methods] This study focuses on the black shale of the Lower Cambrian Qiongzhusi Formation from well W207 in the Sichuan Basin. We conducted a comprehensive analysis of the particle sizes and distribution patterns of the framboidal pyrites. Using this method, in combination with total organic carbon (TOC), total sulfur (TS), pyrite content and previously published iron speciation data, we aimed to reconstruct the paleo-marine redox environment of the Qiongzhusi Formation during its sedimentary period. On this basis, the relationship between the assemblage of pyrite morphological characteristics and redox condition changes is discussed. [Results and Discussions] The redox environmental evolution of the Qiongzhusi Formation can be divided into three stages, i.e., strongly euxinic, intermittently euxinic and dysoxic from the bottom to the top. The morphology of pyrite is abundant, mainly framboidal pyrites, accompanied by a few pyrite microcrystals and euhedral pyrites. The framboidal pyrites are generally small in size, ranging from 2.2 to 18.4 μm with an average value of 6.39 ± 1.7 μm. More than 80% of the particle sizes are in the range of 3 – 8 μm, reflecting that framboidal pyrites were mainly formed in the water bodies of the synsedimentary period. In addition, a robust positive correlation between pyrite content and TOC and TS contents was observed. Pyrite morphology varies in different redox conditions. Specifically, in euxinic environment, the pyrite content is notably high, mainly contributed by abundant framboidal pyrites. In this setting, spherulitic microcrystals are dominant, while pyrite microcrystals and well-defined euhedral pyrites with clear edges are rare. As the redox environment gradually shifted to a more oxidized state, there was a decline in both the pyrite content and the quantity of framboidal pyrites. Concurrently, the microcrystals of the framboidal pyrites evolved from octahedral to cubic forms. At this stage, the pyrite microcrystals and euhedral pyrites both increase in quantity, with the former often exhibiting aggregates and the latter featuring irregular edges. [Conclusions] In conclusion, the observed changes in redox conditions during the sedimentation of the Qiongzhusi Formation reflect a gradual oxidation process in the inner shelf areas of South China. This aligns with the rise in atmospheric oxygen level during the early Cambrian. Notably, sedimentary pyrite displays distinct variations in the distribution of particle sizes and the assemblage of pyrite morphological characteristics as a function of redox conditions. These differences can serve as valuable supplementary indicators for assessing redox conditions in subsequent studies.
Abstract:
[Significance] Grain-sizes of sediment contain multiple factors in transport way, depositional process and environment. Defined as the occurrence frequencies of different diameter particles in sedimentology and geology, grain-size distribution (GSD) records the original sedimentological information. It is one of the based data to reveal modern and ancient depositional environment in river, lake, ocean, desert, loess, etc. Traditional GSDs analytical methods are just adopted to discuss overall features of depositional processes and environments qualitatively or semi-quantitatively, which hardly overcome the defects of quantification and multiple-solutions. [Progress] In this paper, different classification standards of grain-size scale are summarized. Moment and graphical methods of GSD parameters and morphological description standards of frequency curve are compared. Applicability and attention of sedimentary environment analysis traditional methods for GSDs are combed. Using mathematical means, some unconventional research methods are formed to tackle the entirety of GSD. Unsupervised clustering algorithms calculate the similarity of GSDs using frequency, cumulative frequency or statistical parameter of GSDs. Then depositional environments are sorted according to the classes of clustering. Multifractal extracts fractal parameters to represent the complexity of GSDs frequency data. These fractal structures of GSDs could reveal different depositional properties. Operated by multiple sedimentary processes in some sedimentary environments and dynamics, GSD is superposed by multi-subpopulations, and the corresponding frequency curve is bimodal or multimodal. This implies that an inverse model of unmixing would be ideally suited to obtain genetically meaningful interpretations of these subpopulations. There are two kinds of technologies to separate grain-size component from frequency data of GSD. Based on finite mixture model in statistics, single-sample unmixing (SSU) uses probability density function, such as normal, skew normal and Weibull distribution, to unmix single GSD by curve-fitting techniques. Each grain-size component obeys unimodal distribution, and statistical parameters, mean, sorting, skewness, kurtosis and percentage, can be calculated. End-member modelling algorithm (EMMA) decomposes grain-size end-members from GSD dataset. These unimodal or multimodal grain-size end-members are linearly independent and fixed in one GSD dataset. Many improved EMMAs are realized in different open-source tools. In order to introduce applications of above-mentioned unconventional methods, 27 GSDs from central bar of the Kangshan River in Poyang Lake drainage are processed by clustering, multifractal, SSU and EMMA as an example. [Conclusions and Prospects] Sedimentation analysis confronting problems and big data characteristics of GDSs are concluded. Development tendencies of depositional significance analytical methods based on GSDs are prospected. With the advent of various modern grain-size analysis techniques and more sophisticated artificial intelligence procedures in earth sciences, new increasingly intelligent mining methods for GSDs would be emergence to understand the spatio-temporal grain-size patterns in sediments. Some excellent sedimentological related databases have been constructing. Accordingly, open-access database will be established for GSDs, including various kinds of data, intelligent methods and literatures. Under the background of big data, GSD big data technology would provide new engine to mine depositional properties deeply, and integrate into sedimentology big data. Four phases, initial, exploratory, early development and rapid development stage, can describe the research history of GDSs. The coming must be big data stage for mining sedimentological information from GSDs.
[Significance] Grain-sizes of sediment contain multiple factors in transport way, depositional process and environment. Defined as the occurrence frequencies of different diameter particles in sedimentology and geology, grain-size distribution (GSD) records the original sedimentological information. It is one of the based data to reveal modern and ancient depositional environment in river, lake, ocean, desert, loess, etc. Traditional GSDs analytical methods are just adopted to discuss overall features of depositional processes and environments qualitatively or semi-quantitatively, which hardly overcome the defects of quantification and multiple-solutions. [Progress] In this paper, different classification standards of grain-size scale are summarized. Moment and graphical methods of GSD parameters and morphological description standards of frequency curve are compared. Applicability and attention of sedimentary environment analysis traditional methods for GSDs are combed. Using mathematical means, some unconventional research methods are formed to tackle the entirety of GSD. Unsupervised clustering algorithms calculate the similarity of GSDs using frequency, cumulative frequency or statistical parameter of GSDs. Then depositional environments are sorted according to the classes of clustering. Multifractal extracts fractal parameters to represent the complexity of GSDs frequency data. These fractal structures of GSDs could reveal different depositional properties. Operated by multiple sedimentary processes in some sedimentary environments and dynamics, GSD is superposed by multi-subpopulations, and the corresponding frequency curve is bimodal or multimodal. This implies that an inverse model of unmixing would be ideally suited to obtain genetically meaningful interpretations of these subpopulations. There are two kinds of technologies to separate grain-size component from frequency data of GSD. Based on finite mixture model in statistics, single-sample unmixing (SSU) uses probability density function, such as normal, skew normal and Weibull distribution, to unmix single GSD by curve-fitting techniques. Each grain-size component obeys unimodal distribution, and statistical parameters, mean, sorting, skewness, kurtosis and percentage, can be calculated. End-member modelling algorithm (EMMA) decomposes grain-size end-members from GSD dataset. These unimodal or multimodal grain-size end-members are linearly independent and fixed in one GSD dataset. Many improved EMMAs are realized in different open-source tools. In order to introduce applications of above-mentioned unconventional methods, 27 GSDs from central bar of the Kangshan River in Poyang Lake drainage are processed by clustering, multifractal, SSU and EMMA as an example. [Conclusions and Prospects] Sedimentation analysis confronting problems and big data characteristics of GDSs are concluded. Development tendencies of depositional significance analytical methods based on GSDs are prospected. With the advent of various modern grain-size analysis techniques and more sophisticated artificial intelligence procedures in earth sciences, new increasingly intelligent mining methods for GSDs would be emergence to understand the spatio-temporal grain-size patterns in sediments. Some excellent sedimentological related databases have been constructing. Accordingly, open-access database will be established for GSDs, including various kinds of data, intelligent methods and literatures. Under the background of big data, GSD big data technology would provide new engine to mine depositional properties deeply, and integrate into sedimentology big data. Four phases, initial, exploratory, early development and rapid development stage, can describe the research history of GDSs. The coming must be big data stage for mining sedimentological information from GSDs.
Abstract:
The Late Permian was a critical interval in geological history witnessing dramatic changes in tectonics, paleoclimate, paleo-oceanic environment, volcanic activities, and biosphere. Reconstructing the paleogeography, paleoclimate, and paleo-oceanic environment during this interval could provide a more complete picture of the interactions and relationships among different geological factors, as well as their influence on organic matter enrichment and the evolution of the biosphere. Organic-rich rock series were widely deposited during this period in the Sichuan Basin of the upper Yangtze Block, providing the insights into the co-evolution of the geosphere and biosphere. To better address the coeval temporal and spatial changes in paleogeographic settings, paleo-oceanic environment, primary productivity, and organic enrichment, we conducted a detailed sedimentological investigation integrated (major and trace) element contents, organic carbon isotopes and total organic carbon contents in Jianfeng section, located in an intrashelf basin in northeastern Sichuan Basin. The redox-sensitive trace elements data (MoXS, UXS, VXS contents , MoXS/UXS ratios and MoEF-UEF relationship) show that the redox environment of the Jianfeng section has experienced four intervals (I-IV). Interval I (0-10.35 m, spanning from the upper part of Wuchiaping Formation to the lower part of Dalong Formation) was predominated by an oxic environment during deposition, Interval II (10.35-23.35 m, the middle Dalong Formation) experienced a ferruginous-dominant anoxic condition, Interval III (23.35-27.00 m, the upper Dalong Formation) was mainly in a euxinic environment, Interval IV (27.00-34.05 m, from the top of the Dalong Formation to the bottom of the Changhsing Formation) was dominated by an oxic or suboxic environment. Comparing with the Xibeixiang section deposited in deeper water in the Kaijiang-Liangping trough, it was found that the redox environment in the intrashelf basin exhibits spatio-temporal heterogeneity. The micronutrient-limiting elements (ZnXS, CuXS, NiXS) contents demonstrate that primary productivity is high in both intervals II2 and III, coincident with the increasing terrestrial inputs (dedicated by Al、Zr、Ti contents and K/Al ratios). However, the Co×Mn values and Cd/Mo ratios indicated that the upwelling currents arose in intervals II2, II3, III, and IV, and the volcanic activities were only frequent in the Interval IV, which were both not in accordance with the eutrophication and the organic matter enrichment in the middle part of Dalong Formation. Therefore, the high primary productivity of Dalong Formation might be principally affected by a large number of terrestrial inputs. Furthermore, the upwelling currents also made contributions to the prosperity of plankton, but the volcanic activities have poor relationships with it. Combined with the Xibeixiang section in the study area, it is considered that the formation of euxinic water mass was closely related to the high primary productivity, while its spatiotemporal fluctuations were generally controlled by the sea-level fluctuations which could drive its upslope incursion in intervals II and III, downslope retreat in the Interval IV and even demise in the Interval V. In turn, the primary productivity could be further controlled by the upwelling currents particularly on the basinal slope, and terrigenous inputs on the more updip basinal margin. In this case, the primary productivity played a leading role in the enrichment of organic matter.
The Late Permian was a critical interval in geological history witnessing dramatic changes in tectonics, paleoclimate, paleo-oceanic environment, volcanic activities, and biosphere. Reconstructing the paleogeography, paleoclimate, and paleo-oceanic environment during this interval could provide a more complete picture of the interactions and relationships among different geological factors, as well as their influence on organic matter enrichment and the evolution of the biosphere. Organic-rich rock series were widely deposited during this period in the Sichuan Basin of the upper Yangtze Block, providing the insights into the co-evolution of the geosphere and biosphere. To better address the coeval temporal and spatial changes in paleogeographic settings, paleo-oceanic environment, primary productivity, and organic enrichment, we conducted a detailed sedimentological investigation integrated (major and trace) element contents, organic carbon isotopes and total organic carbon contents in Jianfeng section, located in an intrashelf basin in northeastern Sichuan Basin. The redox-sensitive trace elements data (MoXS, UXS, VXS contents , MoXS/UXS ratios and MoEF-UEF relationship) show that the redox environment of the Jianfeng section has experienced four intervals (I-IV). Interval I (0-10.35 m, spanning from the upper part of Wuchiaping Formation to the lower part of Dalong Formation) was predominated by an oxic environment during deposition, Interval II (10.35-23.35 m, the middle Dalong Formation) experienced a ferruginous-dominant anoxic condition, Interval III (23.35-27.00 m, the upper Dalong Formation) was mainly in a euxinic environment, Interval IV (27.00-34.05 m, from the top of the Dalong Formation to the bottom of the Changhsing Formation) was dominated by an oxic or suboxic environment. Comparing with the Xibeixiang section deposited in deeper water in the Kaijiang-Liangping trough, it was found that the redox environment in the intrashelf basin exhibits spatio-temporal heterogeneity. The micronutrient-limiting elements (ZnXS, CuXS, NiXS) contents demonstrate that primary productivity is high in both intervals II2 and III, coincident with the increasing terrestrial inputs (dedicated by Al、Zr、Ti contents and K/Al ratios). However, the Co×Mn values and Cd/Mo ratios indicated that the upwelling currents arose in intervals II2, II3, III, and IV, and the volcanic activities were only frequent in the Interval IV, which were both not in accordance with the eutrophication and the organic matter enrichment in the middle part of Dalong Formation. Therefore, the high primary productivity of Dalong Formation might be principally affected by a large number of terrestrial inputs. Furthermore, the upwelling currents also made contributions to the prosperity of plankton, but the volcanic activities have poor relationships with it. Combined with the Xibeixiang section in the study area, it is considered that the formation of euxinic water mass was closely related to the high primary productivity, while its spatiotemporal fluctuations were generally controlled by the sea-level fluctuations which could drive its upslope incursion in intervals II and III, downslope retreat in the Interval IV and even demise in the Interval V. In turn, the primary productivity could be further controlled by the upwelling currents particularly on the basinal slope, and terrigenous inputs on the more updip basinal margin. In this case, the primary productivity played a leading role in the enrichment of organic matter.
Abstract:
Abstract: [Objective]Currently, research on the Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE; ~183 Ma) in the Ordos Basin mainly focuses on paleoclimate conditions, sedimentary environment evolution, and biodiversity changes, while the mechanisms of sulfur cycling during the T-OAE period in lakes remain unclear.[Methods]In order to further elucidate the sulfur cycling mechanism, Petrology and Geochemistry analyses were conducted on mudstone and black shale samples from the Anya section. The mechanisms of sulfur isotope fractionation during the T-OAE period were explored using major and trace elements and pyrite sulfur isotopes (δ34Spy).[Results and discussions]The pyrite in the Anya section samples mainly exists in the form of tetrahedral crystals, forming framboidal pyrite. The δ34Spy values of the samples exhibit anomalously positive values (ranging from 2.7 to 14.1‰, with an average of 8.3‰). Based on the variations in δ34Spy and total organic carbon (TOC) in conjunction with geochemical indicators (δ13C, TS, Corg/P, (La/Th)N), the evolution of the lake sedimentary environment during the T-OAE period was divided into four stages (high organic matter stages I and II, low organic matter stages I and II). [Conclusions] The δ34Spy values during the T-OAE period in the Anya section of the Ordos Basin coincide with atmospheric precipitation and surface runoff, indicating that the main source of sulfur in the lake is sulfate in the water mass. The sulfur isotopes of the samples are mainly controlled by the redox conditions of the lake bottom water and organic matter, and are independent of sedimentation rate and sulfate concentration. When the dissolved oxygen in the basin bottom water is low and the organic matter content is high, dissolved oxygen infiltrates into the sediment, activating anaerobic oxidants and promoting the reoxidation of H2S, resulting in positive δ34Spy values through Rayleigh fractionation. When the bottom water environment of the lake is oxygen-rich and the organic matter content is low, dissolved oxygen infiltrates into sediments, activating anaerobic oxidants and promoting H2S reoxidation, leading to a positive δ34Spy shift through the Rayleigh fractionation model. When the bottom water environment of the lake is oxygen-deficient and the organic matter content is high, the activity of sulfate-reducing bacteria is enhanced, promoting the MSR reaction and preferentially incorporating 32S, resulting in a negative δ34Spy shift. The sulfur cycle during the T-OAE period in the Anya section is mainly controlled by local/regional sedimentary environments, but the sulfur cycle is also influenced by global warming and intensified hydrological circulation during the T-OAE period
Abstract: [Objective]Currently, research on the Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE; ~183 Ma) in the Ordos Basin mainly focuses on paleoclimate conditions, sedimentary environment evolution, and biodiversity changes, while the mechanisms of sulfur cycling during the T-OAE period in lakes remain unclear.[Methods]In order to further elucidate the sulfur cycling mechanism, Petrology and Geochemistry analyses were conducted on mudstone and black shale samples from the Anya section. The mechanisms of sulfur isotope fractionation during the T-OAE period were explored using major and trace elements and pyrite sulfur isotopes (δ34Spy).[Results and discussions]The pyrite in the Anya section samples mainly exists in the form of tetrahedral crystals, forming framboidal pyrite. The δ34Spy values of the samples exhibit anomalously positive values (ranging from 2.7 to 14.1‰, with an average of 8.3‰). Based on the variations in δ34Spy and total organic carbon (TOC) in conjunction with geochemical indicators (δ13C, TS, Corg/P, (La/Th)N), the evolution of the lake sedimentary environment during the T-OAE period was divided into four stages (high organic matter stages I and II, low organic matter stages I and II). [Conclusions] The δ34Spy values during the T-OAE period in the Anya section of the Ordos Basin coincide with atmospheric precipitation and surface runoff, indicating that the main source of sulfur in the lake is sulfate in the water mass. The sulfur isotopes of the samples are mainly controlled by the redox conditions of the lake bottom water and organic matter, and are independent of sedimentation rate and sulfate concentration. When the dissolved oxygen in the basin bottom water is low and the organic matter content is high, dissolved oxygen infiltrates into the sediment, activating anaerobic oxidants and promoting the reoxidation of H2S, resulting in positive δ34Spy values through Rayleigh fractionation. When the bottom water environment of the lake is oxygen-rich and the organic matter content is low, dissolved oxygen infiltrates into sediments, activating anaerobic oxidants and promoting H2S reoxidation, leading to a positive δ34Spy shift through the Rayleigh fractionation model. When the bottom water environment of the lake is oxygen-deficient and the organic matter content is high, the activity of sulfate-reducing bacteria is enhanced, promoting the MSR reaction and preferentially incorporating 32S, resulting in a negative δ34Spy shift. The sulfur cycle during the T-OAE period in the Anya section is mainly controlled by local/regional sedimentary environments, but the sulfur cycle is also influenced by global warming and intensified hydrological circulation during the T-OAE period
Abstract:
Abstract: Through detailed field investigation in the Zoige Basin, complete stratigraphic profile of sedimentary sequence was found and systematically sampled on the high platform north of the hydrological station at the northern end of the old bridge of the Yellow River in Maqu County at the front of the glacial fans and alluvial fans. Through grain size analysis and Optically Stimulated Luminescence (OSL) dating, the sedimentary environment and surface process changes since the last deglaciation in Zoige Basin were studied. The results showed that during the last glacial before 14.5ka, the ice meltwater and mountain flood process in the Warihe River in the eastern end of the Jishi Mountain were active and accumulation rapidly in the foothill, forming a thick glacial- alluvial fan sand and gravel layer. During the B?lling-Aller?d warm period of 14.7-11.7 ka, the glaciers melted, and the depression in the front of the glacial fans and alluvial fans layer formed a muddy swamp environment with the accumulation of gray-green sandy sediments. However, the climate changed abruptly and worsened during the Younger Dryas period, the upper part of the gray-green bog soil layer was folded and deformed due to surface freeze-thaw action.In the early Holocene (11.7-8.5ka), the prevaling aeolian activities made the shallow depression in this area accumulate to form the interlayer of aeolian sand and mud soil. During the warm and humid period of 8.5-3.1 ka, the palaeosol of swamp meadow black soil was formed in this area. In the late Holocene since 3.1 ka BP, the climate was relatively dry, and the sandstorm deposits were transformed into subalpine meadow paleosols. The results are of great significance to reveal the response of surface processes to environmental changes since the last deglaciation in the Zoige Basin, and to further understand the environmental changes and surface processes in the Tibetan Plateau for global changes.
