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The major controlling factors of organic matter and its enrichment model of the black shale from the Wufeng-Longmaxi Formation were explored by investigating the vertical variation characteristics, as well as major element and trace element abundances in the Wuxi Bailu section. The results show that the sedimentary tectonic setting of the Wufeng-Longmaxi Formation in the north-east margin of the upper Yangtze platform is located on the active continental margin, which is a passive continental margin and continental island arc. The parent rock in the source area is mainly felsic volcanic rocks mixed with small amounts of sedimentary recycling materials. Due to increased plate activity and a drop in sea levels, terrigenous pyroclastic input increased. The palaeoclimate was semi-humid, and a robust dysoxic-reduction environment and a high level of palaeoproductivity, causing the formation of the organic-rich shale in the Wufeng Formation. At the base of the Longmaxi Formation, the sedimentary water body was affected by global transgression, showing a strong anoxic-reductive environment, and the paleoclimate was a warm and humid condition. The palaeoproductivity level was high, resulting in the formation of organic shale. Due to the sea level drop at the top of the Longmaxi Formation, the sedimentary water was in an oxic-reduced environment, but the input of terrigenous pyroclastic matter increased. Because the paleoclimate was warm and humid and the palaeoproductivity level was high, organic-rich shale was formed. The findings demonstrate that terrigenous clastic input circumstances, palaeoproductivity conditions, and paleoredox conditions had the greatest influence on the enrichment of organic matter in the Wufeng-Longmaxi Formation. Thus, organic matter enrichment was controlled by multiple paleoenvironmental factors. [ABSTRACT FROM AUTHOR]

The Permian Lucaogou Formation represents one of the most important hydrocarbon source rock intervals in the Junggar Basin, although the sedimentary paleoenvironment and organic matter enrichment mechanism of the Lucaogou Formation remain controversial. We studied the temporal evolution of the sedimentary paleoenvironment in the Lucaogou Formation by analyzing the elemental composition and total organic carbon content of 27 hydrocarbon source rock samples from the J305 well in the Jimsar Sag. Using these data, we found that the Lucaogou Formation overall was deposited in a semisaline to saline, reducing lake basin under an arid climate. We identified five organic matter-enriched intervals, which can be correlated with the parameters that indicate a wetter climate and a more anoxic lake environment. To compare sedimentary environments spatially, we compiled environmental indicators from 10 cores and outcrops in three sags around the Bogda Mountains. The compilation shows that the organic matter-enriched Jimsar Sag experienced a more arid climate and a more saline and anoxic lake environment during the deposition of the Lucaogou Formation, which was possibly controlled by the paleogeographic position. We conclude that the spatially arid climate and anoxic environment induced organic matter burial in the Jimsar Sag, while temporal events of a more humid climate and more anoxic environment triggered the enrichment of organic matter in some intervals of the Lucaogou Formation. [ABSTRACT FROM AUTHOR]

The shale in Doushantuo Formation is the main source rock of the Ediacaran in Sichuan Basin and the major hydrocarbon source layer of the ancient marine strata for natural gas exploration in recent years. Forty-six samples were collected from two outcrops(Baibaoxi and Shiping)in Chengkou county in northeastern Sichuan Basin.We analyzed the change of the sedimentary environment of the Member Ⅱ and Ⅳ of Doushantuo Formation in terms of their organic geochemical characteristics(total organic carbon, rock pyrolysis, kerogen carbon isotopes)and element geochemical characteristics(major elements, trace elements and rare earth elements).Results show that the shale of Doushantuo Formation in northeastern Sichuan Basin exhibit a high abundance of organic matter, with TOC content ranging from 0.19 % to 20.38 % (mean value is 4.76 %). The dominant type of organic matter is type Ⅰ kerogen, and it has reached over-maturity stage. High δEu anomalies and abnormally richness in V, Mn, Mo, Ba and U elements of the black shale revealed strong hydrothermal action during this period. The distribution pattern of rare earth elements and various redox indexes indicated that the redox fluctuates repeatedly under the conditions of anaerobic-anoxic-sulfidic environment during the depositional period of the Member Ⅱ. The water environment of Member Ⅳ changed from anoxic to anaerobic-sulfidic environment. In general, the water connectivity of the basin is strong, the depositional rate is relative high. The nutrients carried by upwelling from hydrothermal sources proliferate productivity, which promotes the formation of organic shale.

