Current Issue
2025 Vol. 434, No. 4
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2025, 43(4): 1163-1177.
doi: 10.14027/j.issn.1000-0550.2024.083
Abstract:
Objective To investigate the geological conditions and exploration potential of marine shale gas in the Wulalike Formation， western Ordos Basin， and effectively guide the exploration of marine shale gas. Methods A comprehensive analysis of its organic geochemistry， XRD and FE-SEM， combined with the regional structure evolution characteristics and thermal simulation experimentation， was conducted to determine the distribution of marine shale， hydrocarbon generation potential， reservoir properties， preservation conditions and exploration potential. Results （1） The thickness of strata in the Wulalike Formation varies greatly， from 10-80 m in the northern and central areas to more than 1 100 m in the south. The TOC of the Wulalike shale is between 0.10% and 1.40%， and the organic matter is mainly Type I （sapropelic）. The shales in the north and central areas are deeply buried and have undergone a high degree of thermal evolution， bringing it to the gas generation stage now. The southern part was affected by the thrust of the western ordos basin， which generated both oil and gas. （2） Various types of pores are developed in the Wulalike shale （inter- and intragranular pores， microfractures and a small amount of organic matter pores）. Although its overall porosity is lower than that of the Longmaxi shales of the Sichuan Basin， the content of brittle mineral is high， microfractures are relatively well developed， and its permeability is better than in the Longmaxi shales. （3） In the western Ordos Basin， the underlying Kelimoli Formation is mainly limestone， and the overlying Lashizhong Formation is mainly shale with minor limestone content， which provide good sealing capability and favors shale gas enrichment. Conclusions The stratigraphic distribution， hydrocarbon potential， fracturing ability and preservation conditions of the Wulalike Formation indicate that the southern area of the Tiekesumao Sag in the northern part and the western area of the Gufengzhuang Sag in the central part， both have good marine shale gas exploration potential.
Objective To investigate the geological conditions and exploration potential of marine shale gas in the Wulalike Formation， western Ordos Basin， and effectively guide the exploration of marine shale gas. Methods A comprehensive analysis of its organic geochemistry， XRD and FE-SEM， combined with the regional structure evolution characteristics and thermal simulation experimentation， was conducted to determine the distribution of marine shale， hydrocarbon generation potential， reservoir properties， preservation conditions and exploration potential. Results （1） The thickness of strata in the Wulalike Formation varies greatly， from 10-80 m in the northern and central areas to more than 1 100 m in the south. The TOC of the Wulalike shale is between 0.10% and 1.40%， and the organic matter is mainly Type I （sapropelic）. The shales in the north and central areas are deeply buried and have undergone a high degree of thermal evolution， bringing it to the gas generation stage now. The southern part was affected by the thrust of the western ordos basin， which generated both oil and gas. （2） Various types of pores are developed in the Wulalike shale （inter- and intragranular pores， microfractures and a small amount of organic matter pores）. Although its overall porosity is lower than that of the Longmaxi shales of the Sichuan Basin， the content of brittle mineral is high， microfractures are relatively well developed， and its permeability is better than in the Longmaxi shales. （3） In the western Ordos Basin， the underlying Kelimoli Formation is mainly limestone， and the overlying Lashizhong Formation is mainly shale with minor limestone content， which provide good sealing capability and favors shale gas enrichment. Conclusions The stratigraphic distribution， hydrocarbon potential， fracturing ability and preservation conditions of the Wulalike Formation indicate that the southern area of the Tiekesumao Sag in the northern part and the western area of the Gufengzhuang Sag in the central part， both have good marine shale gas exploration potential.
2025, 43(4): 1178-1198.
doi: 10.14027/j.issn.1000-0550.2024.077
Abstract:
Objective A set of black marine graptolitic shales is developed in the Wulalike Formation at the western margin of the Ordos Basin. In recent years， natural gas has continually been discovered in this set of shales and an industrial level of oil flow has been produced， demonstrating that the Wulalike Formation has significant exploration potential. This study examines the development of the graptolites and establishes a biostratigraphic framework as a basis for analyzing the sedimentary evolution of the Wulalike Formation. Methods The age range of the Wulalike Formation was clarified by analyzing the graptolite content of individual cored wells and comparing the graptolite range. Single-well graptolite zones were identified and biostratigraphic frameworks were constructed on that basis， and the distribution of the graptolite zones is discussed. The spatial evolution of the black shale was analyzed in combination with the lithological development. Sea-level variation was also considered based on the response of graptolite morphological complexity to marine transgression. Results （1） The Wulalike Formation was developed during the Darriwilian-Sandbian stages at the end of the middle- and beginning of the Upper Ordovician. The Wulalike Formation is divided into four graptolite zones from bottom to top： the Pterograptus elegans zone， Jiangxigraptus vagus zone， Nemagraptus gracilis zone and Climacograptus bicornis zone. （2） From a comparative analysis of their development， it is believed that all four graptolite zones are developed in the northern Wulalike Formation of the study area in the vicinity of well QT9， while the Pterograptus elegans and Jiangxigraptus vagus zones are absent in the southern part of the Formation. （3） Two marine transgression periods are indicated by the Pterograptus elegans and Nemagraptus gracilis zones in the Wulalike Formation， transitioning upward into the Climacograptus bicornis zone and then into the Lashizhong Formation， during which time the sea level gradually lowered. Conclusions The black graptolitic shale at the bottom of the Wulalike Formation shows strong diachronism from Pterograptus elegans in well QT9 to Nemagraptus gracilis in well YinT2. Along the western margin of the Ordos Block， the strata are gradually younger to the south， and the deposition center gradually migrates southward. The diachronism is typically manifested as “degradational stacking”， evident as an upward-shallowing stacking pattern that indicates a forced marine regression interface. Alternatively， the diachronism of the black shale may be due to marine transgression deposition and progradation in a highstand systems tract， resulting in the progradation of the black shale towards the south or southwest. Due to the limitations of core sampling， in subsequent studies the causes of the diachronism of the black shale will require determination of the formation ages of the limestone at the top and bottom of the Wulalike Formation， along with verification by seismic data. The establishment of a biostratigraphic framework in this study based on the graptolite zonation provides important guidance for understanding the stratigraphic and sedimentary evolution of the Wulalike Formation at the western margin of the Ordos Basin.
Objective A set of black marine graptolitic shales is developed in the Wulalike Formation at the western margin of the Ordos Basin. In recent years， natural gas has continually been discovered in this set of shales and an industrial level of oil flow has been produced， demonstrating that the Wulalike Formation has significant exploration potential. This study examines the development of the graptolites and establishes a biostratigraphic framework as a basis for analyzing the sedimentary evolution of the Wulalike Formation. Methods The age range of the Wulalike Formation was clarified by analyzing the graptolite content of individual cored wells and comparing the graptolite range. Single-well graptolite zones were identified and biostratigraphic frameworks were constructed on that basis， and the distribution of the graptolite zones is discussed. The spatial evolution of the black shale was analyzed in combination with the lithological development. Sea-level variation was also considered based on the response of graptolite morphological complexity to marine transgression. Results （1） The Wulalike Formation was developed during the Darriwilian-Sandbian stages at the end of the middle- and beginning of the Upper Ordovician. The Wulalike Formation is divided into four graptolite zones from bottom to top： the Pterograptus elegans zone， Jiangxigraptus vagus zone， Nemagraptus gracilis zone and Climacograptus bicornis zone. （2） From a comparative analysis of their development， it is believed that all four graptolite zones are developed in the northern Wulalike Formation of the study area in the vicinity of well QT9， while the Pterograptus elegans and Jiangxigraptus vagus zones are absent in the southern part of the Formation. （3） Two marine transgression periods are indicated by the Pterograptus elegans and Nemagraptus gracilis zones in the Wulalike Formation， transitioning upward into the Climacograptus bicornis zone and then into the Lashizhong Formation， during which time the sea level gradually lowered. Conclusions The black graptolitic shale at the bottom of the Wulalike Formation shows strong diachronism from Pterograptus elegans in well QT9 to Nemagraptus gracilis in well YinT2. Along the western margin of the Ordos Block， the strata are gradually younger to the south， and the deposition center gradually migrates southward. The diachronism is typically manifested as “degradational stacking”， evident as an upward-shallowing stacking pattern that indicates a forced marine regression interface. Alternatively， the diachronism of the black shale may be due to marine transgression deposition and progradation in a highstand systems tract， resulting in the progradation of the black shale towards the south or southwest. Due to the limitations of core sampling， in subsequent studies the causes of the diachronism of the black shale will require determination of the formation ages of the limestone at the top and bottom of the Wulalike Formation， along with verification by seismic data. The establishment of a biostratigraphic framework in this study based on the graptolite zonation provides important guidance for understanding the stratigraphic and sedimentary evolution of the Wulalike Formation at the western margin of the Ordos Basin.
2025, 43(4): 1199-1212.
doi: 10.14027/j.issn.1000-0550.2024.096
Abstract:
Objective The western part of the Ordos Basin was located in the Qilian Sea area during the middle-late Ordovician Wulalike period， when a thick layer of black marine shale developed in the region. Many exploration wells in the Ordos Basin have produced industrial-level oil and gas flows from the Wulalike Formation in recent years， indicating a high potential for further exploration and development of marine shale oil and gas. The study of the Ordovician paleogeography in the western part of the basin is conducive to clarifying the distribution of hydrocarbon source rocks and reservoirs and directing exploration for marine shale gas. Methods This paper describes a comprehensive study of the petrological characteristics， sedimentary facies types and sedimentary evolution of the Wulalike Formation， involving field observation of a large number of outcrops and analyses of drill cores， rock thin sections and elemental geochemical data. Results （1） The Wulalike Formation comprises a range of rock types： siliceous shale， calcareous shale， clay shale， mixed shale and granular limestone. Of these， laminated siliceous shale was identified as the most favorable lithofacies for shale gas exploration. （2） Four sedimentary facies types from west to east were identified and analyzed： basin facies， broad sea-shelf facies， slope facies and carbonate platform facies. （3） During the Wulalike period， the western Ordos Basin underwent multiple transgression/regressions， and gradual shallowing of the water modified the sedimentary environment. Conclusions A model of the carbonate platform margin sedimentation established for the western Ordos Basin in the Wulalike period indicates that the broad sea-shelf facies， slope facies and granular shoal facies favor natural gas exploration in the next step.