Abstract: Through detailed field investigation in the Zoige Basin, complete stratigraphic profile of sedimentary sequence was found and systematically sampled on the high platform north of the hydrological station at the northern end of the old bridge of the Yellow River in Maqu County at the front of the glacial fans and alluvial fans. Through grain size analysis and Optically Stimulated Luminescence (OSL) dating, the sedimentary environment and surface process changes since the last deglaciation in Zoige Basin were studied. The results showed that during the last glacial before 14.5ka, the ice meltwater and mountain flood process in the Warihe River in the eastern end of the Jishi Mountain were active and accumulation rapidly in the foothill, forming a thick glacial- alluvial fan sand and gravel layer. During the B?lling-Aller?d warm period of 14.7-11.7 ka, the glaciers melted, and the depression in the front of the glacial fans and alluvial fans layer formed a muddy swamp environment with the accumulation of gray-green sandy sediments. However, the climate changed abruptly and worsened during the Younger Dryas period, the upper part of the gray-green bog soil layer was folded and deformed due to surface freeze-thaw action.In the early Holocene (11.7-8.5ka), the prevaling aeolian activities made the shallow depression in this area accumulate to form the interlayer of aeolian sand and mud soil. During the warm and humid period of 8.5-3.1 ka, the palaeosol of swamp meadow black soil was formed in this area. In the late Holocene since 3.1 ka BP, the climate was relatively dry, and the sandstorm deposits were transformed into subalpine meadow paleosols. The results are of great significance to reveal the response of surface processes to environmental changes since the last deglaciation in the Zoige Basin, and to further understand the environmental changes and surface processes in the Tibetan Plateau for global changes.
Abstract:
Utilizing core observations, thin section examinations, cathode luminescence analysis, scanning electron microscopy, fluid inclusions, carbon and oxygen isotope, hydrogen and oxygen isotope, electron probe analysis, situ laser ablation inductively coupled plasma mass spectrometry analysis, and various other analytical techniques, the diagenesis evolution and genetic model of Permian reservoirs in the southern Dagang exploration area were studied. By combining these results with the histories of burial evolution and hydrocarbon accumulation, it was revealed that the Permian sandstones in the South Dagang area possess high compositional maturity, characterized by quartz sandstones and sublithic sandstones. The high-quality reservoirs are dominated by secondary intergranular pores and kaolinite intergranular pores. The reservoir underwent an early exposure-open to late burial-closed environment, experiencing both meteoric freshwater leaching and deep burial dissolution under the control of tectonic movement. The formation of high-quality reservoirs is influenced by numerous factors. High maturity of medium coarse sandstone is the material basis for their formation. Stratum uplift causes sandstone to be leached by meteoric freshwater, forming a significant number of secondary pores, which is the key to the development of high-quality reservoirs. Early hydrocarbon charging and inhibition of cementation are conducive to the preservation of secondary pores. Reservoirs currently present in the slope and depression zone, but formed in an area of atmospheric water leaching and injection, represent a potential exploration targets. This model's establishment can be used as a reference for pre-drilling prediction of Permian reservoirs in other areas of the Bohai Bay Basin.
Utilizing core observations, thin section examinations, cathode luminescence analysis, scanning electron microscopy, fluid inclusions, carbon and oxygen isotope, hydrogen and oxygen isotope, electron probe analysis, situ laser ablation inductively coupled plasma mass spectrometry analysis, and various other analytical techniques, the diagenesis evolution and genetic model of Permian reservoirs in the southern Dagang exploration area were studied. By combining these results with the histories of burial evolution and hydrocarbon accumulation, it was revealed that the Permian sandstones in the South Dagang area possess high compositional maturity, characterized by quartz sandstones and sublithic sandstones. The high-quality reservoirs are dominated by secondary intergranular pores and kaolinite intergranular pores. The reservoir underwent an early exposure-open to late burial-closed environment, experiencing both meteoric freshwater leaching and deep burial dissolution under the control of tectonic movement. The formation of high-quality reservoirs is influenced by numerous factors. High maturity of medium coarse sandstone is the material basis for their formation. Stratum uplift causes sandstone to be leached by meteoric freshwater, forming a significant number of secondary pores, which is the key to the development of high-quality reservoirs. Early hydrocarbon charging and inhibition of cementation are conducive to the preservation of secondary pores. Reservoirs currently present in the slope and depression zone, but formed in an area of atmospheric water leaching and injection, represent a potential exploration targets. This model's establishment can be used as a reference for pre-drilling prediction of Permian reservoirs in other areas of the Bohai Bay Basin.
Abstract:
[Objective] The second member of the Permian Lucaogou Formation (P2l2) is the main source rock in the Malang Sag of the Santanghu Basin, with high abundance of β-carotane, but its content shows great differences in the section. In order to further clarify the longitudinal variation of β-carotane in P2l2, and then to explore the main controlling factors of its enrichment and its distribution on the plane. [Methods] In this study, organic geochemical analysis was carried out and the source rocks investigated were divided into groups I, II and III from bottom to top according to the β-carotane index. The β-carotane index was relatively low in group I, and gradually increased from bottom to top in group II, while the β-carotane index in group III fluctuated greatly. [Results and Discussions] The results show that the relative content of β-carotane is an effective index to reflect the quality of source rocks in the study area. The organic matter of P2l2 source rocks in Malang Sag is mainly composed of lower aquatic organisms. However, compared with the group I, the group II and group III have a richer supply of prokaryote organic matter. The ratios of Steranes/Hopanes, C28 steranes/C29 steranes and (Pr+Ph)/(nC17+ nC18) indicated that the precursors of β-carotane in groups II and III might be algae with phytol side chains of chlorophyll a, b and c and bacteri. According to the parameters of Pr/Ph, C35 hopanes/C34 hopanes and ETR, it can be concluded that the water salinity of source rocks in groups II and III is high, mainly in a reducing environment. Compared with the fluctuating environment of weak oxidation-weak reduction during the deposition of group I and the relatively low salinity of water, it is more conducive to the preservation of β-carotane. A certain intensity of hydrothermal activity occurred during the deposition of group III source rocks, resulting in a large fluctuation of β-carotane content. [Conclusions] Therefore, the enrichment of P2l2 β-carotane in Malang Sag is mainly controlled by the source of organic matter and sedimentary environment.
[Objective] The second member of the Permian Lucaogou Formation (P2l2) is the main source rock in the Malang Sag of the Santanghu Basin, with high abundance of β-carotane, but its content shows great differences in the section. In order to further clarify the longitudinal variation of β-carotane in P2l2, and then to explore the main controlling factors of its enrichment and its distribution on the plane. [Methods] In this study, organic geochemical analysis was carried out and the source rocks investigated were divided into groups I, II and III from bottom to top according to the β-carotane index. The β-carotane index was relatively low in group I, and gradually increased from bottom to top in group II, while the β-carotane index in group III fluctuated greatly. [Results and Discussions] The results show that the relative content of β-carotane is an effective index to reflect the quality of source rocks in the study area. The organic matter of P2l2 source rocks in Malang Sag is mainly composed of lower aquatic organisms. However, compared with the group I, the group II and group III have a richer supply of prokaryote organic matter. The ratios of Steranes/Hopanes, C28 steranes/C29 steranes and (Pr+Ph)/(nC17+ nC18) indicated that the precursors of β-carotane in groups II and III might be algae with phytol side chains of chlorophyll a, b and c and bacteri. According to the parameters of Pr/Ph, C35 hopanes/C34 hopanes and ETR, it can be concluded that the water salinity of source rocks in groups II and III is high, mainly in a reducing environment. Compared with the fluctuating environment of weak oxidation-weak reduction during the deposition of group I and the relatively low salinity of water, it is more conducive to the preservation of β-carotane. A certain intensity of hydrothermal activity occurred during the deposition of group III source rocks, resulting in a large fluctuation of β-carotane content. [Conclusions] Therefore, the enrichment of P2l2 β-carotane in Malang Sag is mainly controlled by the source of organic matter and sedimentary environment.
Abstract:
The Lower Cambrian organic-rich shale is important marine shale exploration target in western Hubei and southern Guizhou. Understanding the chronostratigraphic relationships and their underlying causes of organic-rich shale intervals in different areas is crucial for shale gas exploration. In this study, we conducted cyclostratigraphic analyses using natural gamma-ray logging data from the Lower Cambrian Niedi-1 well in western Hubei and the Huangye-1 well in southern Guizhou, and determined the primary astronomical periods. Using previously reported zircon age as anchoring points, we constructed an astronomical time scale for the Lower Cambrian. Our results reveal that wavelengths of 36 m and 9 m in the Niedi-1 well, and 16.6 m and 4 m in the Huangye-1 well, correspond to orbital 405 kyr and 100 kyr cycles, respectively. Furthermore, we calculated the durations of the Niutitang Formaion in the Niedi-1 well and Jiumengchong Formation in the Huangye-1 well to be 3.3 Myr and 2.6 Myr, respectively, with age intervals falling within the range of 535.4±1.7 Ma to 536.82±1.7 Ma and 535.2±1.7 Ma to 537.84±1.7 Ma. Using a sedimentary noise model, relative sea-level changes in the Lower Cambrian were reconstructed. Through correlation between sea-level change curves (DYNOT and ρ1), eccentricity cycles and sedimentary cycles, our results indicate that sea-level changes correspond to 1.2 Myr obliquity modulation cycles and two sedimentary sequences (SQ1 and SQ2). In addition, comparing the total organic carbon content (TOC) of the Lower Cambrian organic-rich shale with orbital eccentricity, sedimentation rates and sea-level changes, our findings reveal that the intervals with high TOC value correlate with the maximum eccentricity of 405 kyr, while low TOC value correspond to minimum value in Niedi-1 well. Because of the chaotic behavior of the solar system, accurately predicting the mechanism driving organic-rich shale during the early Cambrian is impossible. Assuming that the maximum eccentricity corresponds to the maximum organic carbon content at that time, strong seasonal variation will, to some extent, trigger the enrichment of black shale over a more extended period. Furthermore, the comparison between relative sea-level changes and organic carbon content reveals no causal relationship between sea-level fluctuations and the enrichment of organic matter.
The Lower Cambrian organic-rich shale is important marine shale exploration target in western Hubei and southern Guizhou. Understanding the chronostratigraphic relationships and their underlying causes of organic-rich shale intervals in different areas is crucial for shale gas exploration. In this study, we conducted cyclostratigraphic analyses using natural gamma-ray logging data from the Lower Cambrian Niedi-1 well in western Hubei and the Huangye-1 well in southern Guizhou, and determined the primary astronomical periods. Using previously reported zircon age as anchoring points, we constructed an astronomical time scale for the Lower Cambrian. Our results reveal that wavelengths of 36 m and 9 m in the Niedi-1 well, and 16.6 m and 4 m in the Huangye-1 well, correspond to orbital 405 kyr and 100 kyr cycles, respectively. Furthermore, we calculated the durations of the Niutitang Formaion in the Niedi-1 well and Jiumengchong Formation in the Huangye-1 well to be 3.3 Myr and 2.6 Myr, respectively, with age intervals falling within the range of 535.4±1.7 Ma to 536.82±1.7 Ma and 535.2±1.7 Ma to 537.84±1.7 Ma. Using a sedimentary noise model, relative sea-level changes in the Lower Cambrian were reconstructed. Through correlation between sea-level change curves (DYNOT and ρ1), eccentricity cycles and sedimentary cycles, our results indicate that sea-level changes correspond to 1.2 Myr obliquity modulation cycles and two sedimentary sequences (SQ1 and SQ2). In addition, comparing the total organic carbon content (TOC) of the Lower Cambrian organic-rich shale with orbital eccentricity, sedimentation rates and sea-level changes, our findings reveal that the intervals with high TOC value correlate with the maximum eccentricity of 405 kyr, while low TOC value correspond to minimum value in Niedi-1 well. Because of the chaotic behavior of the solar system, accurately predicting the mechanism driving organic-rich shale during the early Cambrian is impossible. Assuming that the maximum eccentricity corresponds to the maximum organic carbon content at that time, strong seasonal variation will, to some extent, trigger the enrichment of black shale over a more extended period. Furthermore, the comparison between relative sea-level changes and organic carbon content reveals no causal relationship between sea-level fluctuations and the enrichment of organic matter.
Abstract:
[Significance] The Late Devonian Frasnian-Famennian (F-F) transition is a critical time interval in geological history during which major marine ecological system changes occurred simultaneously, leading to the mass extinction, known as one of the biggest five in Phanerozoic. This mass extinction (also referred to F-F event or Kellwasser event) was characterized by severe losses of low-latitude shallow water benthic faunas, notably for the reef-dwelling coral and stromatoporoid. High-latitude, deep-sea and terrestrial faunas were affected the least. Until recently, various independent or combined hypotheses, including sea level change, marine anoxia, climate change, volcanic/hydrothermal activities, and bolide impact have been proposed as the causes of this mass extinction event. Among them, climate change and marine anoxia are the most intensively researched and discussed hypotheses in recent years based on the Web of Science data. However, there are still some controversies, and the interactions among these environmental factors are still not clear. [Progress] This study systematically reviews the research papers on the paleoclimatic and paleo-oceanic changes during the F-F transition interval and discusses the anoxic model during this critical period based on the related case studies in South China. Conodont oxygen and strontium isotopes, as well as the carbonate carbon isotope records collectively suggested that the climate became cold during the F-F transition interval, including several rapid warming-cooling fluctuations. Conodont oxygen isotope results suggested that the surface sea-water temperature (SST) had dropped by 5~8 ℃. Strontium isotopes also implied that these warming-cooling fluctuations may have been caused by the short and frequent volcanic activities. Palynology data and carbonate platform exposure/karstification were also the evidence of coeval climate cooling. Additionally, marine anoxia has been extensively hypothesized as a potential killing mechanism for the F–F mass extinction based initially on the occurrence of bituminous limestones (or black shales) named as the Lower and Upper Kellwasser Horizons. Studies on pyrite framboid, biomarker compounds, trace elements, nitrogen isotope, sulfur isotope, Uranium isotope and iron speciation suggested the existence of the Kellwasser anoxic events. However, it generally occurred in geography-specific environments, notably in the pericontinental basins/subbasins proximal to the source hinterlands. Moreover, the extent and degree of anoxia in F-F transition also varied in different study sections all over the world. With regards to anoxic model, researches suggested that the Kellwasser anoxic events should be caused by the increased nutrients input related to the enhanced continental weathering. The marine anoxic studies from three F-F sections belonging to different depositional facies in South China also supported this “top down” anoxic model. [Conclusions and Prospects] Hence, the F-F biotic crisis was not caused by single factor. And, frequent and short-term volcanic activities could have induced the enhanced continental weathering and greenhouse gases emissions, which process, on the one hand, led to the frequent warming-cooling climate fluctuations, and on the other hand promoted the nutrient inputs to the ocean, thereby resulting in eutrophication and anoxia in the shallow-water. Therefore, the mutual interactions of various environmental factors may have exerted great biological pressure in the low-latitude shallow sea, and eventually led to the F-F biotic crisis.
[Significance] The Late Devonian Frasnian-Famennian (F-F) transition is a critical time interval in geological history during which major marine ecological system changes occurred simultaneously, leading to the mass extinction, known as one of the biggest five in Phanerozoic. This mass extinction (also referred to F-F event or Kellwasser event) was characterized by severe losses of low-latitude shallow water benthic faunas, notably for the reef-dwelling coral and stromatoporoid. High-latitude, deep-sea and terrestrial faunas were affected the least. Until recently, various independent or combined hypotheses, including sea level change, marine anoxia, climate change, volcanic/hydrothermal activities, and bolide impact have been proposed as the causes of this mass extinction event. Among them, climate change and marine anoxia are the most intensively researched and discussed hypotheses in recent years based on the Web of Science data. However, there are still some controversies, and the interactions among these environmental factors are still not clear. [Progress] This study systematically reviews the research papers on the paleoclimatic and paleo-oceanic changes during the F-F transition interval and discusses the anoxic model during this critical period based on the related case studies in South China. Conodont oxygen and strontium isotopes, as well as the carbonate carbon isotope records collectively suggested that the climate became cold during the F-F transition interval, including several rapid warming-cooling fluctuations. Conodont oxygen isotope results suggested that the surface sea-water temperature (SST) had dropped by 5~8 ℃. Strontium isotopes also implied that these warming-cooling fluctuations may have been caused by the short and frequent volcanic activities. Palynology data and carbonate platform exposure/karstification were also the evidence of coeval climate cooling. Additionally, marine anoxia has been extensively hypothesized as a potential killing mechanism for the F–F mass extinction based initially on the occurrence of bituminous limestones (or black shales) named as the Lower and Upper Kellwasser Horizons. Studies on pyrite framboid, biomarker compounds, trace elements, nitrogen isotope, sulfur isotope, Uranium isotope and iron speciation suggested the existence of the Kellwasser anoxic events. However, it generally occurred in geography-specific environments, notably in the pericontinental basins/subbasins proximal to the source hinterlands. Moreover, the extent and degree of anoxia in F-F transition also varied in different study sections all over the world. With regards to anoxic model, researches suggested that the Kellwasser anoxic events should be caused by the increased nutrients input related to the enhanced continental weathering. The marine anoxic studies from three F-F sections belonging to different depositional facies in South China also supported this “top down” anoxic model. [Conclusions and Prospects] Hence, the F-F biotic crisis was not caused by single factor. And, frequent and short-term volcanic activities could have induced the enhanced continental weathering and greenhouse gases emissions, which process, on the one hand, led to the frequent warming-cooling climate fluctuations, and on the other hand promoted the nutrient inputs to the ocean, thereby resulting in eutrophication and anoxia in the shallow-water. Therefore, the mutual interactions of various environmental factors may have exerted great biological pressure in the low-latitude shallow sea, and eventually led to the F-F biotic crisis.
