Current Articles

2024, Volume 42,  Issue 3

Display Method:
2024, 42(3)
Special Issue Proposal
2024, 42(3): 1-4.
Paleoclimatic and Paleo-Oceanic Environment Evolution in the Frasnian-Famennian Transition: Potential causes of the biotic crisis
ZHANG LiYu, CHEN DaiZhao, LIU Kang
2024, 42(3): 723-737. doi: 10.14027/j.issn.1000-0550.2023.107
Objective The Late Devonian Frasnian-Famennian (F-F) transition is a critical time interval in geological history. This period saw major simultaneous marine ecological system changes that led to one of the Big Five mass extinctions in the Phanerozoic. The F-F event (also referred to as the Kellwasser event) was characterized by severe losses of low-latitude shallow-water benthic faunas, notably reef-dwelling coral and stromatoporoids. High-latitude, deep-sea and terrestrial faunas were the least affected. Various separate or combined hypotheses have been proposed as the causes of this mass extinction (sea-level change, marine anoxia, climate change, volcanic/hydrothermal activities, and bolide impact). Of these, climate change and marine anoxia have been the most intensively researched and discussed hypotheses based on the Web of Science in recent years. However, some controversies remain, and the interactions of certain factors are still unclear. Methods This study systematically reviews available reports on the paleoclimatic and paleo-oceanic changes during the F-F transition and discusses the anoxic model during this critical period, based on related case studies in southern China. Results and Conclusions Conodont oxygen and strontium isotopes, as well as the carbon isotope records for carbonates, collectively suggest a cold climate during the F-F transition interval, with several rapid warming-cooling fluctuations. Conodont oxygen isotopes suggest that the sea-sur face temperature (SST) dropped by 5 °C-8 °C. Strontium isotopes also imply that the temperature fluctuations were due to frequent, short-duration volcanism. Palynological data and carbonate platform exposure/karstification are also suggestive of coeval climate cooling. Additionally, marine anoxia has been extensively hypothesized as a possible killing mechanism in the F-F mass extinction, based initially on the presence of bituminous limestones (or black shales), named the lower and upper Kellwasser horizons. Studies of pyrite framboids, biomarker compounds, trace elements, isotopes of nitrogen, sulfur and uranium, and iron speciation have variously suggested the existence of the Kellwasser anoxic events. However, these generally occurred in geographically specific environments, notably in the pericontinental basins/sub-basins proximal to source hinterlands. Moreover, the range and degree of anoxia in the F-F transition also reportedly differed between study sections around the world. With regard to the anoxic model, studies have suggested that the Kellwasser anoxic events were caused by the increased nutrient input related to enhanced continental weathering. Marine anoxic studies from three F-F sections comprising different depositional facies in southern China also support this “top down” anoxic model. Clearly, the F-F biotic crisis was not caused by any single factor. Frequent short-term volcanic activity may have enhanced continental weathering and associated greenhouse gas emission, leading to frequent warming-cooling climate fluctuations and also increased oceanic nutrient input. In the latter case, eutrophication and anoxia would have occurred in shallow water, and the mutual interaction of the various environmental factors may have exerted biological pressure in the low-latitude shallow sea, and eventually led to the F-F biotic crisis.
Element Geochemical Characteristics and Their Geological Significance of Late Ordovician-Early Silurian Black Shale in Zhaotong Area of Northern Yunnan and Guizhou
LI Juan, CHEN Lei, HU Yue, JI YuBing, LU Chang, DONG JianHua, TAN XiuCheng, YANG Li, YANG Lin, CHEN Xin
2024, 42(3): 738-756. doi: 10.14027/j.issn.1000-0550.2024.038
Objective Located at the southern margin of the Sichuan Basin, the Zhaotong Demonstration Area of northern Yunnan and Guizhou was relatively shallower and closer to the source area than the Changning-Weiyuan area in the basin during the Late Ordovician-Early Silurian period. Therefore, the deposition of the Wufeng Formation-Longmaxi Formation shale in this area may be different from that in the basin,it is of great significance to clarify the siliceous genesis, provenance, and tectonic setting of the Late Ordovician-Early Silurian black shale in the Zhaotong area of northern Yunnan and Guizhou. Methods Using the Taiyang Block in the Zhaotong area as the research region, this study explores the source area background, source rock properties, and siliceous sources of the shale in the Wufeng Formation-Longmaxi Formation in the region by utilizing the testing data of major elements, trace elements, and rare earth elements from four wells within the area. Results The results show that the silica of the black shale mainly came from siliceous organisms and terrestrial clastic materials, and the biogenic silicon first increases and then decreases from bottom to top, showing a trend opposite that of terrestrial source silicon. There was a brief glacial period in the Late Ordovician, until the melting of the Early Silurian glaciers, the occurrence of marine intrusions, and then the gradual falling of sea level, leading to the continuous shallow water bodies. The overall manifestation of terrestrial input is characterized by first decreasing and then increasing, and the sedimentation rate also shows the same trend. The standardized distribution mode curve of chondrite meteorites of rare earth elements in certain samples fluctuated slightly, indicating that there may be a mixture source during the deposition of shale in the Wufeng Formation-Longmaxi Formation in the study area. Conclusions Therefore, the parent materials of the Wufeng Formation-Longmaxi Formation shale in the study area may come from granite and sedimentary rocks of the Kangdian paleocontinent and the central Guizhou uplift, which is affected by certain seafloor hydrothermal fluids during sedimentation, but the original hydrothermal components are extremely small. The relevant indicators comprehensively reflect that the tectonic background of the shale source area of the Wufeng Formation-Longmaxi Formation is a passive continental margin.
Marine Redox Environment and Organic Accumulation in Northeastern Sichuan Basin During the Late Permian
GE XiaoTong, WANG YuanZheng, CHEN DaiZhao, ZHANG GongJing, HUANG TaiYu, LI WangPeng
2024, 42(3): 757-773. doi: 10.14027/j.issn.1000-0550.2023.115
Objective The Late Permian was a critical interval in geological history, with dramatic changes in tectonics, paleoclimate, paleo-oceanic environment, volcanic activities, and the biosphere. Reconstruction of the paleogeography, paleoclimate, and paleo-oceanic environment during this interval could provide a more complete picture of the interactions and relationships among different geological factors, as well as their influence on organic matter enrichment and the evolution of the biosphere. Methods Organic-rich rock series were widely deposited during this period in the Sichuan Basin of the Upper Yangtze Block, providing insights into the co-evolution of the geosphere and biosphere. To better address the coeval temporal and spatial changes in the paleogeographic settings, paleo-oceanic environment, primary productivity, and organic enrichment, we conducted a detailed sedimentological investigation integrated (major and trace) element contents, organic carbon isotopes and total organic carbon (TOC) contents in the Jianfeng section, located in an intrashelf basin in northeastern Sichuan Basin. Results The redox-sensitive trace element data (MoXS, UXS, VXS contents, MoXS/UXS ratios, and MoEF-UEF relationship) show that the redox environment of the Jianfeng section has experienced four intervals (I-IV). Interval I (0-10.35 m, spanning from the upper part of the Wuchiaping Formation to the lower part of the Dalong Formation) was dominated by an oxic environment during deposition, Interval II (10.35-23.35 m, the middle Dalong Formation) experienced a ferruginous-dominant anoxic condition, Interval III (23.35-27.00 m, the upper Dalong Formation) was mainly a euxinic environment, and Interval IV (27.00-34.05 m, from the top of the Dalong Formation to the bottom of the Changhsing Formation) was dominated by an oxic or suboxic environment. Compared with the Xibeixiang section in the deeper water of the Kaijiang-Liangping trough, the redox environment in the intrashelf basin exhibits spatiotemporal heterogeneity. The nutrient-limiting elements (ZnXS, CuXS, NiXS) contents demonstrate that primary productivity is high in both intervals II2 and III, coincident with the increasing terrestrial inputs (dedicated by Al, Zr, and Ti contents and K/Al values). However, the Co×Mn and Cd/Mo values indicated that the upwelling currents arose in intervals II2, II3, III, and IV, and the volcanic activities were only frequent in Interval IV, disagreeing with the eutrophication and organic matter enrichment in the middle Dalong Formation. Therefore, the high primary productivity of the Dalong Formation may be principally affected by a large number of terrestrial inputs. Furthermore, the upwelling currents also made contributions to the prosperity of plankton, but the volcanic activities have poor relationships with it. Conclusions Combined with the Xibeixiang section in the study area, the formation of euxinic water mass was found to be closely related to the high primary productivity, whereas its spatiotemporal fluctuations were controlled by the sea-level changes, which could drive its upslope incursion in intervals II and III, downslope retreat in the Interval IV, and even demise in the Interval V. In turn, the primary productivity could be further controlled by the upwelling currents particularly on the basinal slope, and terrigenous inputs on the more updipping basinal margin. In this case, the primary productivity played a leading role in the enrichment of organic matter.