Objective The western part of the Ordos Basin was located in the Qilian Sea area during the middle-late Ordovician Wulalike period， when a thick layer of black marine shale developed in the region. Many exploration wells in the Ordos Basin have produced industrial-level oil and gas flows from the Wulalike Formation in recent years， indicating a high potential for further exploration and development of marine shale oil and gas. The study of the Ordovician paleogeography in the western part of the basin is conducive to clarifying the distribution of hydrocarbon source rocks and reservoirs and directing exploration for marine shale gas. Methods This paper describes a comprehensive study of the petrological characteristics， sedimentary facies types and sedimentary evolution of the Wulalike Formation， involving field observation of a large number of outcrops and analyses of drill cores， rock thin sections and elemental geochemical data. Results （1） The Wulalike Formation comprises a range of rock types： siliceous shale， calcareous shale， clay shale， mixed shale and granular limestone. Of these， laminated siliceous shale was identified as the most favorable lithofacies for shale gas exploration. （2） Four sedimentary facies types from west to east were identified and analyzed： basin facies， broad sea-shelf facies， slope facies and carbonate platform facies. （3） During the Wulalike period， the western Ordos Basin underwent multiple transgression/regressions， and gradual shallowing of the water modified the sedimentary environment. Conclusions A model of the carbonate platform margin sedimentation established for the western Ordos Basin in the Wulalike period indicates that the broad sea-shelf facies， slope facies and granular shoal facies favor natural gas exploration in the next step.
2025, 43(4): 1213-1232.
doi: 10.14027/j.issn.1000-0550.2024.120
Abstract:
Objective Several recent exploration wells in the Ordovician Wulalike Formation at the western edge of the Ordos Basin have obtained industrial gas flow， breaking through the largely unexplored migmatite zone. Understanding the lithofacies and sedimentary characterisitcs of mignatites can provide theroretical basis and practical data for future oil and gas exploration work in the western edge. Methods Based on typical drilling core and outcrop data， this study comprehensively utilizes thin sections， scanning electron microscopy， and logging to investigate the lithofacies， sedimentary facies， and distribution patterns of the Wulalike Formation in the western edge. [Results and Conclusions] The rocks of the Ordovician Wulaike Formation in the western edge of the Ordos Basin can be divided into three types： Migmatite lithofacies， terrigenous clastic lithofacies， and carbonate lithofacies. The vertical sequence combination is controlled by two periods of marine transgression events， while the lithofacies changes are further controlled by local hydrodynamic conditions. Lithofacies types， vertical sequences， logging， geochemistry and other facies indicators indicate that the Wulalike Formation can develop platforms， continental shelves， slopes， and basins， which can be further divided into 7 subfacies and 15 microfacies. The eastern platform facies feature "intertidal shoal-flat" sequence and the "tidal storm-channel-sand dam" sequence， and the sequence of "continental shelf storm shoreline/shallow sea sand bars" and "continental shelf mud"forms the central continental shelf facies. The western slope basin facies are dominated by turbidite fans， canyon channels， and basin mud. The distribution pattern of sedimentary facies within the third order sequence of the Wulaike Formation indicates that during the formation of sequence SQ1， the water body was generally deep and developed large sections of shale， while during the sedimentation of sequence SQ2， the water body became shallower as a whole. In addition， due to the influence of microbial construction and the connection between the western edge and the middle-east platform， the slope and shelf facies also became narrower overall and retreated westward. Sedimentary model displays that rock types and sedimentary characteristics of the Wulaike Formation are mainly influenced by the source supply， paleogeomorphological features， and hydrodynamic conditions， among which sufficient source supply is the basic condition for migmatites sedimentation in the western edge area.
Objective Several recent exploration wells in the Ordovician Wulalike Formation at the western edge of the Ordos Basin have obtained industrial gas flow， breaking through the largely unexplored migmatite zone. Understanding the lithofacies and sedimentary characterisitcs of mignatites can provide theroretical basis and practical data for future oil and gas exploration work in the western edge. Methods Based on typical drilling core and outcrop data， this study comprehensively utilizes thin sections， scanning electron microscopy， and logging to investigate the lithofacies， sedimentary facies， and distribution patterns of the Wulalike Formation in the western edge. [Results and Conclusions] The rocks of the Ordovician Wulaike Formation in the western edge of the Ordos Basin can be divided into three types： Migmatite lithofacies， terrigenous clastic lithofacies， and carbonate lithofacies. The vertical sequence combination is controlled by two periods of marine transgression events， while the lithofacies changes are further controlled by local hydrodynamic conditions. Lithofacies types， vertical sequences， logging， geochemistry and other facies indicators indicate that the Wulalike Formation can develop platforms， continental shelves， slopes， and basins， which can be further divided into 7 subfacies and 15 microfacies. The eastern platform facies feature "intertidal shoal-flat" sequence and the "tidal storm-channel-sand dam" sequence， and the sequence of "continental shelf storm shoreline/shallow sea sand bars" and "continental shelf mud"forms the central continental shelf facies. The western slope basin facies are dominated by turbidite fans， canyon channels， and basin mud. The distribution pattern of sedimentary facies within the third order sequence of the Wulaike Formation indicates that during the formation of sequence SQ1， the water body was generally deep and developed large sections of shale， while during the sedimentation of sequence SQ2， the water body became shallower as a whole. In addition， due to the influence of microbial construction and the connection between the western edge and the middle-east platform， the slope and shelf facies also became narrower overall and retreated westward. Sedimentary model displays that rock types and sedimentary characteristics of the Wulaike Formation are mainly influenced by the source supply， paleogeomorphological features， and hydrodynamic conditions， among which sufficient source supply is the basic condition for migmatites sedimentation in the western edge area.
2025, 43(4): 1233-1250.
doi: 10.14027/j.issn.1000-0550.2024.079
Abstract:
Objective Investigating the coupling relationships between the differential characteristics of marine shale pore spaces and lithofacies， material composition， pore structure， as well as their implications for shale gas enrichment patterns， is crucial for evaluating the gas-bearing potential of the Wulalike Formation shale in the western Ordos Basin. Methods Taking the Wulalike Formation shale in the western Ordos Basin as the research object， and based on the classification of typical shale lithofacies， this study analyzed the influence of material composition， siliceous genesis， and pore structure on shale pore space through microscopy and scanning electron microscopy observations， helium porosity， pulse permeability， nuclear magnetic resonance， nitrogen and carbon dioxide adsorption， and methane isothermal adsorption tests. The vertical enrichment intervals of shale gas were also investigated. Results In the study area， the porosity of the Wulalike Formation shale shows an increasing trend from the bottom to the top， with no significant correlation between porosity and permeability. Unlike the Longmaxi Formation shale， the target shale interval is generally characterized by high biogenic silica content and low total organic carbon （TOC）. The biogenic silica primarily occupies medium-small pores. Conclusions The relatively poor hydrocarbon generation capacity limits hydrocarbon accumulation in the Wulalike Formation shale. Biogenic silica acts primarily as cementation reducing porosity rather than framework support enhancing porosity. The upper section of the target layer is predominantly mixed shale. Although it exhibits high clastic content and high porosity， it suffers from low TOC content and poor hydrocarbon generation and expulsion capacity. Transitioning towards the bottom， it changes to biogenic silica-rich shale. Although this section has a relatively higher TOC content， the cementation effect of biogenic silica adversely affects reservoir properties. Integrating the characteristics of pore space， it is suggested that the interbedded intervals of multiple shale lithofacies located above the zones where biogenic silica-rich shale develops are expected to indicate potential shale gas enrichment zones even within the low TOC background.
Objective Investigating the coupling relationships between the differential characteristics of marine shale pore spaces and lithofacies， material composition， pore structure， as well as their implications for shale gas enrichment patterns， is crucial for evaluating the gas-bearing potential of the Wulalike Formation shale in the western Ordos Basin. Methods Taking the Wulalike Formation shale in the western Ordos Basin as the research object， and based on the classification of typical shale lithofacies， this study analyzed the influence of material composition， siliceous genesis， and pore structure on shale pore space through microscopy and scanning electron microscopy observations， helium porosity， pulse permeability， nuclear magnetic resonance， nitrogen and carbon dioxide adsorption， and methane isothermal adsorption tests. The vertical enrichment intervals of shale gas were also investigated. Results In the study area， the porosity of the Wulalike Formation shale shows an increasing trend from the bottom to the top， with no significant correlation between porosity and permeability. Unlike the Longmaxi Formation shale， the target shale interval is generally characterized by high biogenic silica content and low total organic carbon （TOC）. The biogenic silica primarily occupies medium-small pores. Conclusions The relatively poor hydrocarbon generation capacity limits hydrocarbon accumulation in the Wulalike Formation shale. Biogenic silica acts primarily as cementation reducing porosity rather than framework support enhancing porosity. The upper section of the target layer is predominantly mixed shale. Although it exhibits high clastic content and high porosity， it suffers from low TOC content and poor hydrocarbon generation and expulsion capacity. Transitioning towards the bottom， it changes to biogenic silica-rich shale. Although this section has a relatively higher TOC content， the cementation effect of biogenic silica adversely affects reservoir properties. Integrating the characteristics of pore space， it is suggested that the interbedded intervals of multiple shale lithofacies located above the zones where biogenic silica-rich shale develops are expected to indicate potential shale gas enrichment zones even within the low TOC background.