Abstract:
The Great Ordovician Biodiversification Event (GOE) is the result of the interaction between marine environments and organisms. The analyses of sedimentary facies and depositional sequences of the Ordovician carbonate platform can reveal its development and evolution and the history of sea level fluctuations, which can provide a sedimentary background and isochronous stratigraphic framework for exploring the spatio-temporal distribution characteristics and the evolution of coeval organisms. Based on the measurement of the outcrop section and microscopic observation, the Lower Ordovician (Tongzi and Honghuayuan formations) successions of the Piaoertian section in northern Guizhou Province were investigated to recognize the lithofacies types, construct the depositional model, identify the sequence stratigraphy, and explore the controls of depositional evolution. Results are as follow: (1) Ten types of lithofacies were identified and further grouped into two depositional belts (or associations) in a carbonate ramp system. The depositional system was further divided into non-skeletal grain-dominated and skeletal grain-dominated end members for the Tongzi and Honghuayuan formations, respectively. (2) These lithofacies are vertically stacked into asymmetrical, meter-scale, shallowing-upward open-marine depositional cycles for both Tongzi and Honghuayuan formations. (3) In view of vertical facies and cycle stacking patterns, three and one half third-order depositional sequences (Sq1~Sq4) were distinguished, which are defined by II sequence boundaries (lithologic transition surfaces). Sq1~Sq3 are full sequences that consist of lower transgressive (TST) and upper regressive (RST) packages, but Sq4 only contains TST. (4) The depositional evolution and sequence development of the Lower Ordovician of the Piaoertian section were mainly controlled by relative sea-level oscillations of different orders and paleogeographic configuration. The relative sea-level oscillations of different orders dictated the vertical development of facies; the paleogeographic configuration primarily defined the spatial distribution of depositional belts (or associations).
The Great Ordovician Biodiversification Event (GOE) is the result of the interaction between marine environments and organisms. The analyses of sedimentary facies and depositional sequences of the Ordovician carbonate platform can reveal its development and evolution and the history of sea level fluctuations, which can provide a sedimentary background and isochronous stratigraphic framework for exploring the spatio-temporal distribution characteristics and the evolution of coeval organisms. Based on the measurement of the outcrop section and microscopic observation, the Lower Ordovician (Tongzi and Honghuayuan formations) successions of the Piaoertian section in northern Guizhou Province were investigated to recognize the lithofacies types, construct the depositional model, identify the sequence stratigraphy, and explore the controls of depositional evolution. Results are as follow: (1) Ten types of lithofacies were identified and further grouped into two depositional belts (or associations) in a carbonate ramp system. The depositional system was further divided into non-skeletal grain-dominated and skeletal grain-dominated end members for the Tongzi and Honghuayuan formations, respectively. (2) These lithofacies are vertically stacked into asymmetrical, meter-scale, shallowing-upward open-marine depositional cycles for both Tongzi and Honghuayuan formations. (3) In view of vertical facies and cycle stacking patterns, three and one half third-order depositional sequences (Sq1~Sq4) were distinguished, which are defined by II sequence boundaries (lithologic transition surfaces). Sq1~Sq3 are full sequences that consist of lower transgressive (TST) and upper regressive (RST) packages, but Sq4 only contains TST. (4) The depositional evolution and sequence development of the Lower Ordovician of the Piaoertian section were mainly controlled by relative sea-level oscillations of different orders and paleogeographic configuration. The relative sea-level oscillations of different orders dictated the vertical development of facies; the paleogeographic configuration primarily defined the spatial distribution of depositional belts (or associations).
Abstract:
[Objective] The beach-bar sand body is an important reservoir developed in the coastal or lakeshore area. Currently, there are more studies on the architecture of lakeshore beach-bar sand body, while there are differences in the depositional environments between the coastal and lakeshore, and there are differences in the sedimentation patterns of beach-bar, so the research on the architecture of beach-bar sand body needs to be deepened. [Methods] In this paper, based on a large amount of literature research on marine beach-bar deposits, the results are summarised in terms of the sedimentary characteristics and the hydrodynamic mechanism of marine beach-bar sand, the factors influencing the architecture, and the architecture pattern and characterization of it. [Results and discussion] The results show that the beach-bar deposits is dominated by sandbar, the development of sandbar is controlled by a variety of hydrodynamics such as nearshore spiral current, longshore current, rip current, swash current, and backward current in the seafloor, a variety types of sandbar have been formed under multiple types of hydrodynamic action, such as linear sandbar, coastal sandbar, sandbar spits, plume sandbar, tongue-like sandbar, oblique sandbar, etc. There are differences in the architecture characteristics and genesis of different types of sandbar. The sedimentary architecture of the sandbar is controlled by multiple factors. Shoreline morphology and wave interaction processes determine the mode of wave action, the supply of sediment sources and changes in relative sea level affect the development and stacking pattern of the sandbar, and tectonics and sedimentary palaeomorphology control the distribution of the sandbar. The sedimentary architecture of the beach-bar was characterized according to three levels: composite beach-bar, single bar and intra-bar accretionary body, and a preliminary beach-bar architecture model and characterization method were established. [Conclusions] Architecture prototypes should be modelled using various methods such as field outcrops, modern sedimentation, subsurface well datas and sediment simulation, which can help enrich the sedimentological theory of marine beach-bar reservoirs and more effectively guide the development of marine beach-bar oil and gas reservoirs.
[Objective] The beach-bar sand body is an important reservoir developed in the coastal or lakeshore area. Currently, there are more studies on the architecture of lakeshore beach-bar sand body, while there are differences in the depositional environments between the coastal and lakeshore, and there are differences in the sedimentation patterns of beach-bar, so the research on the architecture of beach-bar sand body needs to be deepened. [Methods] In this paper, based on a large amount of literature research on marine beach-bar deposits, the results are summarised in terms of the sedimentary characteristics and the hydrodynamic mechanism of marine beach-bar sand, the factors influencing the architecture, and the architecture pattern and characterization of it. [Results and discussion] The results show that the beach-bar deposits is dominated by sandbar, the development of sandbar is controlled by a variety of hydrodynamics such as nearshore spiral current, longshore current, rip current, swash current, and backward current in the seafloor, a variety types of sandbar have been formed under multiple types of hydrodynamic action, such as linear sandbar, coastal sandbar, sandbar spits, plume sandbar, tongue-like sandbar, oblique sandbar, etc. There are differences in the architecture characteristics and genesis of different types of sandbar. The sedimentary architecture of the sandbar is controlled by multiple factors. Shoreline morphology and wave interaction processes determine the mode of wave action, the supply of sediment sources and changes in relative sea level affect the development and stacking pattern of the sandbar, and tectonics and sedimentary palaeomorphology control the distribution of the sandbar. The sedimentary architecture of the beach-bar was characterized according to three levels: composite beach-bar, single bar and intra-bar accretionary body, and a preliminary beach-bar architecture model and characterization method were established. [Conclusions] Architecture prototypes should be modelled using various methods such as field outcrops, modern sedimentation, subsurface well datas and sediment simulation, which can help enrich the sedimentological theory of marine beach-bar reservoirs and more effectively guide the development of marine beach-bar oil and gas reservoirs.
Abstract:
The Yanxi Formation is a set of marine organic rich black fine-grained rock series newly identified in recent years in central Hunan, which has good shale gas exploration potential, the research on the sedimentary characteristics and evolution of this black fine-grained rock series urgently needs to be strengthened. The lithology of Yanxi Formation is mainly siliceous rock and carbonaceous shale, followed by silty shale, siltstone; Its lithofacies assemblage includes: thick pure carbonaceous shale, interbedded carbonaceous shale and siliceous rock, and siliceous rock with thin layers of carbonaceous shale;The Yanxi Formation is generally characterized by a thick southeast and thin northwest distribution in the study area;Through field exploration, lithofacies combination analysis, thin section identification and elements testing analysis, four sedimentary facies types can be identified: deep water basins, deep water continental shelves, shallow water continental shelves, and turbidite fans;The Yanxi Formation is characterized by significant trichotomy vertically. The lower member is interbedded with siliceous rock and carbonaceous shale of unequal thickness. The middle black carbonaceous shale is the most important Source rock of the Yanxi Formation. The upper member is siliceous rock with thin carbonaceous shale. The three members are relatively stable in regional distribution thickness and have good contrast;The tectonic evolution history of the ancient land of South China show that in the early Paleozoic the prototype basin type of the Middle Ordovician in the central Hunan region is a rift type intracontinental sea basin; The analysis of major and rare earth elements indicates that the siliceous rocks in the central Hunan region are mostly biogenic, and the study area was in a passive continental margin environment during the Middle Ordovician. The analysis of trace elements indicate that the central Hunan region was in an anaerobic water retention environment during the Middle Ordovician;Large scale marine invasion occurred in the central Hunan region during the Early Middle Ordovician, the distribution area of deep-water basin facies reached its maximum, while the sedimentary range of carbonaceous shale and siliceous rock reached its maximum simultaneously; During late Ordovician sea level rapidly decreased, and most of the study area was dominated by deep-water continental shelf sedimentation; The prototype basin pattern of Hunan region during the Middle Ordovician is as follows: the northwestern Hunan region is the carbonate platform of the Upper Yangtze region, the central Hunan region is the Jiangnan slope zone (deep water continental shelf), the central southern Hunan is a deep water basin supporting black carbon shale sedimentation, and the southeastern Hunan is the turbidite fan facies area on the northern margin of the Huaxia ancient land.
The Yanxi Formation is a set of marine organic rich black fine-grained rock series newly identified in recent years in central Hunan, which has good shale gas exploration potential, the research on the sedimentary characteristics and evolution of this black fine-grained rock series urgently needs to be strengthened. The lithology of Yanxi Formation is mainly siliceous rock and carbonaceous shale, followed by silty shale, siltstone; Its lithofacies assemblage includes: thick pure carbonaceous shale, interbedded carbonaceous shale and siliceous rock, and siliceous rock with thin layers of carbonaceous shale;The Yanxi Formation is generally characterized by a thick southeast and thin northwest distribution in the study area;Through field exploration, lithofacies combination analysis, thin section identification and elements testing analysis, four sedimentary facies types can be identified: deep water basins, deep water continental shelves, shallow water continental shelves, and turbidite fans;The Yanxi Formation is characterized by significant trichotomy vertically. The lower member is interbedded with siliceous rock and carbonaceous shale of unequal thickness. The middle black carbonaceous shale is the most important Source rock of the Yanxi Formation. The upper member is siliceous rock with thin carbonaceous shale. The three members are relatively stable in regional distribution thickness and have good contrast;The tectonic evolution history of the ancient land of South China show that in the early Paleozoic the prototype basin type of the Middle Ordovician in the central Hunan region is a rift type intracontinental sea basin; The analysis of major and rare earth elements indicates that the siliceous rocks in the central Hunan region are mostly biogenic, and the study area was in a passive continental margin environment during the Middle Ordovician. The analysis of trace elements indicate that the central Hunan region was in an anaerobic water retention environment during the Middle Ordovician;Large scale marine invasion occurred in the central Hunan region during the Early Middle Ordovician, the distribution area of deep-water basin facies reached its maximum, while the sedimentary range of carbonaceous shale and siliceous rock reached its maximum simultaneously; During late Ordovician sea level rapidly decreased, and most of the study area was dominated by deep-water continental shelf sedimentation; The prototype basin pattern of Hunan region during the Middle Ordovician is as follows: the northwestern Hunan region is the carbonate platform of the Upper Yangtze region, the central Hunan region is the Jiangnan slope zone (deep water continental shelf), the central southern Hunan is a deep water basin supporting black carbon shale sedimentation, and the southeastern Hunan is the turbidite fan facies area on the northern margin of the Huaxia ancient land.
Abstract:
[Objective] Different reservoir diagenesis has a significant effect on reservoir heterogeneity, and limits the ability to predict the presence of oil and gas in tight sandstone reservoirs. [Methods] In the study area, the constraints of lithofacies combinations causing differences in diagenesis in Chang 8 tight sandstone in the Zhijing-Ansai area were clarified by observation and identification from thin sections, cathodoluminescence (CL), scanning electron microscopy (SEM), C and O isotope analysis, and laser ablation. [Results and discussion] The reservoir sandstone comprises three lithofacies combinations: type I is siltstone or fine sandstone interbedded with argillaceous rock; type II is fine-to-medium sandstone; and type III is fine sandstone/siltstone/argillaceous rock. The fine particles of the siltstone facies have high mica content, and the rock is strongly compacted. The reservoir contains a large amount of near-argillaceous calcite cement that was developed early. The mudstone is rich in pore water with high concentrations of calcium ions and bicarbonate ions. When the mudstone overlying and underlying the reservoir sandstone is deeply buried, overcompaction causes the muddy and clayey sediments to discharge mineralized water into the sandstone, affecting its diagenesis. The supersaturated calcium carbonate thus forms a dense carbonate cement at the sandstone-mudstone interface. The calcite cement developed in the type II sandstone facies combination was formed later: this is mainly due to the later evolution of organic matter influenced by the decarboxylation of organic matter in the source rocks. The conversion of clay minerals in source rocks occurs when the pore water containing calcium ions undergoes lateral advection along highly permeable pathways, entering the reservoir and providing a material source for the development of calcite in the thick sandstone. Quartz cement is usually developed within the type I lithofacies and is mainly provided by the dissolution of feldspars and the transformation of clay minerals which are the source of silica. The dissolution process usually develops in regions of the rock with relatively coarse particle size and low plastic particle content, thus providing good fluid migration channels. [Conclusions] The differences and origin of diagenesis in the study area are clarified, providing an important basis for further research on the origin of reservoir heterogeneity.
[Objective] Different reservoir diagenesis has a significant effect on reservoir heterogeneity, and limits the ability to predict the presence of oil and gas in tight sandstone reservoirs. [Methods] In the study area, the constraints of lithofacies combinations causing differences in diagenesis in Chang 8 tight sandstone in the Zhijing-Ansai area were clarified by observation and identification from thin sections, cathodoluminescence (CL), scanning electron microscopy (SEM), C and O isotope analysis, and laser ablation. [Results and discussion] The reservoir sandstone comprises three lithofacies combinations: type I is siltstone or fine sandstone interbedded with argillaceous rock; type II is fine-to-medium sandstone; and type III is fine sandstone/siltstone/argillaceous rock. The fine particles of the siltstone facies have high mica content, and the rock is strongly compacted. The reservoir contains a large amount of near-argillaceous calcite cement that was developed early. The mudstone is rich in pore water with high concentrations of calcium ions and bicarbonate ions. When the mudstone overlying and underlying the reservoir sandstone is deeply buried, overcompaction causes the muddy and clayey sediments to discharge mineralized water into the sandstone, affecting its diagenesis. The supersaturated calcium carbonate thus forms a dense carbonate cement at the sandstone-mudstone interface. The calcite cement developed in the type II sandstone facies combination was formed later: this is mainly due to the later evolution of organic matter influenced by the decarboxylation of organic matter in the source rocks. The conversion of clay minerals in source rocks occurs when the pore water containing calcium ions undergoes lateral advection along highly permeable pathways, entering the reservoir and providing a material source for the development of calcite in the thick sandstone. Quartz cement is usually developed within the type I lithofacies and is mainly provided by the dissolution of feldspars and the transformation of clay minerals which are the source of silica. The dissolution process usually develops in regions of the rock with relatively coarse particle size and low plastic particle content, thus providing good fluid migration channels. [Conclusions] The differences and origin of diagenesis in the study area are clarified, providing an important basis for further research on the origin of reservoir heterogeneity.
Genesis of Sinian-Cambrian Unconformity in the Northwestern Tarim Basin: Evidence from Sedimentology
Abstract:
Sinian-Cambrian parallel unconformity is widely developed in Aksu area of the NW Tarim Basin, and there is still controversy over its formation mechanism at present. On the basis of sedimentological research, the genesis of the above unconformity is studied through detailed field observation and microscopic identification, aiming at the sedimentary facies and karst phenomenon at the top of the Upper Sinian Qigebrak Formation. The results show that platform margin and slope facies are developed at the top of Qigebrak Formation in Aksu area. Platform margin includes grain beach and microbial reef, consisting of thick- to massive bedded dolo-oolite, medium- to thick- bedded intraclast dolopackstone and massive dolostromatolite reef. Lithofacies of slope include: dolobreccia, thin- to medium- bedded turbidite and thin- bedded dolowackestone. In the late Sinian, the sedimentary water bodies in the eastern Aksu area were deeper, and the degree of karst development was significantly stronger than that in the western areas, which was dominated by horizontal phreatic zone vetically. In addition, in the early Cambrian, the Yuertusi Formation maintained the same sedimentary trend as the top of the Qigebrak Formation, and both showed stratigraphic sequences of deepening water bodies from west to east. This unconformity was formed by tilting movement induced by tectonic activity at the platform margin in the late Sinian, which resulted in the uplift of the western part of Aksu area. Meanwhile, this movement led to atmospheric freshwater hydrological system flowing eastward, resulting in strong karstification in the eastern platform margin zone. This study could provide guidance for oil and gas explorations of the Sinian dolomite reservoirs in the region.
Sinian-Cambrian parallel unconformity is widely developed in Aksu area of the NW Tarim Basin, and there is still controversy over its formation mechanism at present. On the basis of sedimentological research, the genesis of the above unconformity is studied through detailed field observation and microscopic identification, aiming at the sedimentary facies and karst phenomenon at the top of the Upper Sinian Qigebrak Formation. The results show that platform margin and slope facies are developed at the top of Qigebrak Formation in Aksu area. Platform margin includes grain beach and microbial reef, consisting of thick- to massive bedded dolo-oolite, medium- to thick- bedded intraclast dolopackstone and massive dolostromatolite reef. Lithofacies of slope include: dolobreccia, thin- to medium- bedded turbidite and thin- bedded dolowackestone. In the late Sinian, the sedimentary water bodies in the eastern Aksu area were deeper, and the degree of karst development was significantly stronger than that in the western areas, which was dominated by horizontal phreatic zone vetically. In addition, in the early Cambrian, the Yuertusi Formation maintained the same sedimentary trend as the top of the Qigebrak Formation, and both showed stratigraphic sequences of deepening water bodies from west to east. This unconformity was formed by tilting movement induced by tectonic activity at the platform margin in the late Sinian, which resulted in the uplift of the western part of Aksu area. Meanwhile, this movement led to atmospheric freshwater hydrological system flowing eastward, resulting in strong karstification in the eastern platform margin zone. This study could provide guidance for oil and gas explorations of the Sinian dolomite reservoirs in the region.