Sedimentological and Geochemical Characteristics of Late Permian Organic-Rich Rocks in North Sichuan and West Hubei Provinces
WEI HengYe, HU Die, QIU Zhen, ZHANG Xuan, LIU Wen, KONG WeiLiang, Ahmed Mansour
2024, 42(3): 774-798. doi: 10.14027/j.issn.1000-0550.2023.086
Objective The current focus of non-conventional petroleum theories is the accumulation mechanism of organic matter. One of the most commonly debated issues regards the main influences on organic matter enrichment previous studies lack analysis of the depositional process, which is the likely reason for the debate. Methods This study combines sedimentological and geochemical methods to conduct detailed sequence stratigraphy, and facies and elemental analyses, aiming to address the accumulation mechanism of sedimentary organic matter. Results The Late Permian Wuchiaping Formation and Dalong Formation in the northern Sichuan Basin and the West Hubei Basin are subdivided in ascending order into the First, Second and Third members of the Wuchiaping Formation, and the First and Second members of the Dalong Formation. The Late Permian is subdivided into five third-order stratigraphy sequences, SQ1 to SQ5. Analysis of the sequence stratigraphy revealed the four-stage developmental history of the rift basin in the northern Sichuan Basin and the West Hubei Basin: the initial stage followed by rapid rifting, rifting climax and a shrinking stage. The initial stage mainly coincided with the deposition of SQ2, and consists of dark cherty limestones of slope facies. The rapid rifting stage mainly occurred during SQ3, and comprises calcareous shales of deepwater shelf facies. The rifting climax stage mainly took place during SQ4, consisting of black bedded cherts of basin facies. The shrinking stage mainly occurred during SQ5, and consists of medium- or thin-bedded limestones. The sweet-spot interval developed mainly between the highstand system tract (HST) of SQ3 and the transgressive system tract (TST) of SQ5, during which the nutrient elements Fe, Cu, Ni, Zn and redox-sensitive elements Mo, V, Fe/Al and S were highly enriched. This sweet-spot interval is also associated with highly active hydrothermal activity indicated by Al/(Al+Fe+Mn) ratios < 0.6 and the Al-Fe-Mn diagram. Conclusions The development of high primary productivity occurred prior to the strongly reducing conditions during the deposition of the sweet-spot interval. This suggests that the reducing conditions in the bottom water resulted from high consumption of oxygen during the decomposition of organic matter, and that organic matter enrichment was mainly the result of primary productivity. The nutrient material input was related to hydrothermal activity and volcanism during the formation of the rifting basins. Episodic tectonic activity brought abundant nutrient elements to the rifting sea, enhancing the primary productivity level; the primary productivity level was the onset and basic condition for the extraordinarily high organic matter accumulation in the sweet-spot interval, and the strongly reducing conditions in the bottom water was the key factor for organic matter preservation and burial.
Geochemical Constraints on the Hydrothermal Chert of the Kuhfeng Formation in the Middle Permian in the Lower Yangtze and Its Significance
ZOU Yi, WEI HengYe
2024, 42(3): 799-811. doi: 10.14027/j.issn.1000-0550.2024.019
Objective The organic-enriched black bedded cherts in the Kuhfeng Formation were deposited in the Lower Yangtze during the Middle Permian. The hydrothermal cherts may be the key part for the relationship analysis between chert deposits and the extra-ordinarily high organic matter enrichment. This would be the basic theories for the shale gas exploration in this area. Methods This work uses major, minor, and rare-earth elements, combined with the petrology, to analyze the origin of chert and their tectonic setting for the Kuhfeng Formation in the Yeshan of Tongling and Pingdingshan of Chaohu, Anhui province. Results The black bedded cherts in the Chaohu area contain abundant siliceous sponge spicules and radiolarians. The cinerous cherts in the Kuhfeng Formation in the Tongling area contain extremely rare siliceous fossils and consist of micro-quartz. Hydrothermal proxies, such as Al/(Al+Fe+Mn), Eu/Eu*, Al-Fe-Mn diagram, and LuN/LaN, indicate a hydrothermal and biotic origin for the chert in the Kuhfeng Formation in the Tongling and Chaohu areas, respectively. The La-Th-Sc diagram and crossplot between La/Sc and Ti/Zr indicate oceanic island arc and active continental margin origins for the Tongling and Chaohu areas. Conclusions The chert in the Kuhfeng Formation in the Tongling area was primary deposited by hydrothermal activity. The silicic acid was brought by a hydrothermal vent near the slope environments. The black bedded cherts in the Lower Yangtze area are biotic in origin, but the source of silicon was related to the hydrothermal activity. The organic matter enrichment in the Kuhfeng Formation in the Lower Yangtze area may be related to the phytoplankton flourishment induced by the nutrient element input from hydrothermal activity.
Coloration and Environmental Significance of the Marine Red Bed from the Sowa Formation Carbonate in the Qiangtang Basin
LIU Mu, JI ChangJun, HUANG YuanGeng, DING Yi, CHEN RongQing, CHEN MingSi, YANG Bo, CHEN DaiZhao
2024, 42(3): 812-822. doi: 10.14027/j.issn.1000-0550.2023.062
Objective Marine red beds are distributed across various stages in geological history, are often interpreted as products of special environments, and have attracted widespread attention. The Sowa Formation in the Jurassic Qiangtang Basin is a set of sedimentary rocks composed of clastic and carbonate mixed sediments. In the Sowa Formation exposed in Bilocuo, Shuanghu county, Tibet, the red beds are mainly composed of carbonate sediments. Methods This study explored the causes of th red color and significance in the deposition and diagenesis of the red beds based on sedimentological, mineralogical, and geochemical methods. Results Field sedimentology and downhole thin section identification of these red beds revealed that the rocks are composed of grain-supported limestone, with abundant debris from cephalopods, echinoderms, bivalves, bryozoans, gastropods, and other benthic organisms. Biogenic debris and terrestrial quartz debris are the main components of the ooids, which display characteristics of a high-energy beach environment. Spectroscopic studies and scanning electron microscopy indicate that the main cause of the red color is submicron- to micron-sized pseudo-hexagonal and tetragonal hematite crystals. The coloration minerals are concentrated in the ooid cortex structure, and the cementing material is virtually devoid of coloration minerals. This suggests that during early diagenesis, particularly during the formation of ooids and peloids, iron-loving microorganisms played an important role in capturing iron oxides. Furthermore, the geochemical analysis of carbonate minerals shows that the redness is positively correlated with the flux of terrestrial debris input. Additionally, it exhibits a positive Ce/Ce* anomaly (1.1-1.3) and a pattern of enriched-middle rare earth elements “bulge”, indicating that the iron oxides underwent reductive dissolution. Conclusions Based on these findings, the study reconstructed the red coloration process of these red beds. The input of terrestrial iron-bearing minerals provided the material basis for the red coloration, and then, during early diagenesis, reductive fluids dissolved the iron-bearing minerals, providing free iron ions for iron-loving microorganisms, allowing them to recrystallize and ultimately form hydroxyl iron and cause red coloration through chemical or biological processes.