2025, 43(4): 1251-1263.
doi: 10.14027/j.issn.1000-0550.2024.085
Abstract:
Objective This study examines the microscopic characteristics of laminated shale reservoirs in the Third member of the Middle Ordovician Wulalike Formation at the western margin of the Ordos Basin， to further understand their effectiveness. Methods An integrated approach combining X-ray diffraction （XRD）， scanning electron microscopy （SEM）， mineral mapping， organic geochemical analysis， CO2 and N2 adsorption and mercury injection capillary pressure tests to compare the mineralogy， organic geochemistry and pore structures of different textures within the laminated shales. Results These laminated shales comprise limestone and tuff laminae and silica-rich beds. The limestone and tuff laminae （density of 60-180/m） have lower organic carbon content than the silica-rich beds. The laminated shales contain both pores and fractures. Clay mineral-related intergranular and intragranular pores are abundant in the silica-rich beds， with rare organic pores. Calcite and dolomite intragranular dissolution pores are predominant in the limestone laminae. The tuff laminae contain pyrite intercrystalline pores， clay-related intergranular and intragranular pores and dolomite intragranular dissolution pores. Dissolution pores in carbonate minerals in the limestone and tuff laminae are related to dissolution of acid fluids generated by thermal evolution of organic matter in the silica-rich beds. Near-horizontal fractures are evident along boundaries between limestone and tuff laminae and silica-rich beds. The density of near-horizontal fractures is 63-130/m， and they are 0.2-4.9 mm wide. Limestone and tuff laminae have fewer micropores but more macropores than the silica-rich beds. Limestone laminae contain 2.5-4.3 times the volume of macropores， and these contribute 1.9-2.1 times the total pore volume found in adjacent silica-rich beds. The volume of macropores in the tuff laminae is 1.5-2.3 times greater than in adjacent silica-rich beds. Conclusions In laminated shale reservoirs， oil and gas generated in silica-rich beds migrate along pores and fractures， and preferentially enrich in adjacent limestone and tuff laminae. Limestone and tuff laminae contain more macropores and more free gas than silica-rich beds. The laminated shale reservoirs in the Wulalike Formation contain a high proportion of free gas related to limestone and tuff laminae.
Objective This study examines the microscopic characteristics of laminated shale reservoirs in the Third member of the Middle Ordovician Wulalike Formation at the western margin of the Ordos Basin， to further understand their effectiveness. Methods An integrated approach combining X-ray diffraction （XRD）， scanning electron microscopy （SEM）， mineral mapping， organic geochemical analysis， CO2 and N2 adsorption and mercury injection capillary pressure tests to compare the mineralogy， organic geochemistry and pore structures of different textures within the laminated shales. Results These laminated shales comprise limestone and tuff laminae and silica-rich beds. The limestone and tuff laminae （density of 60-180/m） have lower organic carbon content than the silica-rich beds. The laminated shales contain both pores and fractures. Clay mineral-related intergranular and intragranular pores are abundant in the silica-rich beds， with rare organic pores. Calcite and dolomite intragranular dissolution pores are predominant in the limestone laminae. The tuff laminae contain pyrite intercrystalline pores， clay-related intergranular and intragranular pores and dolomite intragranular dissolution pores. Dissolution pores in carbonate minerals in the limestone and tuff laminae are related to dissolution of acid fluids generated by thermal evolution of organic matter in the silica-rich beds. Near-horizontal fractures are evident along boundaries between limestone and tuff laminae and silica-rich beds. The density of near-horizontal fractures is 63-130/m， and they are 0.2-4.9 mm wide. Limestone and tuff laminae have fewer micropores but more macropores than the silica-rich beds. Limestone laminae contain 2.5-4.3 times the volume of macropores， and these contribute 1.9-2.1 times the total pore volume found in adjacent silica-rich beds. The volume of macropores in the tuff laminae is 1.5-2.3 times greater than in adjacent silica-rich beds. Conclusions In laminated shale reservoirs， oil and gas generated in silica-rich beds migrate along pores and fractures， and preferentially enrich in adjacent limestone and tuff laminae. Limestone and tuff laminae contain more macropores and more free gas than silica-rich beds. The laminated shale reservoirs in the Wulalike Formation contain a high proportion of free gas related to limestone and tuff laminae.
2025, 43(4): 1264-1274.
doi: 10.14027/j.issn.1000-0550.2024.029
Abstract:
Significance Macrocyclic alkanes are monocyclic saturated hydrocarbons with more than ten ring carbons （n>10）. They have been detected in oil shales， crude oils， soils and higher plants， carry unambiguous biological source information， thus serving as precise proxies for original depositional environments. Their carbon-number distribution is variable， and they co-elute at regular intervals with long-chain alkyl alkenes and alkyl cyclohexanes. Moreover， their mass-spectrometric molecular ions （CnH2n） and diagnostic fragment ions （m/z 97， 111， 125） are identical to those compounds， making misidentification common. Accurate structural characterization is therefore urgently needed. Progress We conducted a comprehensive literature review and isolated individual macrocyclic alkanes from torbanite of the Sydney Basin， Australia. High-resolution nuclear magnetic resonance （NMR） spectroscopy elucidated their unique monocyclic saturated skeletons. By analogy with cyclohexane fragmentation， three dominant mass-spectrometric cleavage pathways were proposed， fully explaining the genesis of the characteristic fragments. Possible biochemical synthesis pathway was synthesized from previous reports. Conclusions and Prospects NMR analyses confirm that macrocyclic alkanes possess distinctive monocyclic saturated structures. In torbanite， those derived from Botryococcus appear to originate from long-chain oleic acid that cyclizes via long-chain unsaturated polyaldehyde intermediates. Future work should refine the biosynthetic pathways for higher-plant-derived macrocyclic alkanes， establish relevant ratio parameters， and apply them to geological samples.
Significance Macrocyclic alkanes are monocyclic saturated hydrocarbons with more than ten ring carbons （n>10）. They have been detected in oil shales， crude oils， soils and higher plants， carry unambiguous biological source information， thus serving as precise proxies for original depositional environments. Their carbon-number distribution is variable， and they co-elute at regular intervals with long-chain alkyl alkenes and alkyl cyclohexanes. Moreover， their mass-spectrometric molecular ions （CnH2n） and diagnostic fragment ions （m/z 97， 111， 125） are identical to those compounds， making misidentification common. Accurate structural characterization is therefore urgently needed. Progress We conducted a comprehensive literature review and isolated individual macrocyclic alkanes from torbanite of the Sydney Basin， Australia. High-resolution nuclear magnetic resonance （NMR） spectroscopy elucidated their unique monocyclic saturated skeletons. By analogy with cyclohexane fragmentation， three dominant mass-spectrometric cleavage pathways were proposed， fully explaining the genesis of the characteristic fragments. Possible biochemical synthesis pathway was synthesized from previous reports. Conclusions and Prospects NMR analyses confirm that macrocyclic alkanes possess distinctive monocyclic saturated structures. In torbanite， those derived from Botryococcus appear to originate from long-chain oleic acid that cyclizes via long-chain unsaturated polyaldehyde intermediates. Future work should refine the biosynthetic pathways for higher-plant-derived macrocyclic alkanes， establish relevant ratio parameters， and apply them to geological samples.
2025, 43(4): 1275-1292.
doi: 10.14027/j.issn.1000-0550.2023.108
Abstract:
Significance Beach-bar sand bodies in coastal or lake-shore areas may develop into an important type of hydrocarbon reservoir. There are certain differences in coastal and lake-shore depositional environments and patterns. Numerous studies have been conducted on the architecture of lake-shore beach-bar sand bodies， research on the architecture of beach-bar sand bodies needs to be strengthened. This present study is a broad review of reported research on marine beach-bar deposits in terms of their sedimentary characteristics and the hydrodynamic mechanism of marine beach-bar sand， and it considers the factors influencing the sedimentary architecture pattern. Progress The results show that beach-bar deposits are dominated by sand bar development， which in turn is influenced by several hydrodynamic situations （e.g.， nearshore spiral current， longshore current， rip current， swash current， and reverse current at the seafloor） that form the various types of sand bar （e.g.， linear， coastal， sand bar spits， plume， tongue-like and oblique sand bars， etc.）. Their architectural characteristics and genesis also differ. Among the multiplicity of factors influencing sand bar sedimentary architecture are shoreline morphology， sea-level change， supply of sediment and wave interactions that determine the mode of wave action， all affecting sand bar development and stacking pattern. Tectonics and sedimentary paleomorphology control the geographic placement of the sand bar. Three levels of beach-bar architecture were established： composite， single bars and intra-bar accretionary bodies， and a preliminary beach-bar architecture model is proposed. Conclusions and prospects Architectural prototypes were modeled using data from field outcrops， modern sedimentation， subsurface well information and sediment simulation. These enrich sedimentological theories and serve to guide the development of marine beach-bar oil and gas reservoirs.
Significance Beach-bar sand bodies in coastal or lake-shore areas may develop into an important type of hydrocarbon reservoir. There are certain differences in coastal and lake-shore depositional environments and patterns. Numerous studies have been conducted on the architecture of lake-shore beach-bar sand bodies， research on the architecture of beach-bar sand bodies needs to be strengthened. This present study is a broad review of reported research on marine beach-bar deposits in terms of their sedimentary characteristics and the hydrodynamic mechanism of marine beach-bar sand， and it considers the factors influencing the sedimentary architecture pattern. Progress The results show that beach-bar deposits are dominated by sand bar development， which in turn is influenced by several hydrodynamic situations （e.g.， nearshore spiral current， longshore current， rip current， swash current， and reverse current at the seafloor） that form the various types of sand bar （e.g.， linear， coastal， sand bar spits， plume， tongue-like and oblique sand bars， etc.）. Their architectural characteristics and genesis also differ. Among the multiplicity of factors influencing sand bar sedimentary architecture are shoreline morphology， sea-level change， supply of sediment and wave interactions that determine the mode of wave action， all affecting sand bar development and stacking pattern. Tectonics and sedimentary paleomorphology control the geographic placement of the sand bar. Three levels of beach-bar architecture were established： composite， single bars and intra-bar accretionary bodies， and a preliminary beach-bar architecture model is proposed. Conclusions and prospects Architectural prototypes were modeled using data from field outcrops， modern sedimentation， subsurface well information and sediment simulation. These enrich sedimentological theories and serve to guide the development of marine beach-bar oil and gas reservoirs.