Abstract:
[Objective] The shrinkage and demise of the Late Ordovician carbonate platform in northeastern Yunnan have been subject to various interpretations. To delve deeper into its origins, this paper intends to investigate the Late Ordovician paleoenvironmental evolution in northeastern Yunnan and analyze the influence of paleoenvironmental factors on the demise of the Late Ordovician carbonate platform in this region. [Methods] The Wanhe section in the northeast of Yunnan Province was selected as the research object. Using methods such as indoor mineralogy, petrology, and sedimentary geochemistry analysis, a systematic study was conducted on the microfacies characteristics of the Ordovician Linxiang-Silurian Longmaxi formations in northeastern Yunnan Province. During this period, an analysis of paleoenvironmental changes was conducted to explore the paleoenvironmental impacts during the contraction and extinction of carbonate platforms in the Late Ordovician in northeastern Yunnan Province. [Results] Through the analysis of lithological characteristics, microscopic mineral composition, and paleontological features within the Linxiang-Longmaxi formations at the Wanhe section, six sedimentary microfacies were identified: nodular limestone, laminated microcrystalline limestone, microcrystalline fossiliferous limestone, laminated calcareous mudstone, sandy mudstone and silty sandstone, and calcareous shale and calcareous carbonaceous mudstone. These were further divided into three sedimentary subfacies: shallow carbonate ramp, deep carbonate ramp, and restricted retention shelf. By utilizing sedimentary geochemical methods, we analyzed the paleoceanographic geochemical characteristics and established a geochemical profile. This analysis revealed frequent and significant changes in the paleoceanographic indicators, including paleoproductivity, paleoredox conditions, terrestrial detrital input, and paleosea level fluctuations, between the Ordovician Daduhe and Silurian Longmaxi formations. [Conclusions] Through the analysis of paleoceanographic changes around the Late Ordovician carbonate platform in Northeastern Yunnan Province, the extinction of the Late Ordovician carbonate platform in this region can be attributed to oceanic hypoxia. This hypoxia led to the decline of photosynthetic organisms and subsequently inhibited carbonate production, resulting in the contraction and eventual extinction of the carbonate platform. Additionally, sea-level changes, climate fluctuations, increased input of terrigenous debris, and the Late Ordovician mass extinction event played crucial roles in the process of the carbonate platform disappearance.
[Objective] The shrinkage and demise of the Late Ordovician carbonate platform in northeastern Yunnan have been subject to various interpretations. To delve deeper into its origins, this paper intends to investigate the Late Ordovician paleoenvironmental evolution in northeastern Yunnan and analyze the influence of paleoenvironmental factors on the demise of the Late Ordovician carbonate platform in this region. [Methods] The Wanhe section in the northeast of Yunnan Province was selected as the research object. Using methods such as indoor mineralogy, petrology, and sedimentary geochemistry analysis, a systematic study was conducted on the microfacies characteristics of the Ordovician Linxiang-Silurian Longmaxi formations in northeastern Yunnan Province. During this period, an analysis of paleoenvironmental changes was conducted to explore the paleoenvironmental impacts during the contraction and extinction of carbonate platforms in the Late Ordovician in northeastern Yunnan Province. [Results] Through the analysis of lithological characteristics, microscopic mineral composition, and paleontological features within the Linxiang-Longmaxi formations at the Wanhe section, six sedimentary microfacies were identified: nodular limestone, laminated microcrystalline limestone, microcrystalline fossiliferous limestone, laminated calcareous mudstone, sandy mudstone and silty sandstone, and calcareous shale and calcareous carbonaceous mudstone. These were further divided into three sedimentary subfacies: shallow carbonate ramp, deep carbonate ramp, and restricted retention shelf. By utilizing sedimentary geochemical methods, we analyzed the paleoceanographic geochemical characteristics and established a geochemical profile. This analysis revealed frequent and significant changes in the paleoceanographic indicators, including paleoproductivity, paleoredox conditions, terrestrial detrital input, and paleosea level fluctuations, between the Ordovician Daduhe and Silurian Longmaxi formations. [Conclusions] Through the analysis of paleoceanographic changes around the Late Ordovician carbonate platform in Northeastern Yunnan Province, the extinction of the Late Ordovician carbonate platform in this region can be attributed to oceanic hypoxia. This hypoxia led to the decline of photosynthetic organisms and subsequently inhibited carbonate production, resulting in the contraction and eventual extinction of the carbonate platform. Additionally, sea-level changes, climate fluctuations, increased input of terrigenous debris, and the Late Ordovician mass extinction event played crucial roles in the process of the carbonate platform disappearance.
Abstract:
The proposal of carbonate factory and its classification study are of great significance for promoting the development of carbonate sedimentology. However, the current classification scheme is not sufficient to cover all carbonate sediments, so it is urgent to clarify different carbonate factories at the level of mechanism and process. The carbon utilized by traditional carbonate factories mainly comes from the atmospheric and oceanic inorganic carbon pools, while some special marine and continental carbonate factories mainly fix carbons from exogenous carbon pools, so the two are fundamentally different. The latter is often of great significance for tracing deep water environment and climate events. Taking hydrocarbon seep carbonate as an example, this paper puts forward the concept of “Cold-seep Carbonate Factory” formally. In addition, its sedimentary characteristics, biological composition and biogeochemical processes have been summarized, and its geological significance has been addressed with an example. The type of carbonate factory based on exogenous carbon pool deserves much attention.
The proposal of carbonate factory and its classification study are of great significance for promoting the development of carbonate sedimentology. However, the current classification scheme is not sufficient to cover all carbonate sediments, so it is urgent to clarify different carbonate factories at the level of mechanism and process. The carbon utilized by traditional carbonate factories mainly comes from the atmospheric and oceanic inorganic carbon pools, while some special marine and continental carbonate factories mainly fix carbons from exogenous carbon pools, so the two are fundamentally different. The latter is often of great significance for tracing deep water environment and climate events. Taking hydrocarbon seep carbonate as an example, this paper puts forward the concept of “Cold-seep Carbonate Factory” formally. In addition, its sedimentary characteristics, biological composition and biogeochemical processes have been summarized, and its geological significance has been addressed with an example. The type of carbonate factory based on exogenous carbon pool deserves much attention.
Abstract:
[Objective] The dissolution of quartz particles in tight sandstone has attracted more and more attention. There is a clear correlation between the dissolution boundary characteristics of quartz particles and the dissolution mechanism, but the coupling relationship has not been systematically summarized to determine the types and characteristics of quartz dissolution. In order to clarify the relationship between the dissolution characteristics of quartz particle boundary and the dissolution mechanism. [Method] Based on the methods of thin section identification, cathodoluminescence, and scanning electron microscopy, combined with the test and analysis data, the fine observation and statistical analysis of the quartz boundary dissolution characteristics of the second member of Xujiahe Formation in western Sichuan are carried out. By establishing different types of quartz boundary dissolution, the dissolution process of quartz is determined, and the dissolution mechanism is analyzed. It can provide new ideas and references for the dissolution analysis of sandstone skeleton particles and the genesis of pores in tight sandstone. [Result] The boundary dissolution characteristics of quartz particles are divided into two categories: 1 smooth boundary-dissolved pore-increasing type, which shows that the boundary of quartz particles is smooth and clear, and pores are formed after dissolution; the fuzzy boundary-dissolution metasomatism type is characterized by the fuzzy and rough boundary of quartz particles, which is the result of dissolution and metasomatism of carbonate minerals and clay minerals. According to the different metasomatism minerals, it is further divided into boundary fuzzy-dissolution-carbonate metasomatism type and boundary fuzzy-dissolution-clay mineral metasomatism type. The smooth boundary-dissolution pore-increasing type is the result of organic acid dissolution under acidic conditions. The fuzzy boundary-dissolution-carbonate metasomatism type is the result of metasomatism between carbonate ions and quartz particles due to the difference in ion concentration under alkaline conditions. The boundary fuzzy-dissolution-clay mineral metasomatism type is that under alkaline conditions, clay minerals release alkali metal ions, produce a “salt effect”, accelerate the dissolution rate of quartz particles, and metasomatized quartz particles through clay film. The evolution of quartz particle dissolution boundary tends to change from smooth boundary-dissolution pore-increasing type to fuzzy boundary-dissolution metasomatic type, which not only reveals the characteristics of smooth boundary-dissolution pore-increasing type but also reflects that the evolution of quartz particle boundary is in the direction of favorable reservoir. Among the quartz particles corroded in the second member of the Xujiahe Formation in the study area, the corrosion loss part accounts for 4.33 % ~ 8.67 % of the quartz particle area, with an average of 6.37 %. The proportion of dissolved quartz particles to all quartz particles reached more than 55.33 %, with a maximum of about 72 % and an average of 63.02 %. The content of quartz in the thin section is about 45 % -96 %, with an average of 75.3 %. Therefore, the statistical results of quartz dissolution surface porosity are between 2.05 % and 4.09 %, with an average of 3.19 %. [Conclusion] The dissolution of quartz particles is carried out in a favorable direction for the reservoir and can provide a certain amount of secondary pores, increase the oil and gas reservoir space, and effectively improve the pore structure of tight sandstone.
[Objective] The dissolution of quartz particles in tight sandstone has attracted more and more attention. There is a clear correlation between the dissolution boundary characteristics of quartz particles and the dissolution mechanism, but the coupling relationship has not been systematically summarized to determine the types and characteristics of quartz dissolution. In order to clarify the relationship between the dissolution characteristics of quartz particle boundary and the dissolution mechanism. [Method] Based on the methods of thin section identification, cathodoluminescence, and scanning electron microscopy, combined with the test and analysis data, the fine observation and statistical analysis of the quartz boundary dissolution characteristics of the second member of Xujiahe Formation in western Sichuan are carried out. By establishing different types of quartz boundary dissolution, the dissolution process of quartz is determined, and the dissolution mechanism is analyzed. It can provide new ideas and references for the dissolution analysis of sandstone skeleton particles and the genesis of pores in tight sandstone. [Result] The boundary dissolution characteristics of quartz particles are divided into two categories: 1 smooth boundary-dissolved pore-increasing type, which shows that the boundary of quartz particles is smooth and clear, and pores are formed after dissolution; the fuzzy boundary-dissolution metasomatism type is characterized by the fuzzy and rough boundary of quartz particles, which is the result of dissolution and metasomatism of carbonate minerals and clay minerals. According to the different metasomatism minerals, it is further divided into boundary fuzzy-dissolution-carbonate metasomatism type and boundary fuzzy-dissolution-clay mineral metasomatism type. The smooth boundary-dissolution pore-increasing type is the result of organic acid dissolution under acidic conditions. The fuzzy boundary-dissolution-carbonate metasomatism type is the result of metasomatism between carbonate ions and quartz particles due to the difference in ion concentration under alkaline conditions. The boundary fuzzy-dissolution-clay mineral metasomatism type is that under alkaline conditions, clay minerals release alkali metal ions, produce a “salt effect”, accelerate the dissolution rate of quartz particles, and metasomatized quartz particles through clay film. The evolution of quartz particle dissolution boundary tends to change from smooth boundary-dissolution pore-increasing type to fuzzy boundary-dissolution metasomatic type, which not only reveals the characteristics of smooth boundary-dissolution pore-increasing type but also reflects that the evolution of quartz particle boundary is in the direction of favorable reservoir. Among the quartz particles corroded in the second member of the Xujiahe Formation in the study area, the corrosion loss part accounts for 4.33 % ~ 8.67 % of the quartz particle area, with an average of 6.37 %. The proportion of dissolved quartz particles to all quartz particles reached more than 55.33 %, with a maximum of about 72 % and an average of 63.02 %. The content of quartz in the thin section is about 45 % -96 %, with an average of 75.3 %. Therefore, the statistical results of quartz dissolution surface porosity are between 2.05 % and 4.09 %, with an average of 3.19 %. [Conclusion] The dissolution of quartz particles is carried out in a favorable direction for the reservoir and can provide a certain amount of secondary pores, increase the oil and gas reservoir space, and effectively improve the pore structure of tight sandstone.
Abstract:
[Objective] The Upper Carboniferous Benxi Formation in the Ordos Basin is the first set of sea-land interaction coal-bearing strata after the Caledonian movement, and has great exploration potential, but the coupling relationship between the basin and mountains under the influence of the north-south source system and multi-source composite in the eastern part of the basin is still controversial due to the tectonic activity of the periphery orogenic belt. [Method] In order to further trace the source system and restore the paleogeographical pattern, the sedimentary source filling process of the Benxi Formation in the eastern Ordos Basin was discussed by analyzing the characteristics of the rock chip components, rare earth elements and the U-Pb age characteristics of clastic zircon, combined with the tectonic background and lithological characteristics of the host rock. [Results] The sediments in the northern part of the study area were mainly derived from the Inner Mongolia uplift (Seltenshan, Wula-Daqingshan, Jining area) and the Central Asian orogenic belt in the active continental margin tectonic background. The southern part is mainly supplied by clastic material from the North Qinling, and the tectonic background of the source area is relatively complex, which is mainly a mixture of passive continental margin and continental island arc. In the North Qinling, the tectonic environment of stretching and stretching to collision and extrusion under the background of trench-arc-basin is formed. The parent rocks in the northern and southern source areas are the mixture of granitic rock and alkaline basalt, and the northern and southern provenance systems converge in the Lian45-Chengjiagou area. [Conclusion] As a whole, the Central Asian orogenic belt of Benxi Age has a high degree of uplift and denudation on both sides of the study area, which reflects the characteristics of irregular oblique collision in the northern part of the North China Plate. The northern margin of the North China Plate has a high uplift, which provides the main provenance for the northern part of the study area, and is dominated by the development of a large tide-delta complex system. Under the influence of strong source supply, the sand body extends farther in the northeast of the study area. Compared with the northern provenance, the overall provenance supply in the south was weaker, with the North Qinling Mountain as the main source area, and the southern margin of the North China Plate was uplifted in low amplitude at this time, and the sedimentary system of the barrier coast was mainly developed. In the area where the north and south sources meet, more detrital materials from the Northern Qinling Mountains are accepted, and a large number of tidal sand dams are deposited due to the transformation of eastward and north-east currents.
[Objective] The Upper Carboniferous Benxi Formation in the Ordos Basin is the first set of sea-land interaction coal-bearing strata after the Caledonian movement, and has great exploration potential, but the coupling relationship between the basin and mountains under the influence of the north-south source system and multi-source composite in the eastern part of the basin is still controversial due to the tectonic activity of the periphery orogenic belt. [Method] In order to further trace the source system and restore the paleogeographical pattern, the sedimentary source filling process of the Benxi Formation in the eastern Ordos Basin was discussed by analyzing the characteristics of the rock chip components, rare earth elements and the U-Pb age characteristics of clastic zircon, combined with the tectonic background and lithological characteristics of the host rock. [Results] The sediments in the northern part of the study area were mainly derived from the Inner Mongolia uplift (Seltenshan, Wula-Daqingshan, Jining area) and the Central Asian orogenic belt in the active continental margin tectonic background. The southern part is mainly supplied by clastic material from the North Qinling, and the tectonic background of the source area is relatively complex, which is mainly a mixture of passive continental margin and continental island arc. In the North Qinling, the tectonic environment of stretching and stretching to collision and extrusion under the background of trench-arc-basin is formed. The parent rocks in the northern and southern source areas are the mixture of granitic rock and alkaline basalt, and the northern and southern provenance systems converge in the Lian45-Chengjiagou area. [Conclusion] As a whole, the Central Asian orogenic belt of Benxi Age has a high degree of uplift and denudation on both sides of the study area, which reflects the characteristics of irregular oblique collision in the northern part of the North China Plate. The northern margin of the North China Plate has a high uplift, which provides the main provenance for the northern part of the study area, and is dominated by the development of a large tide-delta complex system. Under the influence of strong source supply, the sand body extends farther in the northeast of the study area. Compared with the northern provenance, the overall provenance supply in the south was weaker, with the North Qinling Mountain as the main source area, and the southern margin of the North China Plate was uplifted in low amplitude at this time, and the sedimentary system of the barrier coast was mainly developed. In the area where the north and south sources meet, more detrital materials from the Northern Qinling Mountains are accepted, and a large number of tidal sand dams are deposited due to the transformation of eastward and north-east currents.
Abstract:
The Permian Upper Urho Formation in the eastern slope area of Fukang Sag is an important oil and gas exploration formation in the Junggar Basin. However, the deep conglomerate reservoir is tight, and the unclear reservoir characteristics and genesis restrict the process of oil and gas exploration and development. Through core observation, casting thin section, physical property analysis, scanning electron microscope, high-pressure mercury injection curve and other data, the petrology characteristics, physical property characteristics, pore characteristics and reservoir genetic mechanism of the tight conglomerate reservoir of the Upper Urho Formation in the slope area of Fukang Sag were studied. The results indicate that: ① the reservoir is an ultra-low porosity and ultra-low permeability lithic sandstone reservoir; ② The pore types are mainly micropores and fractures, and the pore structure is mainly small pores and micro throats, with poor connectivity; ③ Sedimentary microfacies are the basis for the differential development of reservoir physical properties. Conglomerates deposited in clastic flow channels, and fine sandstones deposited in far sand bars have high shale content and poor physical properties, while underwater distributary channels have relatively good physical properties of conglomeratic sandstones; Diagenetic compaction and clay illitization are the main factors causing the tightness and low permeability of the reservoir; Fractures and high pressure are the main reasons for higher initial oil and gas production; ④ The conglomeratic sandstone in the underwater distributary channel at the top of P3w1 and bottom of P3w2 is an important layer for stable oil production, and the Fubei area, far from the influence of provenance of the Xiquan uplift, is a favorable area for further oil and gas exploration.
The Permian Upper Urho Formation in the eastern slope area of Fukang Sag is an important oil and gas exploration formation in the Junggar Basin. However, the deep conglomerate reservoir is tight, and the unclear reservoir characteristics and genesis restrict the process of oil and gas exploration and development. Through core observation, casting thin section, physical property analysis, scanning electron microscope, high-pressure mercury injection curve and other data, the petrology characteristics, physical property characteristics, pore characteristics and reservoir genetic mechanism of the tight conglomerate reservoir of the Upper Urho Formation in the slope area of Fukang Sag were studied. The results indicate that: ① the reservoir is an ultra-low porosity and ultra-low permeability lithic sandstone reservoir; ② The pore types are mainly micropores and fractures, and the pore structure is mainly small pores and micro throats, with poor connectivity; ③ Sedimentary microfacies are the basis for the differential development of reservoir physical properties. Conglomerates deposited in clastic flow channels, and fine sandstones deposited in far sand bars have high shale content and poor physical properties, while underwater distributary channels have relatively good physical properties of conglomeratic sandstones; Diagenetic compaction and clay illitization are the main factors causing the tightness and low permeability of the reservoir; Fractures and high pressure are the main reasons for higher initial oil and gas production; ④ The conglomeratic sandstone in the underwater distributary channel at the top of P3w1 and bottom of P3w2 is an important layer for stable oil production, and the Fubei area, far from the influence of provenance of the Xiquan uplift, is a favorable area for further oil and gas exploration.