Cyclostratigraphic Analysis of the Lower Cambrian Shales in Western Hubei and Southern Guizhou
WEI XiaoSong, YAN DeTian, GONG Yin, NIU Xing, LIANG WanLe, FU HaiJiao, LIU ZiXuan, YANG XiangRong, ZHANG Bao
2024, 42(3): 823-838. doi: 10.14027/j.issn.1000-0550.2023.106
Objective The Lower Cambrian organic-rich shale is an important marine shale exploration target in western Hubei and southern Guizhou. Understanding the chronostratigraphic relationships and their underlying causes of organic-rich shale intervals in different areas is crucial for shale gas exploration. Methods In this study, we conducted cyclostratigraphic analyses using natural gamma-ray logging data from the Lower Cambrian well ND1 in western Hubei and the well HY1 in southern Guizhou and determined the primary astronomical periods. Using previously reported zircon ages as anchoring points, we constructed an astronomical time scale for the Lower Cambrian. Using a sedimentary noise model, relative sea-level changes in the Lower Cambrian were reconstructed. Results Our results reveal that wavelengths of 36 m and 9 m in well ND1 and 16.6 m and 4 m in well HY1 correspond to orbital 405 kyr and 100 kyr cycles, respectively. Furthermore, we calculated the durations of the Niutitang Formaion in well Niedi-1 and the Jiumengchong Formation in well HY1 to be 3.30 and 2.64 Myr, respectively, The astronomical age scales of two options are presented. In Option 1, the top of the Niutitang Formation in well ND1 serves as the starting point for the anchor point, with an age interval of 535.20±1.70 Ma to 538.50±1.70 Ma. Option 2 employs the top of the Jiumengchong Formation in well HY1 as the starting point of the anchor point, with the corresponding age interval spanning from 535.20±1.70 Ma to 537.84±1.70 Ma. Through correlation sea-level change curves (DYNOT and ρ1)with eccentricity cycles and sedimentary cycles, our results indicate that sea-level changes correspond to 1.2 Myr obliquity modulation cycles and two sedimentary sequences (SQ1 and SQ2). Comparing the total organic carbon (TOC) content of the Lower Cambrian organic-rich shale with orbital eccentricity, sedimentation rates, and sea-level changes, our findings reveal that the intervals with high TOC value correlate with the maximum eccentricity of 405 kyr, whereas those with low TOC value correspond to the minimum value in well ND1. In addition, the high TOC value corresponds to both the high sea level period and the low sea level period. Conclusions Because of the chaotic behavior of the solar system, accurately predicting the mechanism that drives organic-rich shale during the Early Cambrian is impossible. Assuming that the maximum eccentricity corresponds to the maximum organic carbon content at that time, strong seasonal variations have triggered the enrichment of black shale over a more extended period. Furthermore, the comparison between relative sea-level changes and organic carbon content reveals no causal relationship between sea-level fluctuations and the enrichment of organic matter.
Continental Sedimentary Microfacies Distribution of Mixed Fine-Grained Rocks and Its Controlling Effect on Sweet Spots
ZHU BaiYu, YIN SenLin, GUO HaiPing, TANG Pan, ZHAO JunWei, CHEN Xu, LEI ZhangShu
2024, 42(3): 839-856. doi: 10.14027/j.issn.1000-0550.2023.051
Objective The Permian Luhcaoao Formation in the Jimusaer Sag has developed a continental mixed sedimentary system. Insufficient research on the sedimentary microfacies types of mixed fine-grained rocks and the distribution pattern of sweet spots has hindered the development of shale oil in this area. Methods Using core, outcrop, logging, and X-ray fluorescence (XRF) element logging data, analytical testing, oil testing, and production are integrated by lithofacies, sedimentary facies coupling analysis, and dynamic static combination. The sedimentary micro-environment, microfacies classification, model, and its controlling factors on deserts were studied. Results (1) From the core, eight types of sedimentary structures were identified: wave cross bedding, horizontal bedding, syngenetic deformation structure, pyrite, calcite strip, birds-eye structure and nodule, suture structure, biological fossil, and dolomite dissolution hole. The sub-facies are divided into semi-deep to deep lacustrine, mixed shallow lacustrine, and clastic shallow lacustrine. These are further subdivided into six microfacies: semi deep lacustrine mud, dolomitic flat, mixed beach bar, mixed shallow lacustrine mud, shallow lacustrine sand bar, and clastic shallow lacustrine mud. The sedimentary characteristics and mechanism of different microfacies are different. (2) A sedimentary microfacies model of mixed fine-grained rocks in saline lakes has been established. No.1 member of the Lucaogou Formation evolved from deep to semi-deep to shallow lacustrine from bottom to top. Typical reservoir microfacies are developed in different layers, including shallow lacustrine sand bar, mixed beach bar, and dolomitic flat. Among them, dolomitic flat has a more continuous and large-scale sheet shape, and mixed beach bar has potato shape, with highly developed and different scales. The shallow lacustrine sand bar is intermittent and scale limited; vertically, it is located in the superposition of thin reservoir microfacies and thick mudstone. The Lucaogou Formation No.2 member is dominated by shallow lacustrine facies. The reservoir microfacies are similar to No.1, but the architecture style is different. The development quantity and scale of dolomitic flat and mixed beach bars are greatly reduced, and shallow lacustrine sand bars were developed, showing large-scale scattered potato shapes and thick layer superposition vertically. The profile shows that the stacking style of microfacies changed at different positions of the plane. (3) The deposition of the target layer is a transition environment from dry and hot brackish water to salt water, with a high level of paleo-productivity. The lower desert section was deposited in an oxygen enriched environment, but the upper section was deposited in an anaerobic sub reduction to reduction environment. (4) Sedimentary microfacies control desert distribution and oilfield development strategies. The mixed beach bar microfacies is a type I sweet spot reservoir with high development degree, weak heterogeneity and the best production effect. Large scale deployment of horizontal well platform operation is adopted for development. The dolomitic flat microfacies is a type III sweet spot reservoir, with strong heterogeneity and general development effect. It is a follow-up replacement oil and gas resource. The shallow lacustrine sand bar is a type II sweet spot reservoir being, with overall dispersion, poor continuity, strong heterogeneity, and good production effect. Conclusions After the detailed study of the spatial distribution of sedimentary microfacies, the horizontal well development is deployed with reference to the parameters of the nuclear magnetic resonance curve.