2025, 43(4): 1293-1307.
doi: 10.14027/j.issn.1000-0550.2024.095
Abstract:
Objective The Emeishan large igneous province （ELIP）， located in southwest China， experienced extensive post-eruption erosion resulting in the accumulation of voluminous volcanic detritus in the adjacent Youjiang Basin during the Late Permian to Early Triassic. These clastic sediments not only record the types and composition characteristics of the denuded volcanic rocks at the top of the ELIP， but also reveal the possible magmatic evolution trend of the denuded volcanic sequence in the ELIP. Therefore， the Lower Triassic clastic rocks in northwest Guizhou are of great significance for the systematic understanding of the denudation sequence and magmatic evolution. Methods To further clarify the Early Triassic erosion evolution， we analyzed the provenance of the Lower Triassic Feixianguan Formation in the Weining area， southwestern Guizhou province， and discussed the erosion and magmatic process in the late stage of the ELIP. Results The mudstones of the Feixianguan Formation are composed of quartz， feldspar， calcite， volcanic lithic fragments， and clay minerals. The structures of volcanic lithic fragments indicate a basaltic and felsic volcanic source. The mudstones have high ICV（the Index of Compositional Variability） values， which reflects that the clastic rocks of the Fexianguan Formation in Weining area are derived from the first cyclic clastic rocks. The mudstone， which of Al2O3/TiO2 ratio are slightly higher than the high-Ti basalt of the ELIP， is characterized by high Fe2O3 and MgO contents. It indicates a dominant source from the Emeishan high-Ti basalt. Detrital zircons from the Fexianguan Formation have an age peak of ca. 260 Ma， which support the ELIP as the main provenance. This is consistent with the provenance of the Late Permian clastic sediments in the Youjiang Basin. Compared with the underlying Longtan Formation in the Upper Permian， the Lower Triassic Fexianguan Formation in Weining area， of which ~260 Ma zircons have higher Th/Nb and U/Yb ratios， have more zircons with ages of >300 Ma. Such features show a systematic variation in sedimentary sequence. Conclusions In the source-sink sedimentary system composed of the Youjiang Basin and ELIP， the zircon geochemical characteristics of detrital sediments preserved in the Late Permian and Early Triassic Induan likely represent the inherent characteristics of the denuded volcanic sequence. Such zircon geochemical characteristics reflect a magmatic evolution process with dimi-nishing crustal assimilation in the late-stage of the ELIP.
Objective The Emeishan large igneous province （ELIP）， located in southwest China， experienced extensive post-eruption erosion resulting in the accumulation of voluminous volcanic detritus in the adjacent Youjiang Basin during the Late Permian to Early Triassic. These clastic sediments not only record the types and composition characteristics of the denuded volcanic rocks at the top of the ELIP， but also reveal the possible magmatic evolution trend of the denuded volcanic sequence in the ELIP. Therefore， the Lower Triassic clastic rocks in northwest Guizhou are of great significance for the systematic understanding of the denudation sequence and magmatic evolution. Methods To further clarify the Early Triassic erosion evolution， we analyzed the provenance of the Lower Triassic Feixianguan Formation in the Weining area， southwestern Guizhou province， and discussed the erosion and magmatic process in the late stage of the ELIP. Results The mudstones of the Feixianguan Formation are composed of quartz， feldspar， calcite， volcanic lithic fragments， and clay minerals. The structures of volcanic lithic fragments indicate a basaltic and felsic volcanic source. The mudstones have high ICV（the Index of Compositional Variability） values， which reflects that the clastic rocks of the Fexianguan Formation in Weining area are derived from the first cyclic clastic rocks. The mudstone， which of Al2O3/TiO2 ratio are slightly higher than the high-Ti basalt of the ELIP， is characterized by high Fe2O3 and MgO contents. It indicates a dominant source from the Emeishan high-Ti basalt. Detrital zircons from the Fexianguan Formation have an age peak of ca. 260 Ma， which support the ELIP as the main provenance. This is consistent with the provenance of the Late Permian clastic sediments in the Youjiang Basin. Compared with the underlying Longtan Formation in the Upper Permian， the Lower Triassic Fexianguan Formation in Weining area， of which ~260 Ma zircons have higher Th/Nb and U/Yb ratios， have more zircons with ages of >300 Ma. Such features show a systematic variation in sedimentary sequence. Conclusions In the source-sink sedimentary system composed of the Youjiang Basin and ELIP， the zircon geochemical characteristics of detrital sediments preserved in the Late Permian and Early Triassic Induan likely represent the inherent characteristics of the denuded volcanic sequence. Such zircon geochemical characteristics reflect a magmatic evolution process with dimi-nishing crustal assimilation in the late-stage of the ELIP.
2025, 43(4): 1308-1321.
doi: 10.14027/j.issn.1000-0550.2023.141
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， and these events have received extensive attention in recent years. An abrupt negative excursion in carbon isotopes （δ13C） was recorded near the Late Pennsylvanian Kasimovian-Gzhelian Boundary （KGB）， accompanied by significant global warming. The KGB warming event against the background of the interglacial period of the icehouse climate is of great interest， but the study of this warming event is still in its initial stages. Methods Here， detailed sedimentological and carbon isotope 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 in South China. Results Four sedimentary lithofacies were identified-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 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
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， and these events have received extensive attention in recent years. An abrupt negative excursion in carbon isotopes （δ13C） was recorded near the Late Pennsylvanian Kasimovian-Gzhelian Boundary （KGB）， accompanied by significant global warming. The KGB warming event against the background of the interglacial period of the icehouse climate is of great interest， but the study of this warming event is still in its initial stages. Methods Here， detailed sedimentological and carbon isotope 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 in South China. Results Four sedimentary lithofacies were identified-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 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
Discussion of the Characteristics and Causes of Different Types of Quartz Grain Boundary Dissolution
2025, 43(4): 1322-1335.
doi: 10.14027/j.issn.1000-0550.2023.103
Abstract:
Objective There is a clear correlation between the boundary condition of quartz particles and their dissolution. This relationship is systematically summarized here to determine the types and characteristics of quartz dissolution. Methods Thin section identification， cathodoluminescence and scanning electron microscopy were used in combination with detailed observation and statistical analysis to determine the quartz boundary dissolution characteristics in rock from the second member of the Xujiahe Formation， western Sichuan. The processes of different types of quartz boundary dissolution were determined and the mechanisms were analyzed. The study provides new ideas and references for dissolution analysis in sandstone skeleton particles， and the genesis of pores in tight sandstone. Results Two types of boundary dissolution in quartz particles were revealed： （1） smooth boundary-dissolved pore-increasing， in which the boundary of the quartz particles is seen to be smooth and clear， and pores are formed after dissolution； and （2） fuzzy metasomatic boundary dissolution seen as indistinct and rough quartz particle boundaries caused by the presence of dissolved and metasomatized carbonates or clay minerals producing either fuzzy boundary-dissolution carbonates or fuzzy boundary-dissolution clay minerals. Type 1 is the result of organic acid dissolution in acidic conditions. Type 2 metasomatism between carbonate ions and quartz particles is due to differences in ion concentration in alkaline conditions. Clay mineral metasomatism occurs when alkaline metal ions are released to produce a ‘salt effect’， accelerating the dissolution rate of quartz particles. Metasomatized quartz particles are covered with a clay film. The dissolution boundaries of the quartz particles tend to change from type 1 to type 2； the alteration favors the formation of a hydrocarbon reservoir. Quartz particles in the second member of the Xujiahe Formation in the study area had a surface area corrosion loss of 4.33%-8.67% （average 6.37%）. Up to 72% of all quartz particles showed dissolved surfaces， averaging 63.02%. The quartz content in thin sections was about 45%-96% （average 75.3%）. A statistical analysis of the results showed quartz dissolution surface porosity between 2.05% and 4.09% （average 3.19%； R2 =0.792 6）. Conclusions The proportion of partially dissolved quartz particles effectively alters the pore structure of tight sandstone， thus favoring reservoir development by providing a reasonable amount of secondary pores and increasing reservoir space for oil and gas.
Objective There is a clear correlation between the boundary condition of quartz particles and their dissolution. This relationship is systematically summarized here to determine the types and characteristics of quartz dissolution. Methods Thin section identification， cathodoluminescence and scanning electron microscopy were used in combination with detailed observation and statistical analysis to determine the quartz boundary dissolution characteristics in rock from the second member of the Xujiahe Formation， western Sichuan. The processes of different types of quartz boundary dissolution were determined and the mechanisms were analyzed. The study provides new ideas and references for dissolution analysis in sandstone skeleton particles， and the genesis of pores in tight sandstone. Results Two types of boundary dissolution in quartz particles were revealed： （1） smooth boundary-dissolved pore-increasing， in which the boundary of the quartz particles is seen to be smooth and clear， and pores are formed after dissolution； and （2） fuzzy metasomatic boundary dissolution seen as indistinct and rough quartz particle boundaries caused by the presence of dissolved and metasomatized carbonates or clay minerals producing either fuzzy boundary-dissolution carbonates or fuzzy boundary-dissolution clay minerals. Type 1 is the result of organic acid dissolution in acidic conditions. Type 2 metasomatism between carbonate ions and quartz particles is due to differences in ion concentration in alkaline conditions. Clay mineral metasomatism occurs when alkaline metal ions are released to produce a ‘salt effect’， accelerating the dissolution rate of quartz particles. Metasomatized quartz particles are covered with a clay film. The dissolution boundaries of the quartz particles tend to change from type 1 to type 2； the alteration favors the formation of a hydrocarbon reservoir. Quartz particles in the second member of the Xujiahe Formation in the study area had a surface area corrosion loss of 4.33%-8.67% （average 6.37%）. Up to 72% of all quartz particles showed dissolved surfaces， averaging 63.02%. The quartz content in thin sections was about 45%-96% （average 75.3%）. A statistical analysis of the results showed quartz dissolution surface porosity between 2.05% and 4.09% （average 3.19%； R2 =0.792 6）. Conclusions The proportion of partially dissolved quartz particles effectively alters the pore structure of tight sandstone， thus favoring reservoir development by providing a reasonable amount of secondary pores and increasing reservoir space for oil and gas.