Abstract:
There are several provenance systems in the southern Ordos Region, such as Qinling and Qilian orogenic belt, and the quantitative characterization of the elements of each source-to-sink system is relatively weak, which restricts the study of the differences of different source-to-sink systems and the structure-sedimentary filling process constrained by them. Taking the Shanxi Formation of Lower Permian as the object of study, the differences of unit elements such as tectonic setting, paleo-slope and depositional system of each source-to-sink system are elucidated by means of sedimentology, geochemistry and detrital zircon dating, etc., and the structure-sedimentary model of the Shanxi Formation under the combined influence of multiple source-to-sink systems in the southern basin is reconstructed. The results show that: (1) There are three major source-to-sink systems in the southern Ordos area: North Qilian, West North Qinling and East North Qinling during the sedimentary period of Shanxi Formation. The North Qilian source-to-sink system is relatively rich in light REE, the mean δEu is 0.60, and the mean paleo-slope along the source direction is 0.045°. Light REE are obviously enriched in the source-to-sink system in the western part of the North Qinling, with an average δEu of 0.75 and an average paleo-slope of 0.04°. The source-to-sink system in the eastern part of the North Qinling is rich in light REE, with an average δEu value of 1.05 and an average paleo-slope of 0.048°; (2) The source-to-sink systems are derived from the Central Asian orogenic belt, the North China Craton basement, the east and west of the North Qinling and the North Qilian orogenic belt, but the North China Craton basement is the main source; (3) Active continental margin dominates the tectonic background of provenance regions in all source-to-sink systems, which followed by passive continental margin. Due to Mianlue Ocean subduction, the southern part of Ordos is an active continental margin tectonic setting with continuous plate convergence. The source area of the source-to-sink system in the eastern part of the North Qinling has the highest uplift degree, and the late Shanxi is relatively sand-rich, dominated by transitional deposits from the delta plain to the front, with the smallest extension scale. The source region of the source-to-sink system in the west part of the North Qinling has the lowest uplift degree, rich sand in the early stage and poor sandy sediments in the late stage. The source region of the North Qilian source-to-sink system has a moderate uplift degree, and is dominated by the delta plain transiting deposits to the front margin.
There are several provenance systems in the southern Ordos Region, such as Qinling and Qilian orogenic belt, and the quantitative characterization of the elements of each source-to-sink system is relatively weak, which restricts the study of the differences of different source-to-sink systems and the structure-sedimentary filling process constrained by them. Taking the Shanxi Formation of Lower Permian as the object of study, the differences of unit elements such as tectonic setting, paleo-slope and depositional system of each source-to-sink system are elucidated by means of sedimentology, geochemistry and detrital zircon dating, etc., and the structure-sedimentary model of the Shanxi Formation under the combined influence of multiple source-to-sink systems in the southern basin is reconstructed. The results show that: (1) There are three major source-to-sink systems in the southern Ordos area: North Qilian, West North Qinling and East North Qinling during the sedimentary period of Shanxi Formation. The North Qilian source-to-sink system is relatively rich in light REE, the mean δEu is 0.60, and the mean paleo-slope along the source direction is 0.045°. Light REE are obviously enriched in the source-to-sink system in the western part of the North Qinling, with an average δEu of 0.75 and an average paleo-slope of 0.04°. The source-to-sink system in the eastern part of the North Qinling is rich in light REE, with an average δEu value of 1.05 and an average paleo-slope of 0.048°; (2) The source-to-sink systems are derived from the Central Asian orogenic belt, the North China Craton basement, the east and west of the North Qinling and the North Qilian orogenic belt, but the North China Craton basement is the main source; (3) Active continental margin dominates the tectonic background of provenance regions in all source-to-sink systems, which followed by passive continental margin. Due to Mianlue Ocean subduction, the southern part of Ordos is an active continental margin tectonic setting with continuous plate convergence. The source area of the source-to-sink system in the eastern part of the North Qinling has the highest uplift degree, and the late Shanxi is relatively sand-rich, dominated by transitional deposits from the delta plain to the front, with the smallest extension scale. The source region of the source-to-sink system in the west part of the North Qinling has the lowest uplift degree, rich sand in the early stage and poor sandy sediments in the late stage. The source region of the North Qilian source-to-sink system has a moderate uplift degree, and is dominated by the delta plain transiting deposits to the front margin.
Abstract:
[Objective] The direct relationship between sedimentary environment and seismic elastic response of organic shale is very important for reservoir quality evaluation and geological modeling of unconventional oil and gas reservoirs. [Methods] The sedimentary characteristics and lithofacies of the Longmaxi Shale in the Zigong area are characterized, and four depositional units (I-A, I-B, I-C and II) are identified, the influence of sedimentary environment on the elastic characteristics of shale of Longmaxi Formation in Zigong area, Sichuan Basin is analyzed. [Results] The results show that the sedimentary environment mainly controls the elastic characteristics of the shale of the Longmaxi Formation in two aspects: first, the difference of rock structure caused by hydrodynamic action; The elastic characteristics of shale in the deep water shelf environment are obviously different from those of argillaceous siltstone in the overlying turbidite environment, the variation of elastic characteristics is mainly controlled by the precession system and water depth.In addition, it is found that quartz and TOC content have a competitive relationship in influencing the elastic characteristics of the reservoir. Finally, the prediction ability of pre-stack AVO inversion to seismic elastic attributes is used to trace the sedimentary evolution process, which provides an insight for directly characterizing sedimentary facies of unconventional shale reservoirs by geophysical attributes. [Conclusions] The sedimentary environment affects the elastic characteristics of shale reservoirs by controlling the structure and composition of rocks. For the influence of quartz and TOC content on reservoir elasticity, the softening effect caused by organic matter is dominant at low quartz content, and the hardening effect of quartz grains is dominant at high quartz content.
[Objective] The direct relationship between sedimentary environment and seismic elastic response of organic shale is very important for reservoir quality evaluation and geological modeling of unconventional oil and gas reservoirs. [Methods] The sedimentary characteristics and lithofacies of the Longmaxi Shale in the Zigong area are characterized, and four depositional units (I-A, I-B, I-C and II) are identified, the influence of sedimentary environment on the elastic characteristics of shale of Longmaxi Formation in Zigong area, Sichuan Basin is analyzed. [Results] The results show that the sedimentary environment mainly controls the elastic characteristics of the shale of the Longmaxi Formation in two aspects: first, the difference of rock structure caused by hydrodynamic action; The elastic characteristics of shale in the deep water shelf environment are obviously different from those of argillaceous siltstone in the overlying turbidite environment, the variation of elastic characteristics is mainly controlled by the precession system and water depth.In addition, it is found that quartz and TOC content have a competitive relationship in influencing the elastic characteristics of the reservoir. Finally, the prediction ability of pre-stack AVO inversion to seismic elastic attributes is used to trace the sedimentary evolution process, which provides an insight for directly characterizing sedimentary facies of unconventional shale reservoirs by geophysical attributes. [Conclusions] The sedimentary environment affects the elastic characteristics of shale reservoirs by controlling the structure and composition of rocks. For the influence of quartz and TOC content on reservoir elasticity, the softening effect caused by organic matter is dominant at low quartz content, and the hardening effect of quartz grains is dominant at high quartz content.
Abstract:
[Objective] Among the past ice chamber climate records, the Late Paleozoic ice age, which developed mainly in the Gondwana continent, is the most similar to the current climate evolution It has become a hot target for comparative studies of Quaternary ice ages and ice chamber climate studies. The Late Paleozoic ice age, as the glacial event with the widest influence range and the richest geological record since the Phanerozoic, recorded the complete greenhouse-icehouse-greenhouse climate change process, which is of great significance for us to understand the evolution of earth climate. The Lhasa Block originated from the Gondwana continent and was located at the northeastern margin of the Gondwana continent during the Late Paleozoic period. Although a lot of studies have been conducted on the spatial and temporal evolution and controlling factors of the Late Paleozoic ice age, the sedimentary evolution history of the Lhasa Block during the Late Paleozoic ice age is still unclear and needs to be strengthened. [Methods] In view of this, this study selected the Late Paleozoic strata in the Xainza area of the Lhasa Block to carry out a 1:200 scale profile survey, which includes rock color, lithological characteristics, rock thickness, sedimentary structures, fossils and contact relationships. Lithofacies and lithofacies combinations were classified for glacial development, and sedimentary architecture analysis was applied to find the lateral and vertical changes of sedimentary facies to identify the sedimentary environment and recover the glacial sedimentary system. [Results and Discussions] The study shows that the Late Paleozoic ice age records of Lhasa block are mainly present at the Lagar Formation, with the age constraint of Late Carboniferous-Early Permian. According to the glacial deposits of Lagar Formation, twenty lithofacies and sixteen typical lithofacies associations are recognized, which are arranged into six sedimentary environments, including shallow sea shelf, baseline fan, subglacial, ice river, ice lake and outwash fan. [Conclusions] The Late Paleozoic glaciers in the central part of the Lhasa Block are located in the nearshore glaciomarine environment, and the glacial depositional system is mainly divided into marine and terrestrial phases. In addition, a number of small glacial-interglacial cyclones can be delineated in the early evolutionary stage and late evolutionary stage of the Lagar Formation based on the variation of glacial and non-glacial environments in the vertical direction. The sedimentary system analysis for the Lagar Formation sedimentary sequences indicates that the Late Paleozoic ice age in the Xainza area of the Lhasa Block experienced a transition from early marine to late terrestrial glaciations, indicating a global trend of gradual climate warming from the Late Carboniferous to the Early Permian, consistent with the global Late Paleozoic ice age evolutionary features. The Late Paleozoic ice age was the closest global ice age to the Quaternary ice age and is an important window for understanding future climate shifts such as glacial melting and global warming on Earth. Conducting research on the Late Paleozoic sedimentary record of the Lhasa Block is of great significance for exploring the spatial and temporal evolution, climate change and driving mechanisms of the global Late Paleozoic ice age.
[Objective] Among the past ice chamber climate records, the Late Paleozoic ice age, which developed mainly in the Gondwana continent, is the most similar to the current climate evolution It has become a hot target for comparative studies of Quaternary ice ages and ice chamber climate studies. The Late Paleozoic ice age, as the glacial event with the widest influence range and the richest geological record since the Phanerozoic, recorded the complete greenhouse-icehouse-greenhouse climate change process, which is of great significance for us to understand the evolution of earth climate. The Lhasa Block originated from the Gondwana continent and was located at the northeastern margin of the Gondwana continent during the Late Paleozoic period. Although a lot of studies have been conducted on the spatial and temporal evolution and controlling factors of the Late Paleozoic ice age, the sedimentary evolution history of the Lhasa Block during the Late Paleozoic ice age is still unclear and needs to be strengthened. [Methods] In view of this, this study selected the Late Paleozoic strata in the Xainza area of the Lhasa Block to carry out a 1:200 scale profile survey, which includes rock color, lithological characteristics, rock thickness, sedimentary structures, fossils and contact relationships. Lithofacies and lithofacies combinations were classified for glacial development, and sedimentary architecture analysis was applied to find the lateral and vertical changes of sedimentary facies to identify the sedimentary environment and recover the glacial sedimentary system. [Results and Discussions] The study shows that the Late Paleozoic ice age records of Lhasa block are mainly present at the Lagar Formation, with the age constraint of Late Carboniferous-Early Permian. According to the glacial deposits of Lagar Formation, twenty lithofacies and sixteen typical lithofacies associations are recognized, which are arranged into six sedimentary environments, including shallow sea shelf, baseline fan, subglacial, ice river, ice lake and outwash fan. [Conclusions] The Late Paleozoic glaciers in the central part of the Lhasa Block are located in the nearshore glaciomarine environment, and the glacial depositional system is mainly divided into marine and terrestrial phases. In addition, a number of small glacial-interglacial cyclones can be delineated in the early evolutionary stage and late evolutionary stage of the Lagar Formation based on the variation of glacial and non-glacial environments in the vertical direction. The sedimentary system analysis for the Lagar Formation sedimentary sequences indicates that the Late Paleozoic ice age in the Xainza area of the Lhasa Block experienced a transition from early marine to late terrestrial glaciations, indicating a global trend of gradual climate warming from the Late Carboniferous to the Early Permian, consistent with the global Late Paleozoic ice age evolutionary features. The Late Paleozoic ice age was the closest global ice age to the Quaternary ice age and is an important window for understanding future climate shifts such as glacial melting and global warming on Earth. Conducting research on the Late Paleozoic sedimentary record of the Lhasa Block is of great significance for exploring the spatial and temporal evolution, climate change and driving mechanisms of the global Late Paleozoic ice age.
Abstract:
In order to study the sedimentary characteristics and geological significance of the typical tempestites in the Lower Ordovician Fenxiang Formation in the chengkou area of the Upper Yangtze Basin. Via detailed field survey and microscopic section analysis, the sedimentary sequence of the tempestites and mode of the Fenxiang Formation are established, which reveal the geological significance. The results show that the tempestites sedimentary structures of Fenxiang Formation in Chengkou area mainly include bottom scour structure, storm gravel layer, grain sequence bedding and mound cross-bedding. At the same time, five kinds of tempestites sedimentary sequences were identified: Sequence I was mainly composed of bottom erosion, gravel layer (A), grain sequence (B) and parallel bedding segment (C), and mainly developed in the platform margin facies; Sequence II consists of grain sequence (B) and parallel bedding segment (C), which developed in the platform margin facies near the slope. Sequence III is composed of bottom erosion and gravel layer (A), grain sequence layer (B), parallel bedding (C) and argillaceous limestone segment (E), which are mainly deposited in the fore-platform slope facies zone. Sequence Ⅳ mainly consists of grain sequence segment (B) and argillaceous micrite segment (E), which are mainly deposited in the lower the fore-platform slope. Sequence V is composed of grain sequence (B), mound bedding segment (D) and argillaceous micrite segment (E), which mainly developed in the deepwater shelf. The development of tempestites indicates that the upper Yangtze platform was located in the low latitude area in the Early Ordovician, and the Chengkou area was dominated by platform margin and slope deposits. The bottom-up sedimentary environment evolved into platform margin → platform front slope → deep water shelf. The development of tempestites indicates that the Upper Yangtze platform was near the equator at low latitude during the Ordovician period. The sedimentary environment of Fenxiang Formation in the Chengkou area of the Upper Yangtze Platform is the platform margin zone. Combined with the regional geological background, it is considered that there are geological conditions for developing large-scale platform margin shoals in the chengkou area of the Yangtze Platform..
In order to study the sedimentary characteristics and geological significance of the typical tempestites in the Lower Ordovician Fenxiang Formation in the chengkou area of the Upper Yangtze Basin. Via detailed field survey and microscopic section analysis, the sedimentary sequence of the tempestites and mode of the Fenxiang Formation are established, which reveal the geological significance. The results show that the tempestites sedimentary structures of Fenxiang Formation in Chengkou area mainly include bottom scour structure, storm gravel layer, grain sequence bedding and mound cross-bedding. At the same time, five kinds of tempestites sedimentary sequences were identified: Sequence I was mainly composed of bottom erosion, gravel layer (A), grain sequence (B) and parallel bedding segment (C), and mainly developed in the platform margin facies; Sequence II consists of grain sequence (B) and parallel bedding segment (C), which developed in the platform margin facies near the slope. Sequence III is composed of bottom erosion and gravel layer (A), grain sequence layer (B), parallel bedding (C) and argillaceous limestone segment (E), which are mainly deposited in the fore-platform slope facies zone. Sequence Ⅳ mainly consists of grain sequence segment (B) and argillaceous micrite segment (E), which are mainly deposited in the lower the fore-platform slope. Sequence V is composed of grain sequence (B), mound bedding segment (D) and argillaceous micrite segment (E), which mainly developed in the deepwater shelf. The development of tempestites indicates that the upper Yangtze platform was located in the low latitude area in the Early Ordovician, and the Chengkou area was dominated by platform margin and slope deposits. The bottom-up sedimentary environment evolved into platform margin → platform front slope → deep water shelf. The development of tempestites indicates that the Upper Yangtze platform was near the equator at low latitude during the Ordovician period. The sedimentary environment of Fenxiang Formation in the Chengkou area of the Upper Yangtze Platform is the platform margin zone. Combined with the regional geological background, it is considered that there are geological conditions for developing large-scale platform margin shoals in the chengkou area of the Yangtze Platform..
Abstract:
The Early Triassic is a special geological historical period after the mass extinction of organisms. The genetic mechanism of oolites and giant oolites and the paleomarine environment they represent have been a controversial issue in the field of geology. Based on field and core observations, using petrological, mineralogical, and geochemical analyses, this paper explores the sedimentary characteristics, genesis, and paleoenvironmental significance of oolites and giant oolites from the Early Triassic Feixianguan Formation in the Yuanba area. The research shows that the oolites and giant oolites of the Feixianguan Formation in the Yuanba area are mainly developed in the platform margin zone of the Fei'er Member, and the types of oolites are mainly concentric oolites and single crystal oolites, indicating that they were formed in a shallow water environment with strong hydrodynamic forces and easy exposure; The type of giant oolites is dominated by concentric oolites, which are often thinly interbedded with oolitic limestone and micrite limestone, accompanied by the appearance of scouring surfaces, indicating that they are developed in areas affected by indirect storm action. Combined with geochemical analysis, it is found that the Sr content of the oolitic concentric ring layer is high, and the common needle or rod shaped crystal structure indicates that its original minerals are mainly aragonite deposits; However, the Sr content of the giant oolitic concentric layer is relatively low, and the irregular granular crystal structure is mainly developed, indicating that its original minerals are mainly calcite deposits. In addition, the oolitic concentric sphere has characteristics such as high Fe content, no significant Ce anomalies, positive Eu anomalies, LREE relative HREE enrichment, and V/(V+Ni)>0.6, indicating that it was formed in a ferritized weak oxygen reduction environment; Giant oolites have characteristics such as low Fe content, negative Ce anomalies, LREE relative HREE depletion, and V/(V+Ni)<0.6, indicating that they were formed in an oxidation-weak oxidation environment. According to comprehensive analysis, during the sedimentary period of the Early Triassic Feixianguan Formation, the seawater was mainly composed of anoxic aragonite sea, which was influenced by the strengthening of continental weathering, especially intermittent storms, which increased the input of terrestrial materials (especially Ca2+and oxidants), resulting in the decrease and oxidation of Mg/Ca in the shallow seawater, and the development of a transient calcite sea. This may be one of the important reasons for the improvement of the marine environment and the gradual recovery of organisms in the Early Triassic.