Geochemical Characteristics and Origins of the Cambrian-Ordovician Dolomites in Northern and Central Tarim Basin
2024, 42(3): 857-876. doi: 10.14027/j.issn.1000-0550.2023.128
Objective The genesis of dolomites is still controversy, and dolomite reservoirs play an important role in carbonate oil and gas exploration, whose reservoir properties (porosity and permeability) are largely influenced by the genesis types and texture characteristics of dolomites. In recent years, many large oil and gas fields have also been found in domestic dolomite reservoirs, such as Tarim Basin, Sulige gas field in Ordos Basin, Puguang and Yuanba gas field in Sichuan Basin and Anyue large or super large gas field. Therefore, systematic researches on the texture types and genetic mechanisms of dolomites yield profound theoretical significance, and also will promote greater advance in carbonate oil and gas exploration in China. The Tarim Basin shares large volume of oil and gas reserves, the exploration target horizons within which has gradually shifted from medium shallow- to medium-buried layers to ultra-deep layers. Especially, industrial oil and gas flows were encountered in the deeply-buried dolomites of the Lower Paleozoic in the Tarim Basin, making the formation, evolution and reservoir characteristics of deep dolomite reservoirs in this basin become the focus of scholars' research. However, the formation mechanism of the Cambrian-Ordovician dolomites in the Tarim Basin has not yet reached a consensus due to deep burial depth, complex genesis and difficult exploration of these dolomites. This study increases the understanding of their origin and provides theoretical support for oil and gas exploration in the region. Methods Detailed petrographic and geochemical (trace-rare earth elements, stable carbon and oxygen isotopes and 87Sr/86Sr ratios) studies of these dolomites were conducted on the Cambrian-Ordovician dolomites of northern and central Tarim Basin based on the international classification scheme of dolomite. Results and Conclusions According to the occurrence of dolomites, the Cambrian-Ordovician dolomites were divided into matrix and cement dolomites. Based on grain sizes, contact relationship between crystal planes (plane or curved surface), and crystal shape (euhedral, subhedral or anhedral), six types of dolomite structures were further identified for the matrix dolomite: (1) very fine to fine crystalline dolomite (Md1); (2) relict mimetic dolomite (Md2); (3) very fine to fine crystalline, planar-e(s) floating dolomite (Md3); (4) fine crystalline, planar-e(s) dolomite (Md4); (5) fine to coarse crystalline, nonplanar-a dolomite (Md5); and (6) coarse crystalline, nonplanar saddle dolomite (Md6). The distribution patterns of rare earth elements (REE) (slight enrichment or depletion of light REE, weak Eu negative or positive anomalies, and weak Ce negative or positive anomalies), δ13C values (-2.83‰ -1.72‰; average -1.64‰) and 87Sr/86Sr values (0.708 7~0.711 6; average 0.709 5) in the six matrices are similar to Cambrian-Ordovician micritic limestone and contemporaneous seawater. The diagenetic fluids of Md1 and Md2 are coeval seawater with varying degrees of evaporation and concentration, and were formed by (pene)contemporaneous dolomitization (including sabkha and reflux infiltration dolomitization) in relatively restricted depositional environments. The parent fluids of Md3 and Md4 are residual seawater, created by shallow-to medium-burial dolomitization. The diagenetic fluids of Md5 are variants of contemporaneous water, formed by deep-burial dolomitization or recrystallization of earlier dolomites. Deep hydrothermal fluids were responsible for the Md6 formation as a result of strong water-rock interactions with the host dolomites, influenced by mutual regulation and re-equilibration between the deep hydrothermal fluids and surrounding rocks.
Genesis of the Sinian-Cambrian Unconformity in the Northwestern Tarim Basin: Evidence from sedimentology
TANG Pan, WANG YuanZheng, LI ShuangJian, YANG Bo, WANG YaPing, YIN SenLin, CHEN GongYang, CHEN DaiZhao
2024, 42(3): 877-891. doi: 10.14027/j.issn.1000-0550.2023.074
Objective A Sinian-Cambrian parallel unconformity has been widely developed in the Aksu area of the northwestern Tarim Basin, and controversy remains over its formation mechanism. Methods Based on sedimentological research, the genesis of the above unconformity was studied through detailed field observation and microscopic identification, focusing on the sedimentary facies and karst phenomenon at the top of the Upper Sinian Qigebrak Formation. Results The results show that platform margin and slope facies are developed at the top of the Qigebrak Formation in the Aksu area. The platform margin includes grain beach and microbial reef, consisting of thick- to massive bedded dolo-oolite, medium- to thick-bedded intraclast dolopackstone and massive dolostromatolite reef. Lithofacies of the slope include: dolobreccia, turbidite, and thin- bedded dolowackestone. In the Late Sinian, the sedimentary water bodies in the eastern Aksu area were deeper, and the degree of karst development was significantly stronger than that in the western areas, which was dominated by horizontal phreatic zone vertically. In addition, in the Early Cambrian, the Yuertusi Formation maintained the same sedimentary trend as the top of the Qigebrak Formation, and both showed stratigraphic sequences of deepening water bodies from west to east. Conclusions This unconformity was formed by tilting movement induced by tectonic activity at the platform margin in the Late Sinian, which resulted in the uplift of the western Aksu area. In addition, this movement led to the atmospheric freshwater hydrological system flowing eastward, resulting in strong karstification in the eastern platform margin zone. This study could provide guidance for oil and gas explorations of the Sinian dolomite reservoirs in the region.
Depositional Facies and Sequence Stratigraphy of the Lower Ordovician Successions in Northern Guizhou Province
GUO Chuan, ZHANG WeiYuan, FU Yong, XIA Peng
2024, 42(3): 892-911. doi: 10.14027/j.issn.1000-0550.2023.101
Objective The Great Ordovician Biodiversification Event (GOE) is the result of the interaction between marine environments and organisms. The analyses of sedimentary facies and depositional sequences of the Ordovician carbonate platform can reveal its development and evolution, as well as the history of sea level fluctuations, which can provide a sedimentary background and isochronous stratigraphic framework for exploring the spatiotemporal distribution characteristics and the evolution of coeval organisms. Methods Based on the measurement of the outcrop sections and microscopic observation, the Lower Ordovician (Tongzi and Honghuayuan Formations) successions of the Piao'ertian section in northern Guizhou province were investigated to recognize the lithofacies types, construct the depositional model, identify the sequence stratigraphy, and explore the controls of depositional evolution. Results and Conclusions (1) Ten types of lithofacies were identified and further grouped into two depositional belts (or associations) in a carbonate ramp system. The depositional system was further divided into non-skeletal and skeletal grain-dominated end members for the Tongzi and Honghuayuan formations, respectively. (2) These lithofacies are vertically stacked into asymmetrical, meter-scale, shallowing-upward open-marine depositional cycles for both Tongzi and Honghuayuan Formations. (3) Based on the vertical facies and cycle stacking patterns, three and one half third-order depositional sequences (Sq1⁃Sq4) were distinguished, which are defined by II sequence boundaries (lithologic transition surfaces). Sq1⁃Sq3 are full sequences that consist of transgressive systems tract (TST) and upper regressive systems tract (RST) packages, but Sq4 only contains TST. (4) The depositional evolution and sequence development of the Lower Ordovician of the Piao'ertian section were controlled by relative sea-level oscillations of different orders and paleogeographic configuration. The relative sea-level oscillations of different orders dictated the vertical development of facies; the paleogeographic configuration primarily defined the spatial distribution of depositional belts (or associations).
Response of Rare Earth Elements in Crude Oil to Biodegradation: A case from the Miaoxi Sag, Bohai Bay Basin
SUN Peng, YANG HaiFeng, WANG FeiLong, TANG GuoMin, WANG GuangYuan, TANG YouJun
2024, 42(3): 912-927. doi: 10.14027/j.issn.1000-0550.2024.061
Objective Rare earth elements (REE) in crude oil contain valuable geochemical information and are widely used in oil-oil (source) comparisons and sedimentary environment reconstructions. However, the effect of biodegradation on REE concentrations and patterns remains unclear. Methods Crude oil in the Miaoxi Sag of the Bozhong Depression in the Bohai Bay Basin has been subjected to varying degrees of biodegradation. In this study, 14 crude oil samples from the area were tested for molecular markers and inorganic elements, using Gas chronmato- graphy-Mass spectrometry (GC-MS) and Inductively coupled plasma-Mass spectrometry (ICP-MS). The degree of biodegradation in the crude oil was recognized by n-alkane intactness and 25-norbornane content. Results It was found that the crude oil samples in the study area were distributed from PM0 to PM7, with a high biodegradation gradient. Large gammacerane content and high levels of C35 hopanes were detected in the high-sulfur oil, which suggests they were derived from the contribution of more reducing and high salinity Es4 source rocks. The low-sulfur oil contained low levels of gammacerane and C35 hopanes, high C27 disteranes and high 4-methyl steranes, mainly derived from Es3 source rocks. Conclusions Comparison revealed that lacustrine crude oils generally have higher REE concentrations than marine crude oils. The REE concentrations and distribution patterns are useful for distinguishing lacustrine oils from marine oils. Sedimentary environment and maturity have limited influence on the REE concentration and distribution pattern in crude oil, whereas biodegradation has a large influence on these factors. The REE concentration in crude oil increases during the biodegradation process, and the REE patterns and some indicators (e.g., LaN/YbN, δEu, δCe) also change regularly with the degree of biodegradation. In summary, REE concentrations are useful markers for evaluating the extent of biodegradation over a slight to extreme range and in various petroleum systems.