2025, 43(4): 1336-1343.
doi: DOI:10.14027/j.issn.1000-0550.2023.081
Abstract:
Objective The second section and one oil formation of the Zhujiang Formation in Wenchang A oilfield belong to the littoral facies sand body. Owing to the frequent turbulence and changes of the water body during the sedimentary period， internal layers and calcareous inter-layers developed. At the ultra-high water-cut stage， the remaining oil distribution is complex because of the influence of the inter-layer， and it is urgent to determine the inter-layer distribution to clarify the subsequent digging direction. However， the thickness of the inter-layer is below 2 m， making it difficult to predict. Methods Based on the type and origin of the inter-layers and according to the guidance of reservoir architecture theory， the "graded division， genesis analysis， interface identification and quantitative characterization" method was used to identify and characterize the inter-layers and remaining oil resolution. Results The 7-3 architecture interface inside the sand body was characterized， and the remaining oil distribution pattern can be divided into two types： " roof type oil" controlled by parallel inter-layers and "angle oil" controlled by crossed inter-layers. The development target is adjusted to the remaining oil enrichment area controlled by the inter-layers， and a strategy is proposed for the first time from "oil digging at the top above water-layers" to "oil digging in the gap". Conclusions The daily oil output can be increased to 127 square meters per day， and the cumulative oil increase is 65 700 cubic meters. This study is important for the remaining oil tapping and oil recovery enhancement at the high-water-cut stage.
Objective The second section and one oil formation of the Zhujiang Formation in Wenchang A oilfield belong to the littoral facies sand body. Owing to the frequent turbulence and changes of the water body during the sedimentary period， internal layers and calcareous inter-layers developed. At the ultra-high water-cut stage， the remaining oil distribution is complex because of the influence of the inter-layer， and it is urgent to determine the inter-layer distribution to clarify the subsequent digging direction. However， the thickness of the inter-layer is below 2 m， making it difficult to predict. Methods Based on the type and origin of the inter-layers and according to the guidance of reservoir architecture theory， the "graded division， genesis analysis， interface identification and quantitative characterization" method was used to identify and characterize the inter-layers and remaining oil resolution. Results The 7-3 architecture interface inside the sand body was characterized， and the remaining oil distribution pattern can be divided into two types： " roof type oil" controlled by parallel inter-layers and "angle oil" controlled by crossed inter-layers. The development target is adjusted to the remaining oil enrichment area controlled by the inter-layers， and a strategy is proposed for the first time from "oil digging at the top above water-layers" to "oil digging in the gap". Conclusions The daily oil output can be increased to 127 square meters per day， and the cumulative oil increase is 65 700 cubic meters. This study is important for the remaining oil tapping and oil recovery enhancement at the high-water-cut stage.
2025, 43(4): 1344-1360.
doi: 10.14027/j.issn.1000-0550.2023.085
Abstract:
Objective The Liushagang Formation is one of the petroliferous reservoirs in the Fushan Sag， Beibuwan Basin. However， the factors controlling reservoir quality have been unclear and there is no matched prediction technology for reservoir quality， which restricts the oil and gas exploration and development process. Nearly 65 years of oil exploration and geological research has revealed multiple sets of oil-bearing target systems in the Weizhou and Liushagang Formations. Further clarification is urgently needed regarding their development and to determine the distribution of reservoirs in this area， in particular to define reservoir control factors and predict the most favorable direction for further oil and gas reservoir exploration， and also to form a set of logging prediction methods appropriate to the study area. Methods Large numbers of core analysis experiments （particle size， sorting， contact mode etc.） and the analysis of thin sections， scanning electron microscopy information， physical properties， petrology and other data were used to estimate reservoir space and the physical properties of the reservoir. Beginning with aspects of early sedimentation， late diagenetic transformation and tectonic action， the sedimentary characteristics， diagenetic types and strength and fracture development of the Liushagang Formation are discussed. The controlling factors of reservoir quality are unraveled from the perspective of “three-element reservoir controls”： sedimentary microfacies， diagenetic facies and fracture facies. Methods of logging characterization and identification standards are also established. Comprehensive histograms and fracture identification divisions of sedimentary microfacies and diagenetic facies logging are completed for a single well， and the comprehensive response characteristics and development criteria for high-quality reservoir logging are clarified. Results It was found that the sedimentary microfacies， diagenetic facies and fracture details are important factors in determining reservoir quality in the Liushagang Formation. The main sedimentary microfacies developed in the study area are underwater distributary channel， estuary bar， far sand bar， and semi-deep to deep lake mudstone-equivalent sedimentary microfacies. The main diagenetic facies are unstable component dissolution facies， clay mineral infill， carbonate cementation and compacted dense facies. The fractures are mainly either horizontal or low-angle. By analyzing the relationships between different types of sedimentary microfacies， diagenetic and fracture facies and reservoir quality parameters， it is concluded that high-quality reservoirs were formed from dissolution facies in the underwater distributary channel and mouth bar in a high-energy depositional environment. The presence of fractures has also significantly improved reservoir quality. Four types of reservoir are classified by integrating the three controlling factors （sedimentary microfacies + diagenetic facies + fracture facies） and combining them with four important reservoir property parameters. The conclusions from oil and gas interpretation and oil test data indicate that the proposed method effectively evaluates and predicts favorable reservoir layers. Conclusions This study provides theoretical guidance for increasing the number of reservoirs and the production of oil and gas from the Liushagang Formation in the Fushan Sag. It also provides insights into the detailed description of reservoirs and the prediction and evaluation of high-quality reservoirs.
Objective The Liushagang Formation is one of the petroliferous reservoirs in the Fushan Sag， Beibuwan Basin. However， the factors controlling reservoir quality have been unclear and there is no matched prediction technology for reservoir quality， which restricts the oil and gas exploration and development process. Nearly 65 years of oil exploration and geological research has revealed multiple sets of oil-bearing target systems in the Weizhou and Liushagang Formations. Further clarification is urgently needed regarding their development and to determine the distribution of reservoirs in this area， in particular to define reservoir control factors and predict the most favorable direction for further oil and gas reservoir exploration， and also to form a set of logging prediction methods appropriate to the study area. Methods Large numbers of core analysis experiments （particle size， sorting， contact mode etc.） and the analysis of thin sections， scanning electron microscopy information， physical properties， petrology and other data were used to estimate reservoir space and the physical properties of the reservoir. Beginning with aspects of early sedimentation， late diagenetic transformation and tectonic action， the sedimentary characteristics， diagenetic types and strength and fracture development of the Liushagang Formation are discussed. The controlling factors of reservoir quality are unraveled from the perspective of “three-element reservoir controls”： sedimentary microfacies， diagenetic facies and fracture facies. Methods of logging characterization and identification standards are also established. Comprehensive histograms and fracture identification divisions of sedimentary microfacies and diagenetic facies logging are completed for a single well， and the comprehensive response characteristics and development criteria for high-quality reservoir logging are clarified. Results It was found that the sedimentary microfacies， diagenetic facies and fracture details are important factors in determining reservoir quality in the Liushagang Formation. The main sedimentary microfacies developed in the study area are underwater distributary channel， estuary bar， far sand bar， and semi-deep to deep lake mudstone-equivalent sedimentary microfacies. The main diagenetic facies are unstable component dissolution facies， clay mineral infill， carbonate cementation and compacted dense facies. The fractures are mainly either horizontal or low-angle. By analyzing the relationships between different types of sedimentary microfacies， diagenetic and fracture facies and reservoir quality parameters， it is concluded that high-quality reservoirs were formed from dissolution facies in the underwater distributary channel and mouth bar in a high-energy depositional environment. The presence of fractures has also significantly improved reservoir quality. Four types of reservoir are classified by integrating the three controlling factors （sedimentary microfacies + diagenetic facies + fracture facies） and combining them with four important reservoir property parameters. The conclusions from oil and gas interpretation and oil test data indicate that the proposed method effectively evaluates and predicts favorable reservoir layers. Conclusions This study provides theoretical guidance for increasing the number of reservoirs and the production of oil and gas from the Liushagang Formation in the Fushan Sag. It also provides insights into the detailed description of reservoirs and the prediction and evaluation of high-quality reservoirs.
2025, 43(4): 1361-1370.
doi: 10.14027/j.issn.1000-0550.2023.087
Abstract:
Objective It is important for reservoir quality evaluation and geological modeling of unconventional oil and gas reservoirs to know the direct relationship between sedimentary environment and seismic elastic response of organic shales. Methods The sedimentary characteristics and lithofacies of the Longmaxi Shale in the Zigong area were characterized， four depositional units （I-A， I-B， I-C and II） were identified， and the influence of sedimentary environment on the elastic characteristics of shale of Longmaxi Formation in Zigong area， Sichuan Basin was analyzed. Results The results show that the sedimentary environment primarily controls the elastic characteristics of the shale in the Longmaxi Formation in two aspects： （1） owing to the difference of rock structure caused by hydrodynamic action， the elastic characteristics of shale in the deep water shelf environment are clearly different from those of argillaceous siltstone in the overlying turbidite environment； （2） in the deep water shelf environment， the variation of elastic characteristics is controlled by the precession system and water depth. In addition， quartz and total organic carbon （TOC） content have a competitive relationship in influencing the elastic characteristics of the reservoir. Finally， the prediction ability of pre-stack amplitude vs. offset （AVO） inversion to seismic elastic attributes is used to trace the sedimentary evolution process， which provides insight into 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. By analyzing the influence of quartz and TOC content on the elastic characteristics of reservoir， the softening effect caused by organic matter was found to be dominant when the quartz content is low， and the hardening effect of quartz grains dominant when the quartz content is high.