The Early Triassic is a special geological historical period after the mass extinction of organisms. The genetic mechanism of oolites and giant oolites and the paleomarine environment they represent have been a controversial issue in the field of geology. Based on field and core observations, using petrological, mineralogical, and geochemical analyses, this paper explores the sedimentary characteristics, genesis, and paleoenvironmental significance of oolites and giant oolites from the Early Triassic Feixianguan Formation in the Yuanba area. The research shows that the oolites and giant oolites of the Feixianguan Formation in the Yuanba area are mainly developed in the platform margin zone of the Fei'er Member, and the types of oolites are mainly concentric oolites and single crystal oolites, indicating that they were formed in a shallow water environment with strong hydrodynamic forces and easy exposure; The type of giant oolites is dominated by concentric oolites, which are often thinly interbedded with oolitic limestone and micrite limestone, accompanied by the appearance of scouring surfaces, indicating that they are developed in areas affected by indirect storm action. Combined with geochemical analysis, it is found that the Sr content of the oolitic concentric ring layer is high, and the common needle or rod shaped crystal structure indicates that its original minerals are mainly aragonite deposits; However, the Sr content of the giant oolitic concentric layer is relatively low, and the irregular granular crystal structure is mainly developed, indicating that its original minerals are mainly calcite deposits. In addition, the oolitic concentric sphere has characteristics such as high Fe content, no significant Ce anomalies, positive Eu anomalies, LREE relative HREE enrichment, and V/(V+Ni)>0.6, indicating that it was formed in a ferritized weak oxygen reduction environment; Giant oolites have characteristics such as low Fe content, negative Ce anomalies, LREE relative HREE depletion, and V/(V+Ni)<0.6, indicating that they were formed in an oxidation-weak oxidation environment. According to comprehensive analysis, during the sedimentary period of the Early Triassic Feixianguan Formation, the seawater was mainly composed of anoxic aragonite sea, which was influenced by the strengthening of continental weathering, especially intermittent storms, which increased the input of terrestrial materials (especially Ca2+and oxidants), resulting in the decrease and oxidation of Mg/Ca in the shallow seawater, and the development of a transient calcite sea. This may be one of the important reasons for the improvement of the marine environment and the gradual recovery of organisms in the Early Triassic.
Abstract:
[Objective] As the development of non-conventional petroleum theories, the organic matter accumulation mechanism has been becoming the hotspot. One of the most debated issues is the main controlling factors of organic matter enrichment. Previous studies were lack of the depositional process analyses, which is likely the reasons of the debate. [Methods] This study combines the sedimentological and geochemical methods to conduct detailed sequence stratigraphy, facies and elemental analyses, aiming to address the sedimentary organic matter accumulation mechanism. [Results] The Upper Permian Wuchiaping and Dalong formations in the northern Sichuan Basin and the West Hubei Basin are subdivided into First Wuchiaping Member, Second Wuchiaping Member, Third Wuchiaping Member, First Dalong Member and Second Dalong Member in an ascending order. The Upper Permian is subdivided into 5 third-order stratigraphy sequences SQ1 to SQ5. Based on the sequence stratigraphy analysis, the development history of the rifting basin in northern Sichuan Basin and the West Hubei Basin is subdivided into four stages: initial stage, rapid rifting, rifting climax, and shrinking stage. The initial stage mainly occurred during the deposition of SQ2, and consists of dark cherty limestones of slope facies. The rapid rifting stage mainly occurred during the SQ3, and consists of calcareous shales of deepwater shelf facies. The rifting climax stage mainly occurred during the SQ4, and consists of black bedded cherts of basin facies. The shrinking stage mainly occurred during the SQ5, and consists of medium-thin-bedded limestones. The sweet-spot interval develops mainly from the highstand system tract (HST) of SQ3 to the transgressive system tract (TST) of SQ5, during which the nutrient elements Fe, Cu, Ni, Zn, and redox-sensitive element Mo, V, Fe/Al and S were highly enriched. This sweet-spot interval is also associated with highly active hydrothermal activity indicated by Al/(Al+Fe+Mn) ratios < 0.6 and the Al-Fe-Mn diagram. [Conclusion] The development of high primary productivity is earlier than the strong reducing conditions during the deposition of the sweet-spot interval, suggesting that the reducing conditions in the bottom water resulted from the high consuming of oxygen during the organic matter decomposition， and that the organic matter enrichment was mainly controlled by primary productivity. The nutrient material input was related to the hydrothermal activity and volcanism during the formation of rifting basins. The tectonic activity brought episodically a lot of nutrient elements to the rifting sea, enhancing the primary productivity level. Thus, the primary productivity level is the onset and basic conditions for the extraordinarily high organic matter accumulation in the sweet-spot interval, and the strong reducing conditions in bottom water was the key factor for organic matter preservation and burial.
[Objective] As the development of non-conventional petroleum theories, the organic matter accumulation mechanism has been becoming the hotspot. One of the most debated issues is the main controlling factors of organic matter enrichment. Previous studies were lack of the depositional process analyses, which is likely the reasons of the debate. [Methods] This study combines the sedimentological and geochemical methods to conduct detailed sequence stratigraphy, facies and elemental analyses, aiming to address the sedimentary organic matter accumulation mechanism. [Results] The Upper Permian Wuchiaping and Dalong formations in the northern Sichuan Basin and the West Hubei Basin are subdivided into First Wuchiaping Member, Second Wuchiaping Member, Third Wuchiaping Member, First Dalong Member and Second Dalong Member in an ascending order. The Upper Permian is subdivided into 5 third-order stratigraphy sequences SQ1 to SQ5. Based on the sequence stratigraphy analysis, the development history of the rifting basin in northern Sichuan Basin and the West Hubei Basin is subdivided into four stages: initial stage, rapid rifting, rifting climax, and shrinking stage. The initial stage mainly occurred during the deposition of SQ2, and consists of dark cherty limestones of slope facies. The rapid rifting stage mainly occurred during the SQ3, and consists of calcareous shales of deepwater shelf facies. The rifting climax stage mainly occurred during the SQ4, and consists of black bedded cherts of basin facies. The shrinking stage mainly occurred during the SQ5, and consists of medium-thin-bedded limestones. The sweet-spot interval develops mainly from the highstand system tract (HST) of SQ3 to the transgressive system tract (TST) of SQ5, during which the nutrient elements Fe, Cu, Ni, Zn, and redox-sensitive element Mo, V, Fe/Al and S were highly enriched. This sweet-spot interval is also associated with highly active hydrothermal activity indicated by Al/(Al+Fe+Mn) ratios < 0.6 and the Al-Fe-Mn diagram. [Conclusion] The development of high primary productivity is earlier than the strong reducing conditions during the deposition of the sweet-spot interval, suggesting that the reducing conditions in the bottom water resulted from the high consuming of oxygen during the organic matter decomposition， and that the organic matter enrichment was mainly controlled by primary productivity. The nutrient material input was related to the hydrothermal activity and volcanism during the formation of rifting basins. The tectonic activity brought episodically a lot of nutrient elements to the rifting sea, enhancing the primary productivity level. Thus, the primary productivity level is the onset and basic conditions for the extraordinarily high organic matter accumulation in the sweet-spot interval, and the strong reducing conditions in bottom water was the key factor for organic matter preservation and burial.
Abstract:
The Liantuo Formation is a key mid-Neoproterozoic stratigraphic unit in the Dahongshan area of Yangtze Block, South China. Its deposition time is coupled with the breakup of the Rodinia supercontinent, and it is the last sedimentary layer before the Sturtian ice Age of the first snowball Earth. However, its deposition time and provenance need to be further studied and to provide fundamental support for dissecting these major geological historical events. Here, we analyzed detrital zircons U-Pb chronology from Liantuo Formation sandstone in this area, which shows sedimentary time is ca. 800-714 Ma and major peaks of detrital zircons are ca. 2500 Ma, ca. 2000 Ma, ca. 880 Ma, and ca. 820 Ma, and the secondary peaks are ca.780 Ma. Combined with the characteristics of paleo-flow direction, gravel composition, and sedimentary facies distribution analysis, the provenance of the Liantuo Formation in the study area is from the depositional recycle of the Dagushi Group and Huashan Group in the lower strata and the Archean to Neoproterozoic magmatic rocks in the northern Yangtze Block, and it is mainly from the northeast direction of the study area. Meanwhile, the western Hubei, southeastern Hubei to northwestern Jiangxi, southwest Hubei to northern Hunan, and Dahongshan of the northern Hubei areas have similar detrital zircon age peaks of Archean, Paleoproterozoic, and Neoproterozoic. And according to the distribution characteristics of magmatic rocks in the north and southeast margin of the Yangtze Block, which suggests that the provenance of the Liantuo Formation in the northern Yangtze Block has a north-northeast supply.
The Liantuo Formation is a key mid-Neoproterozoic stratigraphic unit in the Dahongshan area of Yangtze Block, South China. Its deposition time is coupled with the breakup of the Rodinia supercontinent, and it is the last sedimentary layer before the Sturtian ice Age of the first snowball Earth. However, its deposition time and provenance need to be further studied and to provide fundamental support for dissecting these major geological historical events. Here, we analyzed detrital zircons U-Pb chronology from Liantuo Formation sandstone in this area, which shows sedimentary time is ca. 800-714 Ma and major peaks of detrital zircons are ca. 2500 Ma, ca. 2000 Ma, ca. 880 Ma, and ca. 820 Ma, and the secondary peaks are ca.780 Ma. Combined with the characteristics of paleo-flow direction, gravel composition, and sedimentary facies distribution analysis, the provenance of the Liantuo Formation in the study area is from the depositional recycle of the Dagushi Group and Huashan Group in the lower strata and the Archean to Neoproterozoic magmatic rocks in the northern Yangtze Block, and it is mainly from the northeast direction of the study area. Meanwhile, the western Hubei, southeastern Hubei to northwestern Jiangxi, southwest Hubei to northern Hunan, and Dahongshan of the northern Hubei areas have similar detrital zircon age peaks of Archean, Paleoproterozoic, and Neoproterozoic. And according to the distribution characteristics of magmatic rocks in the north and southeast margin of the Yangtze Block, which suggests that the provenance of the Liantuo Formation in the northern Yangtze Block has a north-northeast supply.
Abstract:
[Objective] During the Early Jurassic, multiple significant perturbations of the carbon-cycle occurred that coincided with abrupt and extreme changes in climate and environment. However, existing research has primarily focused on the western Tethys and northern Europe, with limited attention given to carbon cycle disturbances in the eastern Tethys and the quantitative assessment of their impact on biota. [Methods] This study examined the biological (larger benthic foraminifera), sedimentological, and carbon-isotope (δ13C) stratigraphy from the southern Tibetan Kioto Platform formed in the southeastern Tethys during the Late Pliensbachian–Earliest Toarcian interval to investigate the influence of carbon-cycle perturbations and corresponding climatic and environmental variations on the evolution of carbonate-platform biota in the region. In addition, this study quantitatively analyzed the content of carbonate skeletal grains, the number and species changes of lager benthic foraminifera, and the distribution of Lithiotis Fauna to explore the impact of these events on biota. [Results] Three foraminiferal zones were recognized: Pliensbachian Bosniella oenensis–Cyclorbitosella tibetica (?) and Streptocyclammina liasica, as well as the Earliest Toarcian Siphovalvulina sp. A. According to the sedimentary structure and the quantitative analysis of grains composition and content, this study identified nine carbonate microfacies (MF) from the Pupuga Formation in the Dongqiu section. These microfacies included mudstone (MF1), dolomitic peloidal wackestone (MF2), bioclastic peloidal packstone (MF3), bioclastic wackestone/packstone (MF4), intraclastic packstone–grainstone (MF5), lump grainstone (MF6), Lithiotis rudstone (MF7), bioclastic grainstone (MF8), and oolitic grainstone (MF9). The sedimentary microfacies and microfacies assemblages reveal that the Pupuga Formation in the Dongqiu section represented the shallow–water carbonate platform deposit, characterized by minimal influence from terrigenous input. Furthermore, temporal variation in microfacies demonstrated that the Dongqiu section provided a comprehensive record of a sedimentary succession, delineating a gradual transition from the open platform and platform margin to the inner platform, succeeded by a swift reversion towards the open platform and platform margin. Based on biostratigraphy, two carbon isotope excursion events in the Late Pliensbachian were identified: the margaritatus zone event (ME) characterized by positive carbon isotope excursion and the margaritatus–spinatum zone boundary event (MSBE) with negative carbon isotope excursion as a feature. [Conclusions] During the ME period, there was an increase in skeletal grain content, maintenance of high species diversity and abundance of larger benthic foraminifera, and the occurrence and spread of Lithiotis Fauna. This may indicate that the persistent burial of organic matter consumed atmospheric pCO2 generated during the Late Sinemurian to Early Pliensbachian interval, creating more suitable marine environmental conditions for biotic survival. In contrast, during the MSBE period, there was a decrease in the abundance and size of Lithiotis Fauna and skeletal grain content, along with the extinction of several index larger benthic foraminifera. This evidence indicates a possible relationship between biotic crises and sea-level fall. Although there were changes in the size and frequency of occurrence of the Lithiotis Fauna, it is undeniable that they flourished throughout the Tethyan shallow carbonate platform during the Pliensbachian interval, and the conditions for skeletal production of carbonates were not completely destroyed until the Toarcian oceanic anoxic event.
[Objective] During the Early Jurassic, multiple significant perturbations of the carbon-cycle occurred that coincided with abrupt and extreme changes in climate and environment. However, existing research has primarily focused on the western Tethys and northern Europe, with limited attention given to carbon cycle disturbances in the eastern Tethys and the quantitative assessment of their impact on biota. [Methods] This study examined the biological (larger benthic foraminifera), sedimentological, and carbon-isotope (δ13C) stratigraphy from the southern Tibetan Kioto Platform formed in the southeastern Tethys during the Late Pliensbachian–Earliest Toarcian interval to investigate the influence of carbon-cycle perturbations and corresponding climatic and environmental variations on the evolution of carbonate-platform biota in the region. In addition, this study quantitatively analyzed the content of carbonate skeletal grains, the number and species changes of lager benthic foraminifera, and the distribution of Lithiotis Fauna to explore the impact of these events on biota. [Results] Three foraminiferal zones were recognized: Pliensbachian Bosniella oenensis–Cyclorbitosella tibetica (?) and Streptocyclammina liasica, as well as the Earliest Toarcian Siphovalvulina sp. A. According to the sedimentary structure and the quantitative analysis of grains composition and content, this study identified nine carbonate microfacies (MF) from the Pupuga Formation in the Dongqiu section. These microfacies included mudstone (MF1), dolomitic peloidal wackestone (MF2), bioclastic peloidal packstone (MF3), bioclastic wackestone/packstone (MF4), intraclastic packstone–grainstone (MF5), lump grainstone (MF6), Lithiotis rudstone (MF7), bioclastic grainstone (MF8), and oolitic grainstone (MF9). The sedimentary microfacies and microfacies assemblages reveal that the Pupuga Formation in the Dongqiu section represented the shallow–water carbonate platform deposit, characterized by minimal influence from terrigenous input. Furthermore, temporal variation in microfacies demonstrated that the Dongqiu section provided a comprehensive record of a sedimentary succession, delineating a gradual transition from the open platform and platform margin to the inner platform, succeeded by a swift reversion towards the open platform and platform margin. Based on biostratigraphy, two carbon isotope excursion events in the Late Pliensbachian were identified: the margaritatus zone event (ME) characterized by positive carbon isotope excursion and the margaritatus–spinatum zone boundary event (MSBE) with negative carbon isotope excursion as a feature. [Conclusions] During the ME period, there was an increase in skeletal grain content, maintenance of high species diversity and abundance of larger benthic foraminifera, and the occurrence and spread of Lithiotis Fauna. This may indicate that the persistent burial of organic matter consumed atmospheric pCO2 generated during the Late Sinemurian to Early Pliensbachian interval, creating more suitable marine environmental conditions for biotic survival. In contrast, during the MSBE period, there was a decrease in the abundance and size of Lithiotis Fauna and skeletal grain content, along with the extinction of several index larger benthic foraminifera. This evidence indicates a possible relationship between biotic crises and sea-level fall. Although there were changes in the size and frequency of occurrence of the Lithiotis Fauna, it is undeniable that they flourished throughout the Tethyan shallow carbonate platform during the Pliensbachian interval, and the conditions for skeletal production of carbonates were not completely destroyed until the Toarcian oceanic anoxic event.
Abstract:
[Objective] To understand the sedimentary characteristics of shell limestone-shale mixed strata in the Da’anzhai Member of Sichuan Basin and its influence on favorable shale oil horizon, field profiles in Dazhou Tieshan Jinwo and Liangping Fuluzhen of the northeast Sichuan Basin were studied. [Methods] The lithology and sequence, source and reservoir quality, and favorable strata of the Da’anzhai Member were evaluated in detail by using rock thin sections, X-ray diffraction (XRD) whole rock and organic geochemical analyses. [Results] The results show that: 1) The Da’anzhai Member is a set of limestone-shale mixed strata, and different lithologies are superimposed in an orderly way to form a variety of upward shallower sequences. From shallow lake to semi-deep lake-deep lake, six kinds of upward shallower decimeter to meter lithofacies sequences are identified: ①shale-shell shale, ②shale-thin shell limestone, ③shale-medium shell limestone, ④shale-nodular micrite, ⑤thin shale-shell limestone-crystalline limestone, and ⑥shell limestone-siltstone/fine sandstone. 2) The quality of source and reservoir varies greatly among different lithologic sequences. Black shales in sequence 1–3 are developed with the best source quality. The organic carbon content of black shales in a single sequence shows a trend of higher and lower organic carbon content, and total organic carbon (TOC) gradually decreases with the increase of shell limestone. Different lithologic properties vary greatly. The porosity of shale is higher than that of limestone and siltstone, but the clay content is high, the pore size is small, and the seepage capacity is poor. Limestone easily develops joint fractures, and the density of joints decreases exponentially with the increase of limestone thickness. Therefore, Sequence 2–4 developed middle and thin layer limestone has better reservoir performance. 3) The field oil seedling shows that the shale oil reservoir of the Da’anzhai Member is characterized by the separation of source and reservoir and thin high-quality reservoir. The shale oil seepages in the field are mainly distributed near the joints of the medium–thin layer shell limestone. The favorable reservoir is primarily controlled by the sedimentary conditions, micro-fractures, and configuration of source and reservoir. [Conclusions] Sequences 2 and 3 have the best source and reservoir configuration conditions, which are favorable intervals for shale oil.