The black shale of the Mesoproterozoic Cuizhuang Formation in the Changcheng System in Yongji city, North China Craton, is a potential source rock. Understanding the organic matter enrichment mechanism is crucial for evaluating source rock resources and understanding oil and gas accumulation mechanisms. In this study, we evaluated the sedimentary paleoenvironment and organic matter enrichment mechanisms of shale using thin section observations, mineral composition analysis, organic geochemistry, and elemental geochemistry. We found significant differences in the sedimentary paleoenvironment and organic matter enrichment mechanisms between the lower Cuizhuang Formation and the Beidajian Formation shale. The Cuizhuang Formation was deposited in a late-stage, restricted basin environment during the rift phase, and elemental and geochemical indicators showed that the Cuizhuang Formation was in a suboxiceanoxic water environment, that was influenced by a warm and humid paleoclimate and submarine hydrothermal activities, which promoted the accumulation of organic matter. However, the enrichment of organic matter in the Cuizhuang Formation was mainly controlled by redox conditions. The formation of suboxic-anoxic water environments may be closely related to the warm and humid paleoclimate and submarine hydrothermal activities. Warm conditions promote continental weathering and increase marine productivity, thereby consuming oxygen in the bottom water. Moreover, acidic hydrothermal activity also helps to establish an anoxic environment. Our results reveal the effects controlling various coupled mechanisms dominated by redox conditions, which may explain the development of source rocks in the Cuizhuang Formation. [ABSTRACT FROM AUTHOR]

As a hydrocarbon-rich depression within the Bohai Bay Basin, the Huanghekou Depression is a focal region for exploring hydrocarbons in the eastern China Sea. Previous studies have insufficiently examined the correlation between the enrichment of organic matter and the environments in which it is deposited. Herein, the hydrocarbon potential, palaeoclimate, sedimentary environment, organic matter sources, and organic matter enrichment of the source rocks of the Shahejie Formation in the Huanghekou Depression were investigated using organic and inorganic geochemical indicators. The organic matter type of the source rock in Huanghekou Depression's Shahejie Formation was predominantly Type II, with a minor presence of Type III. Furthermore, the source rock had a poor-to-good comprehensive evaluation grade in E3s1–2, whereas E2s3 and E2s4 had medium-to-good comprehensive evaluations in their source rocks. In terms of maturity, E3s1 was in an intermediate position between the immature and mature stages and E3s2 and E2s3 were between the low-maturity and mature stages, whereas E2s4 transitioned into full maturity. Biomarkers and sensitive element indicators indicated that the organic matter in E3s1–2 was primarily derived from lower aquatic organisms and algae. This palaeoclimate was characterised by aridity, a water body containing saline and semi-saline water, and a strongly reducing environment resulting from water body stratification, leading to oxygen deficiency. The organic matter in E2s3 was primarily derived from aquatic organisms and algal inputs; these deposits were formed in a reduced environment characterised by relatively low salinity, ranging between semi-saline and freshwater conditions. The organic matter enrichment model of the Shahejie Formation was established based on sedimentary environment, palaeoclimatic, and organic matter source analyses, utilising E3s1–2 as preservation models and E2s3 as the productivity model. This study provides a basis for in-depth exploration and advancement of oil and gas reserves. [ABSTRACT FROM AUTHOR]

[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. [ABSTRACT FROM AUTHOR]

[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 CoxMn 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. [ABSTRACT FROM AUTHOR]

[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 paleoproductivity, and the continental input is relatively stable and has no obvious correlation with TOC. Therefore, paleoproductivity 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. [ABSTRACT FROM AUTHOR]