Objective It is important for reservoir quality evaluation and geological modeling of unconventional oil and gas reservoirs to know the direct relationship between sedimentary environment and seismic elastic response of organic shales. Methods The sedimentary characteristics and lithofacies of the Longmaxi Shale in the Zigong area were characterized， four depositional units （I-A， I-B， I-C and II） were identified， and the influence of sedimentary environment on the elastic characteristics of shale of Longmaxi Formation in Zigong area， Sichuan Basin was analyzed. Results The results show that the sedimentary environment primarily controls the elastic characteristics of the shale in the Longmaxi Formation in two aspects： （1） owing to the difference of rock structure caused by hydrodynamic action， the elastic characteristics of shale in the deep water shelf environment are clearly different from those of argillaceous siltstone in the overlying turbidite environment； （2） in the deep water shelf environment， the variation of elastic characteristics is controlled by the precession system and water depth. In addition， quartz and total organic carbon （TOC） content have a competitive relationship in influencing the elastic characteristics of the reservoir. Finally， the prediction ability of pre-stack amplitude vs. offset （AVO） inversion to seismic elastic attributes is used to trace the sedimentary evolution process， which provides insight into 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. By analyzing the influence of quartz and TOC content on the elastic characteristics of reservoir， the softening effect caused by organic matter was found to be dominant when the quartz content is low， and the hardening effect of quartz grains dominant when the quartz content is high.
2025, 43(4): 1371-1385.
doi: 10.14027/j.issn.1000-0550.2023.100
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 a high abundance of β-carotane. However， there is significant variation in its contents across the section. To further clarify the variation characteristics of β-carotane in P2l2 and explore the main controlling factors of its enrichment. Methods Organic geochemical analysis was conducted， and the investigated source rocks were divided into groups I， II and III from bottom to top based on the β-carotane index. The β-carotane index was low in group I and gradually increased from bottom to top in group II， whereas the β-carotane index in group III fluctuated greatly. There are two high-value centers in the plane near the Southwest Thrust Nappe. Results The results show that the relative content of β-carotane effectively reflects the quality of source rocks in the study area. The organic matter of the P2l2 source rocks in Malang Sag is composed of lower aquatic organisms. However， compared with the group I， groups II and III have a richer supply of prokaryote organic matter. The values of steranes/hopanes， C28 steranes/C29 steranes， and （Pr+Ph）/（nC17+nC18） indicated that the precursors of β-carotane in groups II and III may be algae with phytol side chains of chlorophyll a， b， and c and bacteri. Based on the parameters of Pr/Ph， C35 hopanes/C34 hopanes， and extended tricyclic terpane ratio （ETR）， that the water salinity of source rocks in groups II and III is found to be high and primarily 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， this 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 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 a high abundance of β-carotane. However， there is significant variation in its contents across the section. To further clarify the variation characteristics of β-carotane in P2l2 and explore the main controlling factors of its enrichment. Methods Organic geochemical analysis was conducted， and the investigated source rocks were divided into groups I， II and III from bottom to top based on the β-carotane index. The β-carotane index was low in group I and gradually increased from bottom to top in group II， whereas the β-carotane index in group III fluctuated greatly. There are two high-value centers in the plane near the Southwest Thrust Nappe. Results The results show that the relative content of β-carotane effectively reflects the quality of source rocks in the study area. The organic matter of the P2l2 source rocks in Malang Sag is composed of lower aquatic organisms. However， compared with the group I， groups II and III have a richer supply of prokaryote organic matter. The values of steranes/hopanes， C28 steranes/C29 steranes， and （Pr+Ph）/（nC17+nC18） indicated that the precursors of β-carotane in groups II and III may be algae with phytol side chains of chlorophyll a， b， and c and bacteri. Based on the parameters of Pr/Ph， C35 hopanes/C34 hopanes， and extended tricyclic terpane ratio （ETR）， that the water salinity of source rocks in groups II and III is found to be high and primarily 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， this 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 controlled by the source of organic matter and sedimentary environment.
2025, 43(4): 1386-1397.
doi: 10.14027/j.issn.1000-0550.2023.109
Abstract:
Objective Microbial remediation of oil pollution has gained significant attention owing to its environmentally friendly nature and the absence of secondary pollution. To enhance the biological remediation technology for oil pollution， this study screened efficient petroleum hydrocarbon-degrading bacteria to understand and assess their degradation mechanisms. Using soils from the oil-polluted site in the Changqing Oilfield as the microbial source， strains with efficient degradation capabilities towards n-hexadecane were identified. Methods Through morphological characteristics， Gram staining， biochemical and physicochemical tests， 16s rDNA sequence analysis， and resistance experiments， strain Acinetobacter sp. 5-5 and its optimal degradation conditions for n-hexadecane were investigated. Based on zero-order， quasi-first-order， and quasi-second-order kinetic models， the degradation process of n-hexadecane was studied. Additionally， the acidity-alkalinity of the culture medium and the degradation product spectrum were analyzed to identify the degradation pathway of n-hexadecane. Results The results indicated that the strain belonged to Acinetobacter sp.， and at 0.5% （V/V） initial concentration， 0.5% salinity， and pH 7.0， the total degradation rate of n-hexadecane by the petroleum hydrocarbon-degrading bacteria reached 99.24%， with a rapid degradation rate of 82.13% within two days. The kinetics of the degradation process revealed that the quasi-first-order model had the best fitting effect for the substrate degradation of n-hexadecane. The preliminary analysis suggested that the degradation pathway of n-hexadecane in this system involved chain breaking and acid production processes. Conclusions This research demonstrated that petroleum hydrocarbon degrading bacterium Acinetobacter sp. 5-5 had a strong degradation effect on n-hexadecane when the salinity was as high as 0.5%， and the degradation rate reached more than 99%， indicating that this strain had a high potential for alkane degradation and good salinity tolerance and was expected to be widely used in the remediation of petroleum-contaminated salinized soils.
Objective Microbial remediation of oil pollution has gained significant attention owing to its environmentally friendly nature and the absence of secondary pollution. To enhance the biological remediation technology for oil pollution， this study screened efficient petroleum hydrocarbon-degrading bacteria to understand and assess their degradation mechanisms. Using soils from the oil-polluted site in the Changqing Oilfield as the microbial source， strains with efficient degradation capabilities towards n-hexadecane were identified. Methods Through morphological characteristics， Gram staining， biochemical and physicochemical tests， 16s rDNA sequence analysis， and resistance experiments， strain Acinetobacter sp. 5-5 and its optimal degradation conditions for n-hexadecane were investigated. Based on zero-order， quasi-first-order， and quasi-second-order kinetic models， the degradation process of n-hexadecane was studied. Additionally， the acidity-alkalinity of the culture medium and the degradation product spectrum were analyzed to identify the degradation pathway of n-hexadecane. Results The results indicated that the strain belonged to Acinetobacter sp.， and at 0.5% （V/V） initial concentration， 0.5% salinity， and pH 7.0， the total degradation rate of n-hexadecane by the petroleum hydrocarbon-degrading bacteria reached 99.24%， with a rapid degradation rate of 82.13% within two days. The kinetics of the degradation process revealed that the quasi-first-order model had the best fitting effect for the substrate degradation of n-hexadecane. The preliminary analysis suggested that the degradation pathway of n-hexadecane in this system involved chain breaking and acid production processes. Conclusions This research demonstrated that petroleum hydrocarbon degrading bacterium Acinetobacter sp. 5-5 had a strong degradation effect on n-hexadecane when the salinity was as high as 0.5%， and the degradation rate reached more than 99%， indicating that this strain had a high potential for alkane degradation and good salinity tolerance and was expected to be widely used in the remediation of petroleum-contaminated salinized soils.
2025, 43(4): 1398-1411.
doi: 10.14027/j.issn.1000-0550.2024.056
Abstract:
Objective Marine-terrestrial transitional source rocks are the main source rocks in several offshore basins in China， and their differential distribution characteristics restrict the prediction accuracy of source rocks and the effectiveness of oil and gas exploration. In addition， the transport and sedimentation process of terrestrial organic matter determines the quality and distribution of source rocks in a marine-terrestrial transitional environment. Methods Using a combination of flume sedimentary simulation and three-dimensional （3D） laser scanning technology， the dynamic recording and quantitative characterization of the transport process of terrestrial dispersed organic matter under different water salinity conditions were conducted using forward modeling. Machine learning algorithms were used to establish a total organic carbon （TOC） prediction model. Results The results show that terrestrial organic matter in the marine-terrestrial transitional environment is mainly enriched in the delta front and pro-delta. As the transportation distance increases， the abundance of terrestrial organic matter first increased and then decreased. Under the influence of salt flocculation， the transportation distance of terrestrial organic matter in the saltwater environment is closer to the source area， and the sediment thickness is larger. A TOC prediction model was established under experimental conditions based on three deep learning algorithms and， ultimately， the prediction model based on random forest algorithm with outlier removal and experience based sedimentary facies assignment as input features was selected as the optimal model. Conclusions The TOC prediction model under experimental conditions is combined with geological conditions to complete the TOC prediction of source rocks in the Yacheng Formation of the Yanan depression. The results show that the transportation distance of terrestrial organic matter in the Yanan Depression can reach 50 km， and the highest degree of organic matter enrichment occurs at a distance of approximately 31 km from the source area.
Objective Marine-terrestrial transitional source rocks are the main source rocks in several offshore basins in China， and their differential distribution characteristics restrict the prediction accuracy of source rocks and the effectiveness of oil and gas exploration. In addition， the transport and sedimentation process of terrestrial organic matter determines the quality and distribution of source rocks in a marine-terrestrial transitional environment. Methods Using a combination of flume sedimentary simulation and three-dimensional （3D） laser scanning technology， the dynamic recording and quantitative characterization of the transport process of terrestrial dispersed organic matter under different water salinity conditions were conducted using forward modeling. Machine learning algorithms were used to establish a total organic carbon （TOC） prediction model. Results The results show that terrestrial organic matter in the marine-terrestrial transitional environment is mainly enriched in the delta front and pro-delta. As the transportation distance increases， the abundance of terrestrial organic matter first increased and then decreased. Under the influence of salt flocculation， the transportation distance of terrestrial organic matter in the saltwater environment is closer to the source area， and the sediment thickness is larger. A TOC prediction model was established under experimental conditions based on three deep learning algorithms and， ultimately， the prediction model based on random forest algorithm with outlier removal and experience based sedimentary facies assignment as input features was selected as the optimal model. Conclusions The TOC prediction model under experimental conditions is combined with geological conditions to complete the TOC prediction of source rocks in the Yacheng Formation of the Yanan depression. The results show that the transportation distance of terrestrial organic matter in the Yanan Depression can reach 50 km， and the highest degree of organic matter enrichment occurs at a distance of approximately 31 km from the source area.