[Objective] To understand the sedimentary characteristics of shell limestone-shale mixed strata in the Da’anzhai Member of Sichuan Basin and its influence on favorable shale oil horizon, field profiles in Dazhou Tieshan Jinwo and Liangping Fuluzhen of the northeast Sichuan Basin were studied. [Methods] The lithology and sequence, source and reservoir quality, and favorable strata of the Da’anzhai Member were evaluated in detail by using rock thin sections, X-ray diffraction (XRD) whole rock and organic geochemical analyses. [Results] The results show that: 1) The Da’anzhai Member is a set of limestone-shale mixed strata, and different lithologies are superimposed in an orderly way to form a variety of upward shallower sequences. From shallow lake to semi-deep lake-deep lake, six kinds of upward shallower decimeter to meter lithofacies sequences are identified: ①shale-shell shale, ②shale-thin shell limestone, ③shale-medium shell limestone, ④shale-nodular micrite, ⑤thin shale-shell limestone-crystalline limestone, and ⑥shell limestone-siltstone/fine sandstone. 2) The quality of source and reservoir varies greatly among different lithologic sequences. Black shales in sequence 1–3 are developed with the best source quality. The organic carbon content of black shales in a single sequence shows a trend of higher and lower organic carbon content, and total organic carbon (TOC) gradually decreases with the increase of shell limestone. Different lithologic properties vary greatly. The porosity of shale is higher than that of limestone and siltstone, but the clay content is high, the pore size is small, and the seepage capacity is poor. Limestone easily develops joint fractures, and the density of joints decreases exponentially with the increase of limestone thickness. Therefore, Sequence 2–4 developed middle and thin layer limestone has better reservoir performance. 3) The field oil seedling shows that the shale oil reservoir of the Da’anzhai Member is characterized by the separation of source and reservoir and thin high-quality reservoir. The shale oil seepages in the field are mainly distributed near the joints of the medium–thin layer shell limestone. The favorable reservoir is primarily controlled by the sedimentary conditions, micro-fractures, and configuration of source and reservoir. [Conclusions] Sequences 2 and 3 have the best source and reservoir configuration conditions, which are favorable intervals for shale oil.
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[Objective] The latest 3D seismic data displays a large number of strong reflections seismic events progradation in Yanchang Formation, which is quite different from the traditional stratigraphic scheme of "flat rise flat fall and equal thickness distribution". This difference provides a new perspective for re-understanding the stratigraphic framework of Yanchang Formation. [Methods] The seismic facies, sequence stratigraphy and sedimentary system in Longdong area of Ordos Basin are systematically studied through core observation, well seismic calibration and 3D seismic interpretation combined with seismic inversion technology. [Results] The continuous seismic events of strong reflections in Yanchang Formation can indicate the deposition of condensed layers during lake flooding. The seismic facies along provenance and across provenance are quite different. The seismic profiles along the provenance can be divided into three types: sub-parallel structures with medium amplitude and medium continuity, progradational reflections with strong amplitude and strong continuity, and disordered structures with low amplitude and low continuity, among which the progradational type are most obvious. The seismic profiles across the provenance can be divided into three types: parallel structures with strong amplitude and strong continuity, mound structure with medium-strong amplitude and strong continuity, disordered structures with low amplitude and low continuity. The lake basin evolution of Yanchang Formation in Longdong area with some rapid lake transgression and slow lake regression fluctuating processes can be divided into a third-order sequence unit consisting of multi-stage transgressive-regressive (T-R) system tracts. The sequence units prograde and overlapped as the form of wedges toward the center of the lake, and develop the sedimentary sequence of "slope rich in mudstone and both top-set and bottom-set rich in sand bodies". [Conclusion] The conclusion of this paper is quite different from the traditional sedimentary model, which is manifested in three aspects. First, the mudstone marker layer has not changed and the corresponding relationship between individual wells has changed. Second, the extent of lake in Yanchang Formation has not changed, however, the evolution process of basin filling has changed. Third, the overall distribution of sandstone has not changed, while the connectivity between sand layers has changed.
[Objective] The latest 3D seismic data displays a large number of strong reflections seismic events progradation in Yanchang Formation, which is quite different from the traditional stratigraphic scheme of "flat rise flat fall and equal thickness distribution". This difference provides a new perspective for re-understanding the stratigraphic framework of Yanchang Formation. [Methods] The seismic facies, sequence stratigraphy and sedimentary system in Longdong area of Ordos Basin are systematically studied through core observation, well seismic calibration and 3D seismic interpretation combined with seismic inversion technology. [Results] The continuous seismic events of strong reflections in Yanchang Formation can indicate the deposition of condensed layers during lake flooding. The seismic facies along provenance and across provenance are quite different. The seismic profiles along the provenance can be divided into three types: sub-parallel structures with medium amplitude and medium continuity, progradational reflections with strong amplitude and strong continuity, and disordered structures with low amplitude and low continuity, among which the progradational type are most obvious. The seismic profiles across the provenance can be divided into three types: parallel structures with strong amplitude and strong continuity, mound structure with medium-strong amplitude and strong continuity, disordered structures with low amplitude and low continuity. The lake basin evolution of Yanchang Formation in Longdong area with some rapid lake transgression and slow lake regression fluctuating processes can be divided into a third-order sequence unit consisting of multi-stage transgressive-regressive (T-R) system tracts. The sequence units prograde and overlapped as the form of wedges toward the center of the lake, and develop the sedimentary sequence of "slope rich in mudstone and both top-set and bottom-set rich in sand bodies". [Conclusion] The conclusion of this paper is quite different from the traditional sedimentary model, which is manifested in three aspects. First, the mudstone marker layer has not changed and the corresponding relationship between individual wells has changed. Second, the extent of lake in Yanchang Formation has not changed, however, the evolution process of basin filling has changed. Third, the overall distribution of sandstone has not changed, while the connectivity between sand layers has changed.
Abstract:
As an important component of unconventional oil and gas resources, the basic research of shale oil and gas resources has gradually attracted attention. Microstructures of clay sediments have an impact on shale foliation and lamination structure, thus influencing the shale reservoir capacity and development effect. In addition to being affected by physical action, clay particles also undergo chemical adsorption and flocculation to form floccules with varying degrees. The basic unit of floccule is tabular particles or domains, further forming flocs or chains, and finally forming a card house structure under the action of electric charge. In the sedimentary stage, the physical and chemical factors affecting the flocculation include particle size, particle content, external hydrodynamic strength and flow properties, climate, electrolyte concentration, ion type, pH value, organic matter, etc. By studying the physical and chemical factors affecting the microstructures of clay sediments and analyzing the other components of mudstone composite particles such as volcanic rock fragments, shale lithics, muddy intraclasts and fecal pellets, the sedimentary environments of mudstone and shale can be identified and described. This study is still in its infancy and a perfect geological model has not been established; The quantitative research of floccules and other components of mudstone composite particles is insufficient, and big data technology has not been used well; The study on the sedimentary micro-environment and internal association characteristics of marine black shale in western China needs to be deepened; The research technology and method are slightly single. Based on the qualitative and quantitative description of microstructures of clay sediments and demonstration of sedimentary environments of mudstone composite particles, it can achieve fine description of complex and changeable sedimentary micro-environment, deepen understanding of the differentiation characteristics of sedimentary environments of shale such as lakes and seas, and further reveal the differences in shale reservoir quality within the high-precision range, laying a foundation for the selection and development of geology and engineering "desserts" in shale.
As an important component of unconventional oil and gas resources, the basic research of shale oil and gas resources has gradually attracted attention. Microstructures of clay sediments have an impact on shale foliation and lamination structure, thus influencing the shale reservoir capacity and development effect. In addition to being affected by physical action, clay particles also undergo chemical adsorption and flocculation to form floccules with varying degrees. The basic unit of floccule is tabular particles or domains, further forming flocs or chains, and finally forming a card house structure under the action of electric charge. In the sedimentary stage, the physical and chemical factors affecting the flocculation include particle size, particle content, external hydrodynamic strength and flow properties, climate, electrolyte concentration, ion type, pH value, organic matter, etc. By studying the physical and chemical factors affecting the microstructures of clay sediments and analyzing the other components of mudstone composite particles such as volcanic rock fragments, shale lithics, muddy intraclasts and fecal pellets, the sedimentary environments of mudstone and shale can be identified and described. This study is still in its infancy and a perfect geological model has not been established; The quantitative research of floccules and other components of mudstone composite particles is insufficient, and big data technology has not been used well; The study on the sedimentary micro-environment and internal association characteristics of marine black shale in western China needs to be deepened; The research technology and method are slightly single. Based on the qualitative and quantitative description of microstructures of clay sediments and demonstration of sedimentary environments of mudstone composite particles, it can achieve fine description of complex and changeable sedimentary micro-environment, deepen understanding of the differentiation characteristics of sedimentary environments of shale such as lakes and seas, and further reveal the differences in shale reservoir quality within the high-precision range, laying a foundation for the selection and development of geology and engineering "desserts" in shale.
Abstract:
In order to deeply analyze the sedimentary environment of organic matter enrichment in the shale of Wufeng-Longmaxi Formation in Sichuan Basin, this paper studies the spatial and temporal distribution characteristics of shale and its organic matter by comparing the metallogenic model of coal, combining the spatial distribution of primary productivity and carbon fixation rate of different water bodies, as well as the characteristics of sedimentary facies, paleogeomorphology, fossil development, etc. The study found that the shallower and more closed the water body, the higher the primary productivity and carbon fixation rate; Oil-bearing shale is developed in the restricted desalinated shallow water environment, which has low energy, stagnant water and strong reduction environment, and can develop type I organic matter; Shale oil and gas have a similar pattern with coal seams. The Mawei pattern, the closer to the land, the richer the organic matter is, and is associated with coal, evaporite, etc; The carbon and nitrogen sources in the water body are mainly exogenous. The development of sapropelic shale and stone coal from Proterozoic to Early Paleozoic is suitable for the lack of terrestrial higher plants that produce humus at this time; The open shelf or bay environment has low primary productivity, and the seabed is a high hydrodynamic environment affected by ocean currents, which is not conducive to the development of shale and preservation of organic matter; Marine transgression or high water deposition periods are not conducive to organic matter enrichment. It is concluded that the shale of the Wufeng Formation - Longmaxi Formation is developed in a regressive environment, which is a shallow sea-land transitional environment such as lagoon and relatively closed bay restricted by ancient land and underwater low uplift, and has the characteristics of near land and far water; Finally, a two-dimensional table of water depth and closure is established to describe the enrichment law of organic matter in detail. It is proposed that the sufficient condition for the enrichment of organic matter in marine shale is a closed water body, and the necessary condition is a shallow water body (<40 m).
In order to deeply analyze the sedimentary environment of organic matter enrichment in the shale of Wufeng-Longmaxi Formation in Sichuan Basin, this paper studies the spatial and temporal distribution characteristics of shale and its organic matter by comparing the metallogenic model of coal, combining the spatial distribution of primary productivity and carbon fixation rate of different water bodies, as well as the characteristics of sedimentary facies, paleogeomorphology, fossil development, etc. The study found that the shallower and more closed the water body, the higher the primary productivity and carbon fixation rate; Oil-bearing shale is developed in the restricted desalinated shallow water environment, which has low energy, stagnant water and strong reduction environment, and can develop type I organic matter; Shale oil and gas have a similar pattern with coal seams. The Mawei pattern, the closer to the land, the richer the organic matter is, and is associated with coal, evaporite, etc; The carbon and nitrogen sources in the water body are mainly exogenous. The development of sapropelic shale and stone coal from Proterozoic to Early Paleozoic is suitable for the lack of terrestrial higher plants that produce humus at this time; The open shelf or bay environment has low primary productivity, and the seabed is a high hydrodynamic environment affected by ocean currents, which is not conducive to the development of shale and preservation of organic matter; Marine transgression or high water deposition periods are not conducive to organic matter enrichment. It is concluded that the shale of the Wufeng Formation - Longmaxi Formation is developed in a regressive environment, which is a shallow sea-land transitional environment such as lagoon and relatively closed bay restricted by ancient land and underwater low uplift, and has the characteristics of near land and far water; Finally, a two-dimensional table of water depth and closure is established to describe the enrichment law of organic matter in detail. It is proposed that the sufficient condition for the enrichment of organic matter in marine shale is a closed water body, and the necessary condition is a shallow water body (<40 m).
Abstract:
Formation of lacustrine laminae result from water stratification and slow rates of deposition of fine-grained sediments. Carbonate laminae are sensitive to changes in water salinity, alkalinity, and biological activity and can provide important implications for paleoclimate and paleohydrology, which have, however, not received enough attention yet. Here, we take the Oligocene Shangganchaigou Formation of the Xichagou Section, SW Qaidam Basin as an example to study the formation mechanisms of lacustrine laminae, the origins of different carbonate mineral phases in saline lake basins, and their climatic and hydrological significances by observation of conventional thin section, fluorescence thin section and scanning electron microscope, and analysis of X-ray powder diffraction (XRD) and stable carbon and oxygen isotopes. Results show that the Shangganchaigou Formation of the Xichagou Section in the SW Qaidam Basin consist of three kinds of carbonate minerals: aragonite, dolomite, and calcite, all of which show a close association with algal residues. Carbon isotopic values of samples with high contents of aragonite are positive and samples containing rich dolomite show relatively higher δ18O values than those of other samples, indicating that the preservation of aragonite in Oligocene strata was related to algal bloom and dolomite formation resulted from high water salinity. The differed distributions of different carbonate mineral phases reflect a dynamic lake evolving from an early clastic-rich brackish lake to a later saline lake, controlled by early Oligocene humid climate and late Oligocene semi-arid climate.
Formation of lacustrine laminae result from water stratification and slow rates of deposition of fine-grained sediments. Carbonate laminae are sensitive to changes in water salinity, alkalinity, and biological activity and can provide important implications for paleoclimate and paleohydrology, which have, however, not received enough attention yet. Here, we take the Oligocene Shangganchaigou Formation of the Xichagou Section, SW Qaidam Basin as an example to study the formation mechanisms of lacustrine laminae, the origins of different carbonate mineral phases in saline lake basins, and their climatic and hydrological significances by observation of conventional thin section, fluorescence thin section and scanning electron microscope, and analysis of X-ray powder diffraction (XRD) and stable carbon and oxygen isotopes. Results show that the Shangganchaigou Formation of the Xichagou Section in the SW Qaidam Basin consist of three kinds of carbonate minerals: aragonite, dolomite, and calcite, all of which show a close association with algal residues. Carbon isotopic values of samples with high contents of aragonite are positive and samples containing rich dolomite show relatively higher δ18O values than those of other samples, indicating that the preservation of aragonite in Oligocene strata was related to algal bloom and dolomite formation resulted from high water salinity. The differed distributions of different carbonate mineral phases reflect a dynamic lake evolving from an early clastic-rich brackish lake to a later saline lake, controlled by early Oligocene humid climate and late Oligocene semi-arid climate.
Abstract:
The Shizigou anticline structure is located in the west of the Qaidam Basin.There are abundant brine resources in the upper part of the Paleogene Lower Ganchaigou Formation, and the content of K, B and Li in brine is high, which has development prospects. In this paper, through the detection of major, trace elements and strontium isotope of the brine, it is found that: The content of K+ in the brine in the study area is 1.058~15.87g/L, of which 70% exceeds 3g/L ; the chemical type of brine is mainly chloride type; By calculating the characteristic coefficient of brine and analyzing the phase diagram of water-salt system, it is considered that the leaching salt layer is the main cause of the high salinity brine. At the same time, the relationship between strontium isotope characteristics of brine and salt layer and the correlation between K+ and Li+ reflect that the source of deep thermal fluid K also has a certain contribution. Through analysis, the metallogenic model of brine potassium in the study area is preliminarily established: During the Paleogene Xiaganchaigou period, the Shizigou area was in a weak extensional environment, and the sedimentary environment was relatively stable. The halite and potassium-magnesium salt deposits of the upper member of the Xiaganchaigou Formation in the late Eocene were deposited. In the later period, the tectonic activity intensified, and the potassium-rich thermal fluid rose into the lake along the tensile fault, providing some deep material source K, and causing the dissolution of the existing potassium-magnesium salt and some halite deposits, which in turn provided another important source of dissolved potassium for the brine in this area.
The Shizigou anticline structure is located in the west of the Qaidam Basin.There are abundant brine resources in the upper part of the Paleogene Lower Ganchaigou Formation, and the content of K, B and Li in brine is high, which has development prospects. In this paper, through the detection of major, trace elements and strontium isotope of the brine, it is found that: The content of K+ in the brine in the study area is 1.058~15.87g/L, of which 70% exceeds 3g/L ; the chemical type of brine is mainly chloride type; By calculating the characteristic coefficient of brine and analyzing the phase diagram of water-salt system, it is considered that the leaching salt layer is the main cause of the high salinity brine. At the same time, the relationship between strontium isotope characteristics of brine and salt layer and the correlation between K+ and Li+ reflect that the source of deep thermal fluid K also has a certain contribution. Through analysis, the metallogenic model of brine potassium in the study area is preliminarily established: During the Paleogene Xiaganchaigou period, the Shizigou area was in a weak extensional environment, and the sedimentary environment was relatively stable. The halite and potassium-magnesium salt deposits of the upper member of the Xiaganchaigou Formation in the late Eocene were deposited. In the later period, the tectonic activity intensified, and the potassium-rich thermal fluid rose into the lake along the tensile fault, providing some deep material source K, and causing the dissolution of the existing potassium-magnesium salt and some halite deposits, which in turn provided another important source of dissolved potassium for the brine in this area.
Abstract:
[Objective] To study the evolution of river patterns in the distributive fluvial system, establish a database of corresponding parameters, and provide a data basis for predicting the distribution of sedimentary systems within the forecast basin, [Methods] Using modern geographic information software such as Google Earth and Global Mapper, the channel sinuosity was measured according to the grid of equal proportions, analyzes the variation of the channel sinuosity along the course, and establishes the depositional mode of the evolution of the Golmud fluvial fan. [Results and Discussions] A total of 2989 channel sinuosity data were measured, and according to the characteristics of the sinuosity of the Golmud fluvial fan, the Golmud fluvial fan can be divided into four parts: "braided river section", "braided-meander coexistence section", "large high-sinuosity meandering river section" and "small low-sinuosity meandering river section": 1) The slope of the braid river section was 0.7%, the development of the straight river and the braided river with low sinuosity, the braided belt width was large, the channel sinuosity was 1.01-1.43, the average sinuosity was 1.08, and the microfacies such as braided river, abandoned river and flood plain were mainly developed; 2) The slope of the braided-meander coexistence section was 0.63%, which developed both braided river and meandering river, and the development degree and river sinuosity of braided river and meandering river in different positions were different, the channel sinuosity was 1.08-5, and the average sinuosity was 1.52, and microfacies such as braided river, meandering river, abandoned river, crevasse splay, sand dunes and flood plain were mainly developed; 3) The slope of the large high-sinuosity meandering river section was 0.29%, mainly developed large meandering river, the channel was wide, the river bend swing was large, the channel sinuosity was 1.12-5, the average sinuosity was 1.83, and the microfacies such as meandering river, abandoned river, sand dunes and flood plain were mainly developed; 4) The slope of the small low-sinuosity meandering river section was 0.08%, mainly developed small bifurcated meandering river, narrow channel, small river bend swing, the channel sinuosity was 1.07-3.13, the average sinuosity was 1.51, and microfacies such as meandering river, abandoned river, crevasse splay, sand dunes and flood plain were mainly developed. [Conclusions] The depositional model of river-type evolution of macroDFS was established, which provided new knowledge base information for the prediction of the distribution of underground sedimentary systems.