The study of fine-grained sedimentation has consistently concentrated on investigating the mechanisms and principles governing the enrichment of organic matter. However, the lack of unified stratigraphic framework has always existed as fine-grained sedimentation covers two distinct grain-size grades, namely, mud and silt, which has impeded the progress of subsequent production research. This study exemplified this issue by analyzing the first member of the Qingshankou Formation in the southern Songliao Basin. We established reconstructed gamma and density curves that mitigated filter noise interference, integrated high-resolution sequence results with astronomical cycle divisions, and created a high-frequency isochronous stratigraphic framework for clastic fine-grained sedimentation by leveraging the weak sensitivity of sandstone density curves and the robust stability in eccentricity cycle extraction. This approach addresses the inconsistencies in stratigraphic division methodologies and mismatched outcomes stemming from the use varying techniques to delineate mud and silt components within clastic fine-grained sedimentary sequences. Furthermore, it elucidates how tectonic-scale variations in sediment supply coupled with potential accommodation changes dictate macroscopic stacking patterns within strata, whereas climate fluctuations on orbital time scales govern sand-mud progradation degrees within these layers, culminating in periodic rhythmic characteristics characterized by vertical sand-mud interbedding. A model for stratigraphic development pertaining to lake delta systems constrained by a “synchronous heterotopy” paradigm is proposed for the southern Songliao Basin. The organic matter enrichment pattern aligns with its filling dynamics, indicating an “overfilling” type developmental pattern at lower strata levels where organic material predominantly originates from terrestrial plant debris external to the basin; this material accumulates primarily within silty zones along layers—with areas exhibiting heightened enrichment values slightly lagging behind short-eccentric maxima positions. In contrast, under an upper “balanced filling” type developmental framework, sources of organic matter are derived both internally and externally relative to the basin—exhibiting substantial heterogeneity—and regions marked by elevated organic matter concentrations are directly associated with locations identified as short-eccentric maxima.

The shales of the Taiyuan Formation and Shanxi Formation in the North China Basin have good prospects for shale gas exploration and development. In this study, Well KP1 at the northern margin of the North China Basin was used as the research object for rock mineral, organic geochemical, and elemental geochemical analyses. The results show that brittle minerals in the shales of the Taiyuan Formation and Shanxi Formation are relatively rare (<40%) and that the clay mineral content is high (>50%). The average TOC content is 3.68%. The organic matter is mainly mixed and sapropelic. The source rocks of the Taiyuan Formation and Shanxi Formation are mainly felsic, and the tectonic background lies in the continental island arc area. The primary variables that influenced the enrichment of organic materials during the sedimentary stage of the Taiyuan Formation were paleosalinity and paleoproductivity. Paleosalinity acted as the primary regulator of organic matter enrichment during the sedimentary stage of the Shanxi Formation. [ABSTRACT FROM AUTHOR]

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As a hydrocarbon-rich sedimentary basin in China, the Ordos Basin has enormous potential for shale gas resources. The shale of the Upper Carboniferous Benxi Formation is rich in organic matter, however, its palaeoenvironment and organic matter enrichment mode are yet to be revealed. In this study, the geochemical characteristics of the shale of the Benxi Formation in the east-central part of the Ordos Basin were analyzed to investigate its palaeoenvironment. At the same time, the organic matter enrichment modes in different sedimentary facies were compared and analyzed. The results indicate that: 1) the shale of the Benxi Formation was mainly deposited on the continental margin and strong terrestrial clastic input; 2) the deposition period of the Benxi Formation shale had a hot and humid climate with high palaeoproductivity and local volcanic hydrothermal fluid, and a high sedimentation rate with the strong stagnant environment. The bottom water was in dysoxic conditions and a semi-saline deposition environment; 3) multiple factors, such as palaeoproductivity, volcanic hydrothermal, redox conditions, and palaeosalinity interact to influence the enrichment of shale organic matter in Benxi Formation; 4) the organic matter enrichment modes of continental, marine-continental transitional, and marine shales can be classified into three types: "production mode", "hybrid mode of preservation and production", and "preservation mode", respectively. This study provides a reference for the organic matter enrichment mode, shale gas formation conditions, and core area evaluation in these marine-continental transitional shales, and also offers new guidance for exploration ideas for shale gas in different sedimentary facies. [ABSTRACT FROM AUTHOR]