2025, 43(4): 1412-1428.
doi: 10.14027/j.issn.1000-0550.2023.093
Abstract:
Objective Several provenance systems exist in the southern Ordos region， including the Qinling and Qilian orogenic belts. Quantitative knowledge of the elements of each source-to-sink system is relatively weak， which restricts the study of the differences between source-to-sink systems and also the structure-sedimentary filling process constrained by them. Methods This study used sedimentology， geochemistry and detrital zircon dating， etc.， to examine the differences between unit elements in the early Permian Shanxi Formation （e.g.， tectonic setting， paleoslope and depositional system of each source-to-sink system）. These were clarified by reconstructing a structure-sedimentary model of the Shanxi Formation influenced by combinations of the various source-to-sink systems in the southern basin. Results （1） Three major source-to-sink systems were active during the sedimentary period in the southern Ordos area of the Shanxi Formation： North Qilian， and the eastern and western regions of North Qinling. The North Qilian system is enriched in light REE； the mean δEu is 0.60， and the mean paleoslope in the source direction is 0.045°. Light REE are obviously enriched in the source-to-sink system in the western part of North Qinling， with an average δEu of 0.75 and an average paleoslope of 0.04°； and in the eastern part of North Qinling the average δEu value is 1.05 and average paleoslope is 0.048°. （2） The source-to-sink systems were derived from the Central Asian orogenic belt， the North China Craton basement， the eastern and western areas of North Qinling and the North Qilian orogenic belt， but the North China Craton basement was the main source. （3） An active continental margin dominated the tectonic background of the provenances of all source-to-sink systems， and was followed by a passive continental margin. Conclusions Due to Mianlue Ocean subduction， the tectonic setting of the southern Ordos region was an active continental margin with continuous plate convergence. The source area in eastern North Qinling experienced the greatest uplift. The late Shanxi was relatively sand-rich， dominated by transitional deposits from the delta plain to the front， and having the smallest extension. The source region in the western part of North Qinling had the lowest uplift， with rich sand in the early stage and poor sandy sediments later. The source region of North Qilian had moderate uplift， and was dominated by the delta plain transporting deposits to the front margin.
Objective Several provenance systems exist in the southern Ordos region， including the Qinling and Qilian orogenic belts. Quantitative knowledge of the elements of each source-to-sink system is relatively weak， which restricts the study of the differences between source-to-sink systems and also the structure-sedimentary filling process constrained by them. Methods This study used sedimentology， geochemistry and detrital zircon dating， etc.， to examine the differences between unit elements in the early Permian Shanxi Formation （e.g.， tectonic setting， paleoslope and depositional system of each source-to-sink system）. These were clarified by reconstructing a structure-sedimentary model of the Shanxi Formation influenced by combinations of the various source-to-sink systems in the southern basin. Results （1） Three major source-to-sink systems were active during the sedimentary period in the southern Ordos area of the Shanxi Formation： North Qilian， and the eastern and western regions of North Qinling. The North Qilian system is enriched in light REE； the mean δEu is 0.60， and the mean paleoslope in the source direction is 0.045°. Light REE are obviously enriched in the source-to-sink system in the western part of North Qinling， with an average δEu of 0.75 and an average paleoslope of 0.04°； and in the eastern part of North Qinling the average δEu value is 1.05 and average paleoslope is 0.048°. （2） The source-to-sink systems were derived from the Central Asian orogenic belt， the North China Craton basement， the eastern and western areas of North Qinling and the North Qilian orogenic belt， but the North China Craton basement was the main source. （3） An active continental margin dominated the tectonic background of the provenances of all source-to-sink systems， and was followed by a passive continental margin. Conclusions Due to Mianlue Ocean subduction， the tectonic setting of the southern Ordos region was an active continental margin with continuous plate convergence. The source area in eastern North Qinling experienced the greatest uplift. The late Shanxi was relatively sand-rich， dominated by transitional deposits from the delta plain to the front， and having the smallest extension. The source region in the western part of North Qinling had the lowest uplift， with rich sand in the early stage and poor sandy sediments later. The source region of North Qilian had moderate uplift， and was dominated by the delta plain transporting deposits to the front margin.
2025, 43(4): 1429-1445.
doi: 10.14027/j.issn.1000-0550.2023.099
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 it 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 owing to the tectonic activity of the periphery orogenic belt. Methods 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 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 derived from the Inner Mongolia uplift （Seltenshan， Wula-Daqingshan， Jining area） and the Central Asian orogenic belt in the active continental margin. The southern part is supplied by clastic material from the North Qinling， and the tectonic background of the source area is relatively complex， which is a mixture of a passive continental margin and continental island arc. In the North Qinling， the tectonic environment includes stretching to collision and extrusion until a trench-arc-basin is formed. The parent rocks in the northern and southern source areas are a mixture of granitic rock and alkaline basalt， and the northern and southern provenance systems converge in the well Lian 45-Chengjiagou area. Conclusions Overall， the Central Asian orogenic belt of the Benxi Formation has a high degree of uplift and denudation on both sides of the study area， reflecting the characteristics of irregular oblique collision in the northern North China Plate. The northern margin of the North China Plate has 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. With a strong source supply， the sand body extends farther in the northeast of the study area. The southern part of the Ordos Basin is the source area of the North Qinling Mountains， and the southern margin of the North China Plate rises at this time， developing the barrier coastal sedimentary system. 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 owing to the transformation of the 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 it 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 owing to the tectonic activity of the periphery orogenic belt. Methods 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 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 derived from the Inner Mongolia uplift （Seltenshan， Wula-Daqingshan， Jining area） and the Central Asian orogenic belt in the active continental margin. The southern part is supplied by clastic material from the North Qinling， and the tectonic background of the source area is relatively complex， which is a mixture of a passive continental margin and continental island arc. In the North Qinling， the tectonic environment includes stretching to collision and extrusion until a trench-arc-basin is formed. The parent rocks in the northern and southern source areas are a mixture of granitic rock and alkaline basalt， and the northern and southern provenance systems converge in the well Lian 45-Chengjiagou area. Conclusions Overall， the Central Asian orogenic belt of the Benxi Formation has a high degree of uplift and denudation on both sides of the study area， reflecting the characteristics of irregular oblique collision in the northern North China Plate. The northern margin of the North China Plate has 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. With a strong source supply， the sand body extends farther in the northeast of the study area. The southern part of the Ordos Basin is the source area of the North Qinling Mountains， and the southern margin of the North China Plate rises at this time， developing the barrier coastal sedimentary system. 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 owing to the transformation of the eastward and north-east currents.
2025, 43(4): 1446-1460.
doi: 10.14027/j.issn.1000-0550.2023.113
Abstract:
Objective The Congo Fan Basin is an important strategic area for overseas energy. The reservoir distribution and its mechanism are key issues for exploration in deepwater and ultra-deepwater areas. Methods Based on the maximum area of multi-block three-dimensional seismic and well data thus far， this study investigated the morphological characteristics， distribution and mechanism of turbidity channels in the main target strata （Oligocene-Miocene） in the Lower Congo Basin. Results The tertiary turbidity channel is vertically composed of four strata： channel lag deposit， high NTG （Net to Gross） stacked channel fill， low NTG channel-levee complex， and muddy filling， which correspond to four channel evolution stages： erosion， filling， overflow， and dying. Confined-erosion-valley channel， confined-erosion-stacking channel complex， weakly confined-erosion-stacking channel complex， non/weakly confined-stacking-single channel， and stacking-lobes are developed from source to sea successively. From the Oligocene to Miocene， the channel scale of the Congo Fan Basin first increased and then decreased， forming the four stages. The sedimentary system deposited and moved northward over the past million years. The Early Oligocene supply surging and the West African continental margin uplift initiated the large-scale development of a deep water turbidite. The climatic events and sea level drop in the Middle Miocene led to the maximum development of the sedimentary system in the Middle Miocene. The multi-breaks formed by faults and salt structures on the slope as well as the geomorphology controlled the unloading area of the sand. The influence of salt structures on sand distribution was reflected in macro-scale regional sand enrichment and small-scale local control of “eight kinds of three types" on channels. Conclusions The revelation of reservoir distribution and multi-factor control mechanism is important for energy strategic and scientific research development.
Objective The Congo Fan Basin is an important strategic area for overseas energy. The reservoir distribution and its mechanism are key issues for exploration in deepwater and ultra-deepwater areas. Methods Based on the maximum area of multi-block three-dimensional seismic and well data thus far， this study investigated the morphological characteristics， distribution and mechanism of turbidity channels in the main target strata （Oligocene-Miocene） in the Lower Congo Basin. Results The tertiary turbidity channel is vertically composed of four strata： channel lag deposit， high NTG （Net to Gross） stacked channel fill， low NTG channel-levee complex， and muddy filling， which correspond to four channel evolution stages： erosion， filling， overflow， and dying. Confined-erosion-valley channel， confined-erosion-stacking channel complex， weakly confined-erosion-stacking channel complex， non/weakly confined-stacking-single channel， and stacking-lobes are developed from source to sea successively. From the Oligocene to Miocene， the channel scale of the Congo Fan Basin first increased and then decreased， forming the four stages. The sedimentary system deposited and moved northward over the past million years. The Early Oligocene supply surging and the West African continental margin uplift initiated the large-scale development of a deep water turbidite. The climatic events and sea level drop in the Middle Miocene led to the maximum development of the sedimentary system in the Middle Miocene. The multi-breaks formed by faults and salt structures on the slope as well as the geomorphology controlled the unloading area of the sand. The influence of salt structures on sand distribution was reflected in macro-scale regional sand enrichment and small-scale local control of “eight kinds of three types" on channels. Conclusions The revelation of reservoir distribution and multi-factor control mechanism is important for energy strategic and scientific research development.