Paleogeographic Pattern of the Carbonate Platform in the Middle-Upper Yangtze Area During the Deposition of the Ediacaran Dengying Formation and Distribution Pattern of the Reservoir Facies
DING Yi, LIU ShuGen, WEN Long, WEN HuaGuo, CHEN DaiZhao, SONG JinMin, WANG Han, WANG LinKang, CHEN MingSi, CHEN RongQing, LI ZhiWu
2024, 42(3): 928-943. doi: 10.14027/j.issn.1000-0550.2023.135
Objective The dispute in the paleogeographic pattern of the carbonate platform and distribution of reservoir facies during the deposition of the Dengying Formation in the middle-upper area, hinders the expansion of the exploration domain from the Mianyang-Changning intracratonic sag to other areas. Methods Detailed lithofacies and facies analysis was conducted on more than 30 sections (wells), Results further confirming that the mound-shoal is marked by massive peloidal/ooidal dolograinstone and columnar, domal dolostromatolite, which were deposited around the platform (or inner ramp) margin of the Middle-Upper Yangtze Platform gradually shifting outward into tempestite of the middle-outer ramp or slump dolobreccia, argillaceous dolomite and chert of slope-basin facies. Owing to the mound-shoal barrier, its back was dominated by tidal flat and lagoon. Of these, the tidal flat facies were marked by microbial dololaminite, domal dolostromatolite, dolothrombolite with minor peloidal dolograinstone, and dolomudstone, which are commonly arranged into meter-centimeter scale cycles. The lagoon facies were composed mainly of dolomudstone and peloidal dolowackestone. In general, the mound-shoal facies is present in the lower Dengying Formation along the platform to slope transition (or inner ramp to middle ramp transition). In the Upper Yangtze Platform, it occurs in the Deng 1 (e.g., wells Pengtan 1, Ziyang 1, and sections Qingping, Beidoushan) or Deng 2 members at the Baijiaba, Yulin, and Meiziwan sections, or in both the Deng 1 and Deng 2 members at the Songlin-Yankong areas. On Middle Yangtze Platform, it occurs in the Hamajing (e.g., sections Sancha, Bailuya, and Tianping) or Baimatuo members of the upper Dengying Formation at the Miaohe section. Specifically, the mound-shoal facies shows repeated transgressions and regressions and occurs in different stratigraphic position of the Dengying Formation at many sections (Baiguoping, Xiaofenghe, Dengyingxia, Wushan, Xuejiadian, and Dengjiaya). In contrast, the tidal flat and lagoon facies are widely present on the whole Middle-Upper Yangtze Platform, commonly associated with the grape-like dolomite at the lower-middle part of the Deng 2 member (or correlated stratigraphy). The spatiotemporal distributions of these facies show a progradational trend of the Dengying Formation on the Middle-Upper Yangtze Platform, which consists of 2.5 depositional sequences with three progradations (shallowing) and two retrogradations (deepening). The regressive system tract of Sequence 1 is formed by the lower Dengying Formation (the Deng 1 to middle Deng 2 or Hamajing members), characterized by the upward evolution of lagoon or mound-shoal facies into tidal flat facies. On the Upper Yangtze Platform, Sequence 2 occurs in the upper Deng 2 member, marked by the regression of mound-shoal facies and the presence of more lagoonal dolomudstone, followed by the reoccurrence of tidal flat facies. On the Middle Yangtze Platform, Sequence 2 is present in the Shibantan member, represented by the deepening into middle ramp limestone or lagoonal dolomudstone and then shallowing into mound-shoal or tidal flat facies. Sequence 3 on the Middle Yangtze Platform shows a similar deepening (transgression) and shallowing (regression) cycle. On the Upper Yangtze Platform, the transgressive systems tract of Sequence 3 is located in the siliciclastic rock of the Deng 3 member and the following transgressive systems tract is marked by the transition of lagoonal dolomudstone to tidal flat dolomicrobialite. Conclusions Owing to the denudation of the Dengying Formation at the platform margin-slope area and Upper Yangtze Platform beyond the current plate boundary, the platform marginal mound-shoal facies are locally present in the Deng 4 member. In this case, certain areas (e.g., the Songlin-Yankong area and periphery of the Middle Yangtze Platform) with platform marginal mound-shoal facies in the lower-middle Dengying Formation (Deng 1 to 2 or Hamajing to Shibantan members) are important exploration targets. Moreover, the lower Deng 2 and upper Deng 4 members (or coeval strata) formed during progradations composed of microbial dolomite throughout the Middle-Upper Yangtze Platform (or inner ramp) interior are also important exploration targets. Because the microbial dolomite was widely developed throughout the Middle-Upper Yangtze Platform interior, decoding the formation mechanism and distribution pattern of high-quality reservoir is the key to finding new hydrocarbon provinces.
Geochemical Characteristics and Formation Mechanism of Sediments from the Challenger Deep in the Southern Mariana Trench
XIE TongTong, PENG XiaoTong, LIU ShuangQuan, XU HengChao, XU WenJing
2024, 42(3): 944-955. doi: 10.14027/j.issn.1000-0550.2022.104
Objective To investigate the hadal sediments main type, geochemical characteristics and formation in the Challenger Deep of the southern Mariana Trench, Methods the mineralogical and geochemical analysis were uesd to study the sediments and altered basalt from north slope(overlying plate) and axis to the south slope(subduction plate) at the Challenger Deep of the Mariana Trench. Results and Conclusions The hadal sediments can be divided into four types: red deep-sea clay, siliceous sediments, micromanganese-rich sediments, and calcareous sediments. The red deep-sea clay is the most common type of hadal sediment, which contribute the most to subducted sediments. Major and trace element geochemical characteristics are similar to those of altered basalt of the oceanic crust, which imply that red clay is mainly derived from the basalt alteration of the subducted and overlying plates, rather than the volcanic and terrestrial inputs. Moreover, intensified organic matter diagenesis has an important influence on the geochemical properties of the hadal sediments, which is the main reason the Challenger Deep sediments have lower rare earth elements and enrich more micromanganese nodules than the adjacent deep-sea sediments.