The organic‐rich shale of the seventh Oil Member (Chang 7 Member for short) of the Middle‐Late Triassic Yanchang Formation is extensively distributed in the southern margin of the Ordos Basin. Understanding the palaeoenvironmental conditions during the sedimentary period of the Chang 7 Member is crucial to exploring the main factors that control the enrichment of organic matter in high‐quality source rocks. In this study, lithofacies analysis was conducted, and samples were collected from the N36 well located in the southern part of the Ordos Basin. Subsequently, element geochemical analysis was performed to reconstruct the palaeoenvironment, and total organic carbon (TOC) tests were conducted to analyse the characteristics of organic matter enrichment. The findings indicate that the sedimentary period of the Chang 7 Member was characterized by a relatively warm and humid palaeo‐climate and a brackish water environment with oxygen‐poor or sub‐oxygen‐rich conditions, and a gradual decrease in palaeo‐water depth. The parent rock types were mainly felsic igneous rock and intermediate igneous rock, which provide limited nutritional elements for primary producers. The enrichment of organic matter in the Chang 7 Member is influenced by several factors, mainly including palaeo‐salinity, palaeo‐water depth, palaeo‐redox conditions, terrestrial inputs, and palaeo‐climate. The warm and humid palaeo‐climate, higher palaeo‐productivity, relatively higher palaeo‐salinity, lower terrigenous input, deeper palaeo‐water depth, and better preservation conditions are conducive to the development of high‐quality source rock in the Chang 7 Member. [ABSTRACT FROM AUTHOR]

The paleolake level, which is controlled by the moisture balance (precipitation minus evaporation) within the lake basin, is a significant factor in determining the deposition of lacustrine organic-rich shale (LORS) across geological time, and hence influences shale oil enrichment. However, the impact of lake-level-fluctuations on shale oil enrichment of LORS is not well understood. Based on an integration of bulk geochemistry, organic petrography, pyrolysis gas chromatography, and element compositions, we address this issue using the Paleogene Biyang Depression in East China as an example. High lake levels, combined with anoxic–suboxic conditions, brackish–saline water, high productivity, and low detrital influx, are favorable for LORS deposition, which is characterized by a large distribution area and thickness, a high potential for oil generation and emplacement, and a high free shale oil content. In contrast, LORS deposited during low lake levels, with suboxic–dysoxic conditions, fresh–brackish water, low productivity, and high detrital influx, has a small distribution area and thickness, a low potential for oil generation and emplacement, and a low free shale oil content (a comparable maturity was present in all the studied LORS). Our data suggests that the elevated lake level led to higher salinity, stronger reduction conditions, higher productivity, and lower clastic inflow in the paleolake, forming LORS with higher shale oil potential. It has a positive effect on shale oil enrichment of LORS. The findings are also applicable to regional shale oil exploration. [ABSTRACT FROM AUTHOR]

The late Paleozoic Fengcheng Formation shale (LPF shale) in the Junggar Basin, NW China, is the oldest alkaline source rock discovered in the world, providing a unique perspective with which to explore organic matter (OM) enrichment in alkaline lake environments. Combined with the organic carbon isotope profile and paleoenvironmental proxies, this study reveals that the LPF shale was deposited in an arid climate with high salinity and a strong reducing environment, accompanied by frequent volcanic activity. High TOC values are concentrated in two intervals with frequent fluctuations in OM types. A negative excursion due to changes in sedimentary OM source is found in the δ13Corg profile. The excursion corresponds to the OM enrichment interval and is accompanied by abnormally high values of Sr/Ba and Sr/Cu. This implies that the extreme arid climate has led to high salinity, resulting in strong reducibility and changes in paleontological assemblages, which in turn controlled the differential enrichment of OM. The Fengcheng Fm. high‐quality source rocks are the result of the combined action of climatic events, volcanism, high‐salinity water environment and superior hydrocarbon‐generating organisms. The results provide new insights into the formation conditions of terrestrial alkaline high‐quality source rocks and the factors controlling alkaline OM enrichment. [ABSTRACT FROM AUTHOR]