2025, 43(4): 1461-1474.
doi: 10.14027/j.issn.1000-0550.2023.119
Abstract:
Objective Building an isochronous shale facies combination framework for stratigraphic science based on Milankovitch theory and designing an accurate shale facies characterization method are important for evaluating the vertical and horizontal heterogeneity changes of shale facies combinations. This is achieved by studying regional sedimentary patterns. Methods We accurately calculated the composition of shale minerals through Lithoscanner elemental scanning logging and multi mineral models， classifying the lithofacies. Additionally， the Ipsom supervised neural network algorithm was used to conduct gas well lithofacies identification without Lithoscanner elemental scanning logging data. We extract the Milankovitch cycles from the strata through time series analysis methods such as spectral analysis， optimal sedimentation rate estimation， and Gaussian filtering. Results The 1st member of Longmaxi Formation in the Weirong area contains 12 lithofacies. Using the top surface of the Guanyinqiao section as an anchor point， eight fourth order sequences and 24-26 fifth order sequences can be identified. Using the upper and lower boundaries of the fourth order sequence as the framework for lithofacies assemblages， the E1 to E4 lithofacies assemblages in the middle and lower parts of the Longyi member were shown to exhibit multiple lithofacies types and rapid changes in the proportion of the main lithofacies. The vertical and horizontal changes of the E5 to E8 lithofacies assemblages are relatively small. Conclusions By studying the differences and changes in lithofacies assemblages to determine the sedimentary pattern of shale in the Weirong Long Formation， we determined that the vertical and horizontal differences in lithofacies assemblages are primarily influenced by marine productivity and land source supply in central Sichuan. Overall， the quantitative research results of shale facies combinations and the local sedimentary patterns provide new ideas for predicting favorable lithofacies locations within the region and more accurately predicting favorable areas for oil and gas exploration.
Objective Building an isochronous shale facies combination framework for stratigraphic science based on Milankovitch theory and designing an accurate shale facies characterization method are important for evaluating the vertical and horizontal heterogeneity changes of shale facies combinations. This is achieved by studying regional sedimentary patterns. Methods We accurately calculated the composition of shale minerals through Lithoscanner elemental scanning logging and multi mineral models， classifying the lithofacies. Additionally， the Ipsom supervised neural network algorithm was used to conduct gas well lithofacies identification without Lithoscanner elemental scanning logging data. We extract the Milankovitch cycles from the strata through time series analysis methods such as spectral analysis， optimal sedimentation rate estimation， and Gaussian filtering. Results The 1st member of Longmaxi Formation in the Weirong area contains 12 lithofacies. Using the top surface of the Guanyinqiao section as an anchor point， eight fourth order sequences and 24-26 fifth order sequences can be identified. Using the upper and lower boundaries of the fourth order sequence as the framework for lithofacies assemblages， the E1 to E4 lithofacies assemblages in the middle and lower parts of the Longyi member were shown to exhibit multiple lithofacies types and rapid changes in the proportion of the main lithofacies. The vertical and horizontal changes of the E5 to E8 lithofacies assemblages are relatively small. Conclusions By studying the differences and changes in lithofacies assemblages to determine the sedimentary pattern of shale in the Weirong Long Formation， we determined that the vertical and horizontal differences in lithofacies assemblages are primarily influenced by marine productivity and land source supply in central Sichuan. Overall， the quantitative research results of shale facies combinations and the local sedimentary patterns provide new ideas for predicting favorable lithofacies locations within the region and more accurately predicting favorable areas for oil and gas exploration.
2025, 43(4): 1475-1488.
doi: 10.14027/j.issn.1000-0550.2023.120
Abstract:
Objective Significant breakthroughs have been made in the exploration of Jurassic continental shale oil in the Sichuan Basin. Owing to the small grain size， rapid facies changes， and strong heterogeneity of continental mud shale facies， it is 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 and fifteen short eccentricity cycles. The long and short eccentricity cycles 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. （2） Considering 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 and exogenous shell-bearing mudstone and shell limestone facies） and seven lithofacies types （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， and 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 primarily 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. Conclusions Long eccentricity is a key factor controlling the distribution of lithofacies and organic matter enrichment. It controls the ordered development of the 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 the exogenous shell-bearing mudstone.
Objective Significant breakthroughs have been made in the exploration of Jurassic continental shale oil in the Sichuan Basin. Owing to the small grain size， rapid facies changes， and strong heterogeneity of continental mud shale facies， it is 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 and fifteen short eccentricity cycles. The long and short eccentricity cycles 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. （2） Considering 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 and exogenous shell-bearing mudstone and shell limestone facies） and seven lithofacies types （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， and 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 primarily 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. Conclusions Long eccentricity is a key factor controlling the distribution of lithofacies and organic matter enrichment. It controls the ordered development of the 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 the exogenous shell-bearing mudstone.
2025, 43(4): 1489-1506.
doi: 10.14027/j.issn.1000-0550.2023.076
Abstract:
Purpose The organic-rich mudstone of lacustrine facies developed in the Chang 73 sub-member of the Yanchang Formation in the Ordos Basin. It is important to clarify the genesis mechanism and depositional process of organic-rich mud shale and establish the deposition model for shale oil exploration. Methods Based on the typical outcrop observation， core description， thin section observation， X-ray diffraction， main trace element geochemical analysis， and total organic carbon （TOC） analysis in the Chang 73 sub-section of the Ordos Basin， we investigated the depositional environment and distribution characteristics of the organic-rich mud shale. Results Six mud shale facies developed in the Chang 73 sub-member： middle organic matter discontinuous striated mudstone phases （Mdm）， middle organic matter grain sequence stratified siltstone mudstone phase （Mgm）， organic-rich fuzzy-grained chalky shale phase （Rfs）， organic-rich deformed-microwave shale phase （Rds）， organic-rich clear-grained shale phase （Rcs）， and organic-rich tuffaceous blocky shale phase （Rts）， with increasing organic matter content. The Chang 73 sub-member was deposited in a freshwater， anoxic reduction environment with a warm and humid climate， and the overall deposition rate is low. Conclusions The large scale， high abundance of organic matter and good continuity of organic-rich shale deposit developed in the center and southeast of the lake basin and are significantly correlated with salinity and detritus input. More detritus input leads to the oxidative degradation and dilution of organic matter. The local brackish hydration and strong reduction environment caused by volcanic hydrothermal activities is conducive to the enrichment and preservation of organic matter.
Purpose The organic-rich mudstone of lacustrine facies developed in the Chang 73 sub-member of the Yanchang Formation in the Ordos Basin. It is important to clarify the genesis mechanism and depositional process of organic-rich mud shale and establish the deposition model for shale oil exploration. Methods Based on the typical outcrop observation， core description， thin section observation， X-ray diffraction， main trace element geochemical analysis， and total organic carbon （TOC） analysis in the Chang 73 sub-section of the Ordos Basin， we investigated the depositional environment and distribution characteristics of the organic-rich mud shale. Results Six mud shale facies developed in the Chang 73 sub-member： middle organic matter discontinuous striated mudstone phases （Mdm）， middle organic matter grain sequence stratified siltstone mudstone phase （Mgm）， organic-rich fuzzy-grained chalky shale phase （Rfs）， organic-rich deformed-microwave shale phase （Rds）， organic-rich clear-grained shale phase （Rcs）， and organic-rich tuffaceous blocky shale phase （Rts）， with increasing organic matter content. The Chang 73 sub-member was deposited in a freshwater， anoxic reduction environment with a warm and humid climate， and the overall deposition rate is low. Conclusions The large scale， high abundance of organic matter and good continuity of organic-rich shale deposit developed in the center and southeast of the lake basin and are significantly correlated with salinity and detritus input. More detritus input leads to the oxidative degradation and dilution of organic matter. The local brackish hydration and strong reduction environment caused by volcanic hydrothermal activities is conducive to the enrichment and preservation of organic matter.
2025, 43(4): 1507-1521.
doi: 10.14027/j.issn.1000-0550.2023.096
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 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 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.
2025, 43(4): 1522-1534.
doi: DOI:10.14027/j.issn.1000-0550.2023.123
Abstract:
Objective Recently， the shale gas exploration of the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin experienced a significant breakthrough， which has aroused widespread attention towards the redox conditions and the organic matter enrichment mechanisms of the Qiongzhusi Formation during its deposition. Redox conditions in the 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 focused 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， combined with total organic carbon （TOC）， total sulfur （TS）， pyrite content， and previously published iron speciation data， we reconstructed the paleo-marine redox environment of the Qiongzhusi Formation during its deposition. On this basis， the relationship between the assemblage of pyrite morphological characteristics and redox condition changes was discussed. Results The redox environmental evolution of the Qiongzhusi Formation can be divided into three stages： 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 small in size， ranging from 2.2-18.4 μm， with an average value of 6.39±1.70 μm. More than 80% of the particle sizes range from 3-8 μm， reflecting that framboidal pyrites were formed in the water bodies during 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 and 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 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 increased in quantity， with the former 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 experienced a significant breakthrough， which has aroused widespread attention towards the redox conditions and the organic matter enrichment mechanisms of the Qiongzhusi Formation during its deposition. Redox conditions in the 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 focused 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， combined with total organic carbon （TOC）， total sulfur （TS）， pyrite content， and previously published iron speciation data， we reconstructed the paleo-marine redox environment of the Qiongzhusi Formation during its deposition. On this basis， the relationship between the assemblage of pyrite morphological characteristics and redox condition changes was discussed. Results The redox environmental evolution of the Qiongzhusi Formation can be divided into three stages： 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 small in size， ranging from 2.2-18.4 μm， with an average value of 6.39±1.70 μm. More than 80% of the particle sizes range from 3-8 μm， reflecting that framboidal pyrites were formed in the water bodies during 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 and 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 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 increased in quantity， with the former 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.