Rapidity of the Climatic and Environmental Changes During the Past Millennium Inferred from Stalagmite δ13C Records in the Middle Reaches of the Yangtze River
LIU Jing, ZHANG WeiHong, XU Hao, SHAO QingFeng, LU HaiXin, LI MaoXia, CHEN JianShun
2024, 42(3): 956-969. doi: 10.14027/j.issn.1000-0550.2023.028
Objective This study examined the ecological and hydrological variability and regime shift during the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA) in the middle reaches of the Yangtze River. This region is climatically governed by the Asian monsoon, with increased precipitation in summer and decreased in winter. Our study helps deepen our understanding of the history of the regional ecological and hydrological changes and their connections to the monsoon climate. Methods Based on two high-resolution stalagmite (YX262 and YX275) δ13C records from Yongxing cave, Hubei province in the middle reaches of the Yangtze River, we reconstructed the history of the local paleoclimate and environment from 1 044-1 954 A.D. by creating a composite of the two records using the iscam program. Results The δ13C records show two completely different states during the MCA and LIA, with the lower δ13C values characterizing the MCA period, and the higher δ13C values the LIA. During the transition from MCA to LIA, δ13C shows an abrupt change. This phenomenon indicates a regime shift from a strong to weak state for the vegetation respiration activity and precipitation variation. This variation is consistent with many stalagmite δ13C records in southwest China and the middle reaches of the Yangtze River. Here, our stalagmite δ13C record shows that the shift of the vegetation cover was rapid from the MCA to LIA. A Rampfit analysis shows that the stalagmite δ13C shift occurred between 1 434-1 460 A.D., lasting 26 years, which is more rapid and significantly longer than the stalagmite δ18O shift. This variation is attributed to both climate deterioration and human activity influences, leading to a reduction of vegetation cover during this period. On the one hand, the rapid transformation of the stalagmite δ13C record in Yongxing cave may be related to the great migration event in the early Ming Dynasty. A large number of migrants swarmed into Hubei province, inevitably leading to large-scale deforestation and land reclamation and changing the ecological environment of the surface soil vegetation. Those behaviors cause positive stalagmite δ13C. On the other hand, the Yongxing δ13C record is correlated with changes in the total solar radiation, intertropical convergence zone(ITCZ), temperature in eastern China, and El Niño-Southern Oscillation (ENSO) on the MCA and LIA event scales. During the MCA period, the total solar irradiance was larger, and the ITCZ was farther north, with a high frequency of El Nino states. However, during the LIA period, the total solar irradiance was relatively small, and the position of ITCZ was more southerly, in the low frequency period of El Nino states. This correlation indicates that the changes of hydrological circulation and vegetation cover in the middle reaches of the Yangtze River had a dynamic relationship with the regional and global climate change during the past millennium, and the impacts of human activities on surface vegetation may be related to the background of regional or global climate change. Conclusions By studying the δ13C record of stalagmites in Yongxing cave in the past millennium, the eco-hydrological characteristics of MCA and LIA were found to be significantly different, which may be related to global climate change and human activities. The study not only clearly establishes the time boundary between MCA and LIA in the middle reaches of the Yangtze River, but also deepens our understanding of the characteristics and causes of eco-hydrological environment changes in the region during the two periods.
A High-Resolution Stalagmite δ13C Record for the Past 4 750 Years from Dark Cave, Guizhou, SW China
GAO Kui, HE YaoQi, QIU WanYin, ZHANG Xin, FENG TangHui, JIANG XiuYang
2024, 42(3): 970-980. doi: 10.14027/j.issn.1000-0550.2022.068
Objective Global warming may increase the risk of expansion of karst rock desertification (KRD) in Southwest(SW) China, where karst landscapes are widespread, so it is important to reconstruct the evolution of KRD in this region. Methods In this study, the regional eco-evolution of the past 4 750 years was reconstructed based on 29 230Th ages and 954 δ13C records collected from stalagmite HD12 in Dark cave, Qianxi county, Guizhou province. Results and Conclusions Two significant positive δ13C offsets were found at 4 322-3 526 a B.P.and 803-82 a B.P., indicating deterioration of the ecological environment at those times. A significant positive δ13C record of the HD12 stalagmite at about 803 a B.P. persisted for about 290 years with an amplitude of 4.2‰, indicating expansion of KRD in the area. The consistent positive δ13C in stalagmites from several caves in SW China during this period suggests that the large-scale population migration and climatic drought after the Jingkang event (823 a B.P.) promoted the expansion of KRD in this region. During 4 322-3 526 a B.P., the positive shift of HD12 stalagmite δ13C values had an amplitude of 4.9‰. The positive shift at 4 322-3 977 a B.P. corresponds to the 4.2 ka event, and for 3 777-3 526 a B.P. it corresponds to the 3.7 ka event. These two periods of drought are also reflected in several stalagmite and lake records in SW China, suggesting that the weakened Asian summer monsoon and reduced precipitation at those times may have caused a large reduction in vegetation cover and severe soil degradation in the region.
Downstream River Channel Sinuosity Variation of the Distributive Fluvial System: A case study from the Golmud fluvial fan
SUN JiaHui, ZHANG ChangMin, WANG HaoNan, ZHANG XiangHui, FENG WenJie
2024, 42(3): 981-993. doi: 10.14027/j.issn.1000-0550.2023.061
Objective We aimed to study the evolution of river patterns in the distributive fluvial system, establish a database of corresponding parameters, and provide a data basis for predicting the distribution of sedimentary systems within the forecast basin. Methods Using modern geographic information software such as Google Earth and Global Mapper, the channel sinuosity was measured based on a grid of equal proportions, analyzing the variation of the channel sinuosity along the course and establishing the depositional mode of the evolution of the Golmud fluvial fan. Results A total of 2 989 channel sinuosity data were measured, and based on the characteristics of the sinuosity of the Golmud fluvial fan, the Golmud fluvial fan can be divided into four sections: "braided river," "braided-meander coexistence," "large high-sinuosity meandering river," and "small low-sinuosity meandering river section": (1) The slope of the braided river section was 0.70%, developing both the straight and braided river with low sinuosity. The braided belt width was large. The channel sinuosity was 1.01-1.43, the average sinuosity was 1.08, and microfacies such as braided river, abandoned river, and flood plain were developed; (2) The slope of the braided-meander coexistence section was 0.63%, developing both braided and meandering river, and the development degree and river sinuosity varied in different positions. The channel sinuosity was 1.08-5.00, and the average sinuosity was 1.52; microfacies such as braided river, meandering river, abandoned river, crevasse splay, sand dunes, and flood plain were developed; (3) The slope of the large high-sinuosity meandering river section was 0.29%, mainly developing a large meandering river. The channel was wide, the river bend swing was large, the channel sinuosity was 1.12-5.00, and the average sinuosity was 1.83; microfacies such as meandering river, abandoned river, sand dunes, and flood plain were developed; (4) The slope of the small low-sinuosity meandering river section was 0.08%, mainly developing a small bifurcated meandering river, narrow channel, and small river bend swing; the channel sinuosity was 1.07-3.13, with an average sinuosity of 1.51, and microfacies such as meandering river, abandoned river, crevasse splay, sand dunes, and flood plain were developed. Conclusions The depositional model of river-type evolution of the Golmud fluvial fan was established, which provided a new knowledge base for the prediction of the distribution of underground sedimentary systems.
Sedimentary Model and Evolution Law of Meandering Rivers: A case study from the lower Minghuazhen Formation of L oilfield in Bohai Bay Basin
XU ZhongBo, LIU YingXian, LIN GuoSong, YAO YuanRong, GAN LiQin
2024, 42(3): 994-1002. doi: 10.14027/j.issn.1000-0550.2022.074
Objective There are many meandering river depositional models in the lower Minghuazhen Formation in L oilfield. Their sedimentary models and evolutionary laws are not yet clear. Methods The meandering river characteristics and their paleogeological background in different periods are systematically analyzed by using paleo-ntological, core, sidewall core, well logging data, and seismic data. Then, the evolution law is summarized. Results The study shows that during the sedimentary period of the lower Minghuazhen Formation, there are 5 types of meandering rivers developing successively in the study area. The river type of the L44 sublayer is a braided-meandering symbiotic model. The river type of the L42-L40 sublayer is a high-curvature meandering river. The river type of the L32 sublayer is a restricted high-curvature meandering river. The river type of the L30 sublayer is a string-beads meandering river. The river type of the L2 oil group is a low-curvature meandering river under lake-level. Conclusions During the sedimentary period of the lower Minghuazhen Formation, the paleogeological background of the studied area also evolved continuously. Because of the coupling effect of different paleogeological factors, such as sedimentary base level cycles, paleoclimate, paleogeomorphology, and the water environment in sedimentary area, various types of meandering rivers were formed in the different periods.