The Lucaogou Formation in the Jimsar Sag is the main target layer of shale oil exploration and development in continental basins. However, its paleo-environmental information and organic matter enrichment mechanism still need to be determined. In order to investigate the paleo-environment of the Permian Lucaogou Formation, based on core and thin section observation, we carried out major and trace element analyses on 26 source rocks from Well J10025 by using X-ray fluorescence spectroscopy(XRF) and inductively coupled plasma mass spectrometry(ICP-MS). This analysis, combined with GC-MS biomarker data and typical sedimentary structures, reflects the paleo-environmental characteristics of the Luchaogou Formation. Geochemical indicators such as C-value, Sr/Ba, V/Cr, Pr/Ph, P, and Co show that the Lucaogou Formation was generally developed in a saline lake environment under an arid to semi-arid climate, with relatively deep water during deposition and hypoxic to anoxic conditions. The upper member was deposited in a semi-arid, brackish lake with weak reducibility, deep water body, and high productivity. In contrast, the lower member was deposited in an extremely dry, highly saline, and strongly reductive shallow lake with low productivity. The paleo-environment of the upper member fluctuated wildly, while that of the lower member was relatively stable. The correlation analyses between paleo-environmental indicators and TOC show that the preservation conditions and dilution rate of organic matter have limited influence on organic matter enrichment of the Lucaogou Formation. At the same time, the primary productivity is the main controlling factor.

Early Cambrian shale is an important petroleum source rock around the world. Because of little drilling data and poor seismic data, until recently, organic matter enrichment of the Lower Cambrian Yuertusi and Xishanbulake formations shale is still an enigma in the Tarim Basin, northwestern China. Total organic carbon (TOC), major and trace element data of Cambrian shale samples from five boreholes have been analyzed to decipher the mechanism of the organic matter enrichment. The results show that the shales deposited in the western restricted intraplatform have much higher TOC contents (3.2%–19.8%, on average 11.0%) than those from the eastern basin (2.2%–10.2%, on average 4.5%). The paleoproductivity proxies (Ba, Ba/Al, P/Al) in the western restricted platform are much higher than those in the eastern basin. The trace element indicators such as V/Cr, Ni/Co, Mo–TOC and MoEF–UEF suggest an anoxic environment across the basin, but a more restricted environment in the western intraplatform. The paleoproductivity rather than anoxic condition and hydrothermal activity are concluded to have resulted in the differentiation of the organic matter enrichment from the western intraplatform to eastern basin in the early Cambrian shales; the restricted environment was favorable for paleoproductivity and preservation of organic matter. [ABSTRACT FROM AUTHOR]

The vertical geochemical variations in total organic carbon (TOC) content and major and trace elements of the Lower Cambrian Shuijingtuo Formation from the Yichang Slope in the Upper Yangtze were investigated to assess the environmental conditions (redox conditions, water mass restriction, terrigenous input, relative sea-level terrestrial, and paleoproductivity) and to determine the primary controlling factors for organic matter enrichment. The Shuijingtuo Shale is divided into three intervals. The lithofacies of interval I are mainly black siliceous shale with high TOC content, and interval II is mainly black siliceous shale with moderate TOC content. Interval III consists of black, clay-rich siliceous shale and dark-gray calcareous shale and is characterized by a low TOC content. The effects of diagenesis and hydrothermal activity on the elements were evaluated prior to analyzing the environmental condition. There are good positive correlations between TOC and U/Al or Mo/Al ratios, suggesting that major/trace elements still retain the geochemical signature of the sedimentary environment. Meanwhile, the geochemical proxies consisting of Al, Fe, Mn, and Ti indicate that the study area did not experience hydrothermal deposits. The redox proxies (U/Th, Corg:Ptot, and MoEF–UEF) indicate that the interval I samples were formed under a strong reducing condition. The diagram of TOC vs. Mo indicates that the water mass was moderately restricted during the deposition of interval I shales. Proxies of terrigenous input and relative sea-level (Zr/Al and Zr/Rb) suggest that the organic-rich shales at the bottom were deposited under a relatively high sea-level and experienced minimal input of terrigenous debris from the source area. Additionally, the paleoproductivity indicated by Sibio and (Ni + Cu + Zn)/Al was high for interval I samples. During the interval II period, the relative sea-level began to decline, the seawater still remained in reducing conditions, and there was no change in the productivity, but the input of terrigenous debris increased significantly. In the interval III depositional period, the relative sea-level continued to decrease, the seawater shifted to a dysoxic condition, and the paleoproductivity was also at a lower level. The evolution of the sedimentary environment indicates that the high TOC content in the interval I samples is mainly attributed to the strong reducing condition, the preservation condition and debris dilution together control the organic matter content within the interval II samples, and the low TOC content within the interval III samples is constrained by a combination of the poor preservation conditions and lower paleoproductivity. [ABSTRACT FROM AUTHOR]