[Objective] To study the evolution of river patterns in the distributive fluvial system, establish a database of corresponding parameters, and provide a data basis for predicting the distribution of sedimentary systems within the forecast basin, [Methods] Using modern geographic information software such as Google Earth and Global Mapper, the channel sinuosity was measured according to the grid of equal proportions, analyzes the variation of the channel sinuosity along the course, and establishes the depositional mode of the evolution of the Golmud fluvial fan. [Results and Discussions] A total of 2989 channel sinuosity data were measured, and according to the characteristics of the sinuosity of the Golmud fluvial fan, the Golmud fluvial fan can be divided into four parts: "braided river section", "braided-meander coexistence section", "large high-sinuosity meandering river section" and "small low-sinuosity meandering river section": 1) The slope of the braid river section was 0.7%, the development of the straight river and the braided river with low sinuosity, the braided belt width was large, the channel sinuosity was 1.01-1.43, the average sinuosity was 1.08, and the microfacies such as braided river, abandoned river and flood plain were mainly developed; 2) The slope of the braided-meander coexistence section was 0.63%, which developed both braided river and meandering river, and the development degree and river sinuosity of braided river and meandering river in different positions were different, the channel sinuosity was 1.08-5, and the average sinuosity was 1.52, and microfacies such as braided river, meandering river, abandoned river, crevasse splay, sand dunes and flood plain were mainly developed; 3) The slope of the large high-sinuosity meandering river section was 0.29%, mainly developed large meandering river, the channel was wide, the river bend swing was large, the channel sinuosity was 1.12-5, the average sinuosity was 1.83, and the microfacies such as meandering river, abandoned river, sand dunes and flood plain were mainly developed; 4) The slope of the small low-sinuosity meandering river section was 0.08%, mainly developed small bifurcated meandering river, narrow channel, small river bend swing, the channel sinuosity was 1.07-3.13, the average sinuosity was 1.51, and microfacies such as meandering river, abandoned river, crevasse splay, sand dunes and flood plain were mainly developed. [Conclusions] The depositional model of river-type evolution of macroDFS was established, which provided new knowledge base information for the prediction of the distribution of underground sedimentary systems.
Abstract:
[Objective] Statistics of published literature and analysis of previous research results show that the reservoir physical properties and oil and gas enrichment of laminar fine-grained sedimentary rocks are obviously superior to those of undeveloped/weakly developed laminar fine-grained sedimentary rocks and have a very good positive correlation with them. Compared with the laminar structure of Marine fine-grained sedimentary rocks, the laminar lacustrine fine-grained sedimentary rocks are limited by the sedimentary environment being closer to the provenance area, the frequent change of sedimentary environment hydrodynamics, and the more obvious control by climate and tectonic activities. As a result, the reservoir physical properties and oil and gas enrichment characteristics of different types of laminar rocks are not the same. The summary of relevant differences is still unclear and needs to be revealed.[Method] By focusing on the relevant literature at home and abroad in the past ten years, the types and causes of pores and fractures of lamellar fine-grained sedimentary rocks are reviewed, and the controlling effects of lamellar composition, continuity and sequence combination on reservoir physical properties and hydrocarbon enrichment characteristics are described respectively. [Results] It is believed that the laminar structure has the advantages of frequent turnover of vertical material composition, abnormal development of internal micro-fractures and different evolution of organic matter under the laminar structure, which further improves the ability of oil and gas enrichment on the basis of fine-grained sedimentary rocks. . [Prospects] It is pointed out that there is still a lack of difference and comparison of reservoir physical properties of fine grained sedimentary rocks which are all laminated due to different factors such as pores and fractures in each other's layers, and it will be the development direction in the future to use computer to invert macro reservoir sedimentary model based on micro laminar data to guide unconventional reservoir development
[Objective] Statistics of published literature and analysis of previous research results show that the reservoir physical properties and oil and gas enrichment of laminar fine-grained sedimentary rocks are obviously superior to those of undeveloped/weakly developed laminar fine-grained sedimentary rocks and have a very good positive correlation with them. Compared with the laminar structure of Marine fine-grained sedimentary rocks, the laminar lacustrine fine-grained sedimentary rocks are limited by the sedimentary environment being closer to the provenance area, the frequent change of sedimentary environment hydrodynamics, and the more obvious control by climate and tectonic activities. As a result, the reservoir physical properties and oil and gas enrichment characteristics of different types of laminar rocks are not the same. The summary of relevant differences is still unclear and needs to be revealed.[Method] By focusing on the relevant literature at home and abroad in the past ten years, the types and causes of pores and fractures of lamellar fine-grained sedimentary rocks are reviewed, and the controlling effects of lamellar composition, continuity and sequence combination on reservoir physical properties and hydrocarbon enrichment characteristics are described respectively. [Results] It is believed that the laminar structure has the advantages of frequent turnover of vertical material composition, abnormal development of internal micro-fractures and different evolution of organic matter under the laminar structure, which further improves the ability of oil and gas enrichment on the basis of fine-grained sedimentary rocks. . [Prospects] It is pointed out that there is still a lack of difference and comparison of reservoir physical properties of fine grained sedimentary rocks which are all laminated due to different factors such as pores and fractures in each other's layers, and it will be the development direction in the future to use computer to invert macro reservoir sedimentary model based on micro laminar data to guide unconventional reservoir development
Abstract:
[Objective] The study of "Carbonate Factory" emphasizes the specific effects of biological communities on the carbonate production process and scale moderation, which contributes significantly to our understanding of large-scale carbonate sedimentation. Although the concept and application of the "Carbonate Factory" have gained attention in the field of carbonate sedimentology, there is still a lack of systematic understanding of how to interpret the origin, developmental processes, and influencing factors of specific types of carbonate sedimentation in geological history, particularly in deep time. [Methods] This study examines the Gushan Formation of the Cambrian Miaolingian in the Yunmengshan area (Henan Province) as an example of a microbial carbonate factory. A multi-scale grid-based quantitative evaluation method is used to describe the specific form, internal structure, and construction process of the carbonate factory. [Results and Discussions] Our results show that benthic microbial communities were the primary builders of the microbial carbonate factory in the study area. They formed unique structures, including the stromatolite, thrombolite, leiolite, and microbial-induced sedimentary structure (MISS). Various particle types, including ooids, flat pebbles, and microbialite breccias, were preserved within the microbial buildups. The development of the microbial carbonate factories occurred episodically during the Guzhangian Age (Miaolingian) and had relatively small scales (height of <1 m); however, they extended widely in space and exhibited different macroscopic shapes such as subspherical, domical, and tabular. The interior structures of the microbial carbonate factories consisted of individual forming styles as well as vertical combinations of composite forms. [Conclusions] By analyzing various factors influencing the development of the factories, we found that higher nutrient levels may have played a crucial role in the development of microbial carbonate factories in the tide-flat environment. Additionally, the flooding event during the Guzhangian Age led to the eventual closure of the factories. This study provides an ancient example similar to the modern Hamelin Pool microbial carbonate factory in Australia, which has good reference value for understanding the types, characteristics, and evolutionary patterns of carbonate factories in the Cambrian period.
[Objective] The study of "Carbonate Factory" emphasizes the specific effects of biological communities on the carbonate production process and scale moderation, which contributes significantly to our understanding of large-scale carbonate sedimentation. Although the concept and application of the "Carbonate Factory" have gained attention in the field of carbonate sedimentology, there is still a lack of systematic understanding of how to interpret the origin, developmental processes, and influencing factors of specific types of carbonate sedimentation in geological history, particularly in deep time. [Methods] This study examines the Gushan Formation of the Cambrian Miaolingian in the Yunmengshan area (Henan Province) as an example of a microbial carbonate factory. A multi-scale grid-based quantitative evaluation method is used to describe the specific form, internal structure, and construction process of the carbonate factory. [Results and Discussions] Our results show that benthic microbial communities were the primary builders of the microbial carbonate factory in the study area. They formed unique structures, including the stromatolite, thrombolite, leiolite, and microbial-induced sedimentary structure (MISS). Various particle types, including ooids, flat pebbles, and microbialite breccias, were preserved within the microbial buildups. The development of the microbial carbonate factories occurred episodically during the Guzhangian Age (Miaolingian) and had relatively small scales (height of <1 m); however, they extended widely in space and exhibited different macroscopic shapes such as subspherical, domical, and tabular. The interior structures of the microbial carbonate factories consisted of individual forming styles as well as vertical combinations of composite forms. [Conclusions] By analyzing various factors influencing the development of the factories, we found that higher nutrient levels may have played a crucial role in the development of microbial carbonate factories in the tide-flat environment. Additionally, the flooding event during the Guzhangian Age led to the eventual closure of the factories. This study provides an ancient example similar to the modern Hamelin Pool microbial carbonate factory in Australia, which has good reference value for understanding the types, characteristics, and evolutionary patterns of carbonate factories in the Cambrian period.
Abstract:
[Objective] After more than 50 years of exploration, Langgu Sag has entered the stage of oil and gas exploration and development with lithologic and structural-lithologic reservoirs as the main targets. Previous studies have been carried out on the large-scale sedimentary characteristics, hydrocarbon accumulation factors and models of Langgu Sag. However, relatively little is known of the spatial distribution characteristics and the distribution rules of the fine sand bodies. The main controlling factors of hydrocarbon accumulation and their distribution are not uniform, which restricts the evaluation and production of subtle reservoirs. [Methods] This study comprehensively used core data, well logging, seismic data, analytical tests and production data to systematically study the Shahejie Formation in the Jiuzhou-Wanzhuang area. The study aims to identify and classify sedimentary facies and microfacies types, accurately characterize the spatial distribution of sand bodies, and analyze reservoirs formation control factors such as source rocks, traps and fault dredging systems. The hydrocarbon accumulation model is established, leading to the prediction of favorable concealed lithologic or structural-lithologic reservoirs distribution zones. [Results and Discussions] The lithology is mainly fine-grained clastic rock; the sedimentary sequence is not typical, a deformation structure is clearly developed, and floating mud gravel of sandy clastic flow origin can be seen. This reflects the characteristics of the dynamic conditions of traction flow in the distal fan delta. In the study area, there are two provenance supply systems in the south and north, and a braided channel extends from the SE of the Daxing Fault to the interior of the lake in a finger-like way, forming two depositional centers in Jiuzhou and Wanzhuang. In the study area, an underwater distributary channel at the front of the fan delta extends for some distance and migrates frequently. The estuary is unstable, with an underdeveloped or small-scale estuarine bar. Mature source rocks, effective traps and drainage systems are the main controls of hydrocarbon accumulation in the middle section of Sha-3 in the study area. The abundance of organic matter shows that the oil source of the lower submember of Sha-3 in the study area comes from underlying source rocks of the lower submember of Sha-3. Using forward modeling and RGB attribute fusion, sensitive attribute optimization was performed to predict the distribution range of high-quality sandstone reservoirs combined with a series of anticlinal tectonic settings to form good structural traps along with lithologic up-dip pinch-out traps. As oil source faults, the Daxing and Jiuzhou Faults and their secondary branches are the main channels connecting the oil and gas resources of the lower submember of Sha-3 and the reservoirs of the middle submember of Sha-3, and they also control the formation of traps as a whole. [Conclusions] The results show that the study area is primarily characterized by fan-delta systems and lake systems. Within the fan-delta system, various microfacies types were identified (e.g., braided channels, submarine distributary channels, delta front sandbars, and sheet-like sands). The spatial distribution of sand bodies is characterized by thick layers of distributary channel-sandbar complexes with finger-like distribution and continuous thin sheet-like sands. The organic configurations of oil source faults, structures and even lithologic traps are the main causes of hydrocarbon accumulation in the study area, and the reservoir lithology within the traps determines the oil, gas and water distribution. Finally, favorable lithologic or structural-lithologic reservoirs development target areas are predicted in the SE wing of the Jiuzhou plunging nose area, NW wing of the Jiuzhou plunging nose structure and NE wing of the Tongxi ancient structural ridge.
[Objective] After more than 50 years of exploration, Langgu Sag has entered the stage of oil and gas exploration and development with lithologic and structural-lithologic reservoirs as the main targets. Previous studies have been carried out on the large-scale sedimentary characteristics, hydrocarbon accumulation factors and models of Langgu Sag. However, relatively little is known of the spatial distribution characteristics and the distribution rules of the fine sand bodies. The main controlling factors of hydrocarbon accumulation and their distribution are not uniform, which restricts the evaluation and production of subtle reservoirs. [Methods] This study comprehensively used core data, well logging, seismic data, analytical tests and production data to systematically study the Shahejie Formation in the Jiuzhou-Wanzhuang area. The study aims to identify and classify sedimentary facies and microfacies types, accurately characterize the spatial distribution of sand bodies, and analyze reservoirs formation control factors such as source rocks, traps and fault dredging systems. The hydrocarbon accumulation model is established, leading to the prediction of favorable concealed lithologic or structural-lithologic reservoirs distribution zones. [Results and Discussions] The lithology is mainly fine-grained clastic rock; the sedimentary sequence is not typical, a deformation structure is clearly developed, and floating mud gravel of sandy clastic flow origin can be seen. This reflects the characteristics of the dynamic conditions of traction flow in the distal fan delta. In the study area, there are two provenance supply systems in the south and north, and a braided channel extends from the SE of the Daxing Fault to the interior of the lake in a finger-like way, forming two depositional centers in Jiuzhou and Wanzhuang. In the study area, an underwater distributary channel at the front of the fan delta extends for some distance and migrates frequently. The estuary is unstable, with an underdeveloped or small-scale estuarine bar. Mature source rocks, effective traps and drainage systems are the main controls of hydrocarbon accumulation in the middle section of Sha-3 in the study area. The abundance of organic matter shows that the oil source of the lower submember of Sha-3 in the study area comes from underlying source rocks of the lower submember of Sha-3. Using forward modeling and RGB attribute fusion, sensitive attribute optimization was performed to predict the distribution range of high-quality sandstone reservoirs combined with a series of anticlinal tectonic settings to form good structural traps along with lithologic up-dip pinch-out traps. As oil source faults, the Daxing and Jiuzhou Faults and their secondary branches are the main channels connecting the oil and gas resources of the lower submember of Sha-3 and the reservoirs of the middle submember of Sha-3, and they also control the formation of traps as a whole. [Conclusions] The results show that the study area is primarily characterized by fan-delta systems and lake systems. Within the fan-delta system, various microfacies types were identified (e.g., braided channels, submarine distributary channels, delta front sandbars, and sheet-like sands). The spatial distribution of sand bodies is characterized by thick layers of distributary channel-sandbar complexes with finger-like distribution and continuous thin sheet-like sands. The organic configurations of oil source faults, structures and even lithologic traps are the main causes of hydrocarbon accumulation in the study area, and the reservoir lithology within the traps determines the oil, gas and water distribution. Finally, favorable lithologic or structural-lithologic reservoirs development target areas are predicted in the SE wing of the Jiuzhou plunging nose area, NW wing of the Jiuzhou plunging nose structure and NE wing of the Tongxi ancient structural ridge.
Abstract:
Abstract: The pressure differences between source and reservoir rocks is not only the driving force for unconventional oil and gas accumulation, but also an indispensable key content in the study of the genesis of shale oil sweet spots. In addition, laminar structures are widely developed in continental shale, and the degree of development results in differences in the accumulation dynamics of reservoir rocks, which affect the accumulation of shale oil and gas. However, there are relatively few studies on the accumulation dynamics of shale oil. The sweet spot section of the Permian Lucaogou Formation in the Jimusar Sag was taken as the research object, and the intrinsic relationship between the development degree of laminar structure and shale oil and gas accumulation was revealed from the perspective of accumulation dynamics. Through evaluation of source rocks, classification of petrographic types and characterization of pores, etc. The characteristics of source rocks, different types of reservoir rocks, and source-reservoir assemblages in the study area were obtained. Using the equivalent depth method and fluid inclusion simulation, the pressure difference between source rocks and reservoir rocks during the accumulation period was recovered, and the accumulation dynamics of different types of reservoir rocks were obtained. The results show that the study area is dominated by source-reservoir interbedded combinations, and the hydrocarbon generation of high-quality source rocks creates a strong source-reservoir pressure difference between source and reservoir, which promotes the continuous migration of oil and gas to adjacent reservoir spaces; Interbedded silty and argillaceous laminae are widely developed in the reservoir rocks, which constitute a large area of frequent contact between source and reservoir. The degree of development results in differences in the accumulation dynamics of different types of reservoir rocks, and the laminar reservoir has developed laminar structure and the migration distance of oil and gas is shortened, so that it has stronger accumulation power and oil-bearing property. The development of laminar reservoir rocks in the lower sweet spot is a favorable area for studying oil and gas migration and accumulation in the shale sweet spot.
Abstract: The pressure differences between source and reservoir rocks is not only the driving force for unconventional oil and gas accumulation, but also an indispensable key content in the study of the genesis of shale oil sweet spots. In addition, laminar structures are widely developed in continental shale, and the degree of development results in differences in the accumulation dynamics of reservoir rocks, which affect the accumulation of shale oil and gas. However, there are relatively few studies on the accumulation dynamics of shale oil. The sweet spot section of the Permian Lucaogou Formation in the Jimusar Sag was taken as the research object, and the intrinsic relationship between the development degree of laminar structure and shale oil and gas accumulation was revealed from the perspective of accumulation dynamics. Through evaluation of source rocks, classification of petrographic types and characterization of pores, etc. The characteristics of source rocks, different types of reservoir rocks, and source-reservoir assemblages in the study area were obtained. Using the equivalent depth method and fluid inclusion simulation, the pressure difference between source rocks and reservoir rocks during the accumulation period was recovered, and the accumulation dynamics of different types of reservoir rocks were obtained. The results show that the study area is dominated by source-reservoir interbedded combinations, and the hydrocarbon generation of high-quality source rocks creates a strong source-reservoir pressure difference between source and reservoir, which promotes the continuous migration of oil and gas to adjacent reservoir spaces; Interbedded silty and argillaceous laminae are widely developed in the reservoir rocks, which constitute a large area of frequent contact between source and reservoir. The degree of development results in differences in the accumulation dynamics of different types of reservoir rocks, and the laminar reservoir has developed laminar structure and the migration distance of oil and gas is shortened, so that it has stronger accumulation power and oil-bearing property. The development of laminar reservoir rocks in the lower sweet spot is a favorable area for studying oil and gas migration and accumulation in the shale sweet spot.