Sedimentary Environments and Paleoclimate of the Middle⁃Late Jurassic in the Northeastern Sichuan Basin
ZHOU Min, LI XiangHui, WANG JingYu
2024, 42(3): 1003-1015. doi: 10.14027/j.issn.1000-0550.2022.079
Objective The Jurassic was a period of typical fluctuating greenhouse climate that resulted in the deposition of sediments in terrestrial basins. In China, a number of large terrestrial basins were formed during the Jurassic, and the discrepancies in sedimentation records have been preserved in several basins. This study focused on the Middle⁃Upper Jurassic terrestrial redbeds in the northeastern Sichuan Basin to determine changes in the sedimentary environment and paleoclimate. Methods Based on the observation of outcrops, analyses of microscopic clastic composition and C-O isotopes, and to estimate the atmospheric CO2 concentration in this area. Results Medium-to-fine clastic rocks are the predominant lithologies, and five sandstones are further classified by composition and lithological content. Three sedimentary environmental units are recognized: meandering river, lake, and paleosol, which represents a simpler system than in the western Sichuan Basin. The upwards facies sequence is pedogenic, beginning with low sand/mud ratio meandering mudrocks in the Middle Jurassic Shaximiao Formation, lacustrine mudrocks with siltstones in the lower Suining Formation of the lower Upper Jurassic, and upward-coarsening and thickening high sand/mud ratio meandering sandstones in the upper Suining and Penglaizhen Formations of the Middle-Upper Jurassic. Two and a half cycles of semi-arid and arid climate were distinguished in the Middle⁃Upper Jurassic sequence by F/Q and F/L indices, corresponding to the lower Shaximiao Formation, the upper Shaximiao Formation + lower Suining Formation, and the upper Suining Formation + Penglaizhen Formation, respectively. From the pCO2, it is postulated that the Middle Jurassic was relatively cool, becoming warmer in the early Late Jurassic and hot in the middle-to-late Late Jurassic, roughly consistent with the framework of global marine climates. Conclusion The climate was (semi-)arid overall in northeastern Sichuan, similar to that in other regions of the Sichuan Basin. It will be necessary to further verify that an intermittent semi-wet climate occurred in western Sichuan and that a hot climate prevailed in northeastern Sichuan.
Paleoenvironment and Its Petroleum Geological Significance of the Jurassic Da'anzhai Member in the Central-Eastern Sichuan Basin
GUO YiHao, ZENG DeMing, ZHANG Rui, WANG XingZhi, HUANG Dong, ZHANG BenJian, XIE ShengYang
2024, 42(3): 1016-1031. doi: 10.14027/j.issn.1000-0550.2023.052
Objective We attempted to determine the cause of the large difference in paleoenvironment characte⁃ristics during the Jurassic Da'anzhai period and the unclear distribution law of the shale source and reservoir in the Sichuan Basin. Methods Based on the geological drilling cores and field profiles of the Jurassic Da'anzhai member in the central and eastern Sichuan Basin, combined with elemental geochemical analysis methods, we performed a comparative analysis of the paleo-climate,paleo-redox conditions,and the paleo-salinity in the Da'anzhai period. Results The results show that the Da'anzhai member can be divided into shale, shell shale, mudstone, silty mudstone, argillaceous siltstone, shell limestone, and argillaceous shell limestone facies. The paleoenvironment of different sub-members in the Da'anzhai member is different to a certain extent. During the sedimentation period of the Third sub-member, the climate was dry to semi dry, and the water environment was fresh to semi-saline under sub-reducing conditions, located in a shallow to semi-deep lake environment. During the sedimentation of the Second sub-member, the climate was semi dry to semi humid, with more precipitation and lower temperatures, and the water environment was fresh water with sub-reducing conditions. It was a semi-deep lake environment. During the sedimentation of the First sub-member, the climate was dry to semi-dry, and the water environment was semi-saline to saline under oxidation sub-reduction conditions, located in a shallow lacustrine environment. Wells RA1 and YT1 both exhibited the following conditions: the climate was dry→wet→dry, the precipitation was less→more→less, the temperature was high→low→high, the reducibility of water was weak→strong→weak, and the paleo-salinity was high→low→high. From the sedimentation period of the Third sub-member to the early sedimentation period of the Second sub-member, the lacustrine basin depositional center migrated from the eastern to central Sichuan Basin. The middle sedimentation period of the Second sub-member was a transitional stage during which the migration direction of the lake basin depositional center changed. From the late sedimentation period of the Second sub-member to the sedimentation period of the First sub-member, the lacustrine basin depositional center migrated from the central to eastern Sichuan Basin. The average total organic carbon (TOC) of the Second sub-member is 1.70%, and the average porosity is 4.93%, significantly higher than the First and Third sub-members. The fluorescence display is strong, and it is the main source and reservoir of shale oil in the Da'anzhai section. The paleoenvironmental factors have a significant influence on the source and reservoir configuration. The more humid climate, more precipitation, lower temperature, stronger reducibility and lower salinity are conducive to shale deposition, organic matter enrichment, and pore development. Conclusions The study on the paleoenvironment of the Da'anzhai member in the central-eastern Sichuan Basin is important to the regularity of shale deposition,organic matter enrichment,and pore development degree. This research understanding can provide a theoretical basis for shale oil exploration in the Sichuan Basin.
Lithology and Distribution Characteristics of the Oolitic Beach from the Feixianguan Formation, Eastern Sichuan Basin
YE Yu, CHENG Chao, JIANG YuQiang, YI JuanZi, DENG HongBing, LI Xi, GU YiFan, CHEN Yan
2024, 42(3): 1032-1046. doi: 10.14027/j.issn.1000-0550.2022.089
Objective To solve the problems such as unclear lithological variations of the Feixianguan Formation on the western side of the marine trough's southern segment and within the plateau interior of the eastern part of the Sichuan Basin, Methods the lithological types and characteristics of the Feixianguan Formation were studied by comprehensively using multiple geological data such as cores, drilling and logging, and an intelligent identification method of lithological logging based on machine learning was proposed, which solved the technical problems of fine identification of lithology in the old area, and revealed the lithology, distribution and evolution of the Feixianguan Formation in the area. Results and Conclusions (1) The Feixianguan Formation is mainly composed of lithology such as mudstone, mud crystal limestone, argillary limestone, granule limestone, granulous dolomite, mud crystal dolomite, paste dolomite, and gypsum rock; (2) Comparison found that the improved gradient-boosted decision tree algorithm, namely Stochastic Gradient Boosting Decision Tree (SGBDT), is superior to other algorithms for constructing lithology models, and is more suitable for carbonate rock complex lithology identification; (3) The granule limestone developed intensively in the area south of the Kaijiang-Liangping Sea Trough between Feixianguan I period and Feixianguan Ⅲ period, and the granulous dolomite was concentrated in the Feixianguan Ⅱ period and distributed and scattered; (4) The distribution of the oolitic shoal in the area exhibits significant variation. The main development of the plateau ancient geomorphological high point and the edge of the platform in the Feixianguan I period. In the Feixianguan Ⅱ period, most of the the oolitic shoal at the edge of the platform were developed, and a small number of ancient landforms in the platform were developed.And the main development of the plateau ancient geomorphological high point in the Feixianguan Ⅲ period.