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Most scholars believe that transgression events can contribute positively to organic matter enrichment and shale gas sweet spot development, while whether or not regression events are conducive to shale gas sweet spot development remains to be further discussed. Variations in organic carbon content (TOC), and major and trace elements at the Rhuddanian–Aeronian stage in the Luzhou area, the southern margin of Sichuan Basin, were analyzed in this paper. We discuss differences in paleoenvironment organic matter enrichment and shale sweet spot development during transgression and regression. A transgressive system tract (TST) occurred during the early Rhuddanian stage, while early highstand system tracts (Ehst-1 and Ehst-2) occurred during the late Rhuddanian stage and Aeronian stage, and a late highstand system tract (LHST) developed during the late Aeronian stage. A rise in sea level during the TST in the upper Yangtze resulted in an anoxic environment, where a continuous upwelling current brought about a large number of nutrients in the seawater, significantly increasing paleoproductivity. Strong tectonic subsidence, weak chemical weathering, and a rising sea level together led to a low terrigenous debris supply in the catchment area. Therefore, paleoproductivity and redox conditions were the primary controlling factors of organic matter enrichment at the TST stage, with a clastic supply of secondary importance. With the advance of the Guangxi orogeny, the organic matter enrichment at the EHST-1, EHST-2, and LHST stages was mainly controlled by redox conditions and debris supply. A comparison of the key physical parameters and geochemical indicators of shale reservoirs from these four system tracts suggests that two shale sweet spot types (type I and II) were developed during the Rhuddanian–Aeronian stage, occurring in the TST and EHST-2 stages. High TOC and high microcrystalline quartz content are key to developing type I sweet spots, while enhanced anoxic conditions in the bottom water caused by ephemeral, small-scale sea level rises are the main determinant of class II sweet spots in the later EHST stage. [ABSTRACT FROM AUTHOR]

As a hydrocarbon-rich depression within the Bohai Bay Basin, the Huanghekou Depression is a focal region for exploring hydrocarbons in the eastern China Sea. Previous studies have insufficiently examined the correlation between the enrichment of organic matter and the environments in which it is deposited. Herein, the hydrocarbon potential, palaeoclimate, sedimentary environment, organic matter sources, and organic matter enrichment of the source rocks of the Shahejie Formation in the Huanghekou Depression were investigated using organic and inorganic geochemical indicators. The organic matter type of the source rock in Huanghekou Depression’s Shahejie Formation was predominantly Type II, with a minor presence of Type III. Furthermore, the source rock had a poor-to-good comprehensive evaluation grade in E3s1–2, whereas E2s3 and E2s4 had medium-to-good comprehensive evaluations in their source rocks. In terms of maturity, E3s1 was in an intermediate position between the immature and mature stages and E3s2 and E2s3 were between the low-maturity and mature stages, whereas E2s4 transitioned into full maturity. Biomarkers and sensitive element indicators indicated that the organic matter in E3s1–2 was primarily derived from lower aquatic organisms and algae. This palaeoclimate was characterised by aridity, a water body containing saline and semi-saline water, and a strongly reducing environment resulting from water body stratification, leading to oxygen deficiency. The organic matter in E2s3 was primarily derived from aquatic organisms and algal inputs; these deposits were formed in a reduced environment characterised by relatively low salinity, ranging between semi-saline and freshwater conditions. The organic matter enrichment model of the Shahejie Formation was established based on sedimentary environment, palaeoclimatic, and organic matter source analyses, utilising E3s1–2 as preservation models and E2s3 as the productivity model. This study provides a basis for in-depth exploration and advancement of oil and gas reserves.