Fractal Characteristics and Interfering Factors of Microscopic Pores in Marine-Continental Transitional Coal Shale: A case study of the Taiyuan Formation in the northern Qinshui Basin
NIE WanCai, ZHANG TingShan, WANG MingWei, WU Wen, TAN XiuCheng
2024, 42(3): 1047-1057. doi: 10.14027/j.issn.1000-0550.2022.078
Objective To analyze the pore fractal characteristics and interference factors from the coal measure shale of the Taiyuan Formation in the northern Qinshui Basin. Methods Total organic carbon (TOC) content, maturity, X-ray diffraction, and low-temperature nitrogen adsorption experiments were conducted on samples of the Taiyuan Formation in the Yangquan block. The fractal dimension of a sample pore was calculated based on the (Frenkel Halsey Hill, FHH) theoretical model, which analyzes the influence of mineral content, organic geochemical characteristics, and pore structure parameters on the pore fractal dimension. Results The results show that the TOC content of coal measure shale in the Taiyuan Formation is between 0.57% and 6.40%, and the average is 3.18%; the vitrinite reflectance of organic matter (Ro) is between 1.96% and 3.24%, and the average is 2.49%. The coal-measure shale micropores have double fractal characteristics, of which fractal (D1) is between 2.507 9 and 2.663 9, dimension (D2) is between 2.527 1 and 2.809 4, and the organic matter content and maturity are positively correlated with D1 and D2. The pore structure parameters have a positive correlation with D1 and D2, but the correlation coefficient with D2 is higher than that of D1, indicating that micropores have stronger influence on pore structure parameters. The input of terrigenous clastic quartz with high sorting and roundness leads to regular pore morphology, resulting in a negative correlation between quartz content and D1 and D2. Carbonate minerals and feldspar primarily provide macropores, and their contents are related to shale. Both D1 and D2 are negatively correlated, and the pore size of the clay minerals decreases under long-term compaction, whereas the number of micropores increases, and the pore shape is complex, which is positively correlated with fractal dimensions D1 and D2. Conclusions The micropores of the coal measure shale in marine continental transitional facies have dual fractal characteristics. The increase of organic matter content, maturity, pore structure parameters, and clay mineral content can lead to the increase of micropore fractal dimension, and an increase in terrigenous clastic quartz, feldspar, and carbonate mineral content can lead to a decrease in micropore fractal dimension.
Sedimentary Evolution and Controlling Factors of Fan Delta and Slump Fan Complex Deposition in a Steep Slope Belt of a Faulted Basin: A case study from the Cretaceous deposits of the A Sag in Melut Basin, Central Africa
CHEN BinTao, MA Lun, HONG Liang, SHI ZhongSheng, PANG WenZhu, XUE Luo, WANG Lei, DAI HanSong, ZHAO YanJun
2024, 42(3): 1058-1072. doi: 10.14027/j.issn.1000-0550.2022.087
Objective As an important type of coarse-grained sedimentary system and oil and gas reservoir in faulted basins, fan delta and slump fan complex deposition has been the hot topic of research on lacustrine sedimentary and reservoir formation, such as distribution area, evolution period and accumulation model. Methods Based on high resolution 3D seismic and drilling and logging data in the A Sag of Melut Basin in central Africa region, the sedimentary characteristics and evolution of fan delta-slump fan complex are studied by using core observation, sequence division, seismic reflection characteristics and seismic attribute analysis. Results and Conclusions The results showed that the types of sedimentary facies of fan in the study area included fan delta and slump fan caused by gravity flow, slump fan had two types of morphology characteristic, the first one was line source slump fan controlled by slope break, the second one was multistage slump fan with single point source controlled by steep topographic slope. Combined with seismic profiles and well logging features, five periods of fans were recognized in the Cretaceous study section. The single-stage fan all had the characteristics of retrogradation, the vertical evolution of multiple periods of fans was characterized by “first progradation and later retrogradation”. The fan distribution was the largest at the end of Renk Formation deposition in the Lower Cretaceous, reaching 148 km2. However, the fan evolution was mainly influenced by tectonic activity, palaeogeomorphology, provenance supply, and base-level cycle. The fan distribution was larger in the period of strong tectonic activity, abundant provenance supply and large short-axis tectonic uplift, and the slump fan was most developed in the early period of relative base-level rising cycle and decreasing cycle. A new understanding of the sedimentary pattern transformation occurred in the Early Cretaceous and Late Cretaceous in the A Sag was put forward. The exploration potential of the fan delta and slump fan complex deposition in the faulted depression is clarified, which guided the exploration deployment.
Controlling Factors of Organic Enrichment in the Shuijingtuo Formation in the Lower Cambrian of the Chengkou Area, Sichuan Basin
GU Heng, WANG Jian, WEI HengYe, FU XiuGen
2024, 42(3): 1073-1091. doi: 10.14027/j.issn.1000-0550.2022.091
Objective The Lower Cambrian Shuijingtuo Formation black shale is an important source rock and shale gas exploration target in the Sichuan Basin. However, the enrichment mechanism of organic matter in this series of black shale is still unclear. Methods Here, we reported total organic carbon (TOC) contents, sedimentary microfacies, major and trace elements, and pyritic framboid size distributions in the lower part of Shuijingtuo Formation in the Chengkou area, northeastern Sichuan Basin on the Upper Yangtze Platform, providing new evidence for reconstructing the paleogeography, paleoenvironment, and main controlling factors of organic matter enrichment. The results show that the black shale succession in the Shuijingtuo Formation developed five sedimentary microfacies which related to the deep-water shelf slope environment. Productivity index (BaXS,P/Ti)and continental input index Ti, Al indicate that the sedimentary environment of black shale in the Shuijingtuo Formation is at a low level of paleo-productivity, and the continental input is relatively stable and has no obvious correlation with TOC. Therefore, paleo-productivity and continental input are not the main controlling factors of organic matter enrichment in the black shale of the Shuijingtuo Formation. The redox-sensitive element index (U/Th,UEF,MoEF) and pyrite framboid size distributions show that the organic matter content is positively correlated with the redox index, indicating that the organic matter enrichment in Shuijingtuo Formation is mainly controlled by the change of redox conditions of the water column. Conclusions Based on the above understanding, it can be inferred that the organic matter enrichment of the black shale of the Shuijingtuo Formation was controlled by the fluctuation of redox conditions of the bottom water caused by the large-scale transgression event during the stage 2 to stage 3 Cambrian (529-514 Ma), and the organic matter enrichment model was proposed here.
Detection and Geological Significance of Carotenoid-Derived Alkanes in the Source Rocks : A case of well Malu 1, Lucaogou Formation, Santanghu Basin
FU Yin, WANG ZuoDong, ZHANG Ting, WANG WenJun, LI XiaoBin, WANG ZhiYong
2024, 42(3): 1092-1106. doi: 10.14027/j.issn.1000-0550.2024.013
Objective Both γ-carotane and β-carotane are carotenoid-derived alkanes, which are widely distributed in sediments and crude oil, but carotenoid-derived alkanes with low carbon number are rare in the study of source rocks. This study explored the sources of γ-carotane, β-carotane and carotenoid-derived alkanes with low carbon numbers, indicating their significance in geological carriers. Methods Twelve representative source rock samples were collected from well Malu 1 in the Lucaogou Formation of the Malang Sag, Santanghu Basin. The geochemical characteristics were demonstrated in detail by gas chromatography-mass spectrometry, rock pyrolysis, vitrinite reflectance (Ro), and total organic carbon (TOC) analysis to obtain the biogenic information of the target compound. Results and Conclusions The source rocks of the Lucaogou Formation from well Malu 1 are rich in organic matter, good in type, and in the mature stage. Moreover, the source rocks are rich in γ- and β-carotane with high abundance and have carotenoid-derived alkanes with low carbon number (Carbon number ranges from C13 to C25). Based on the comprehensive analysis of the distribution characteristics of a series of biomarkers such as n-alkanes, terpanes, and steranes, bacteria may be the main biological precursors of γ- and β-carotane. In addition, combining the evidence of thermal evolution of hydrocarbon source rocks, microbial fossils, and volcanic eruptions, the present study proposes that the carotenoid-derived alkanes with low carbon number may be the products of γ- and β-carotane affected by microorganisms or thermal evolution. This study provides new insights the exploration of organic matter source, sedimentary environment and thermal maturation of source rocks.