The shales of the Taiyuan Formation and Shanxi Formation in the North China Basin have good prospects for shale gas exploration and development. In this study, Well KP1 at the northern margin of the North China Basin was used as the research object for rock mineral, organic geochemical, and elemental geochemical analyses. The results show that brittle minerals in the shales of the Taiyuan Formation and Shanxi Formation are relatively rare (50%). The average TOC content is 3.68%. The organic matter is mainly mixed and sapropelic. The source rocks of the Taiyuan Formation and Shanxi Formation are mainly felsic, and the tectonic background lies in the continental island arc area. The primary variables that influenced the enrichment of organic materials during the sedimentary stage of the Taiyuan Formation were paleosalinity and paleoproductivity. Paleosalinity acted as the primary regulator of organic matter enrichment during the sedimentary stage of the Shanxi Formation.

The Wufeng Formation-Longmaxi Formation (O3w-S1l1) black shales within the Yangtze foreland basin (South China) hold significant potential as unconventional oil and gas resources. However, several challenges, including sluggish sedimentation rates, thin sedimentary layers, and notable homogeneity, hinder a comprehensive grasp of the organic matter enrichment (OME) mechanism within these shales. These challenges impede the exploration of unconventional oil and gas resources. This study aims to identify the O3w-S1l1 black shales in the Weiyuan area through a combined approach of organic and inorganic geochemical analyses. By doing so, it delves into the controlling mechanism behind OME in the black shales of the Yangtze foreland basin. The findings of this research reveal that the O3w-S1l1 black shales primarily consist of sapropelic kerogen and exhibit signs of over-maturation, with TOC content ranging from 0.43% to 8.21%. These shales are classified as organic-rich, mainly composed mixed and siliceous lithofacies. The presence of silica in the shales originates mainly from biogenic sources, and the highest TOC content coincides with a Sixs contribution of around 30% to the total silica content. During the late Katian, a global sea level drop led to an oxic paleoenvironment, transitioning into a euxinic paleoenvironment as sea levels rose in the early Rhuddanian. Notably, TOC content exhibited a significant correlation with paleoceanographic conditions (e.g., V/Cr, Ni/Co) and paleoproductivity levels (e.g., P/Al, Babio), while displaying a negative correlation with paleoclimate conditions (e.g., CIA, C-value), terrigenous detrital input intensity (e.g., Sixs, Ti/Al), and sedimentation rate ((La/Yb)N). Specifically, favourable conditions for OME encompass robust reducing seawater conditions, high paleoproductivity, a humid climate, reduced influx of terrigenous debris, and relatively elevated sedimentation rates. Regional tectonic movements (Guangxi movement) and fluctuations in relative sea levels exert influence over the deposition of O3w-S1l1 black shales. This study provides a valuable case study for exploring and developing shale gas resources within the Yangtze foreland basin.

The Lower Permian Fengcheng formation is a significant source of rocks and a reservoir for the Mahu Sag in the Junggar Basin. Recently, the paleo-environment deposition factors of the P1f formation have become a popular research topic. This research was conducted using data from the F7 well of Mahu Sag, based on the geochemical analysis results of TOC (total organic carbon), REE (rare earth elements), and major and trace elements of 53 samples from P1f (Lower Permian Fengcheng formation), and some deposition factors are discussed. The P1f deposition process was classified into four stages based on paleo-environment elemental indicators. This research describes the deposition process of the evolution of alkaline lakes. The early and preliminary stages of alkali lake evolution are considered as late P1f1 to middle P1f2; the paleoclimate of this process was dry, the reduction conditions increased, and the paleo-productivity and lake salinity were enhanced. The terminal stage of alkali lake evolution is considered as late P1f2 to middle P1f3; in this period, the paleoclimate changed with seasonal cycles, resulting in a decrease in water salinity and an increase in oxidation; the paleo-productivity of the alkaline lake was at a medium level. Until the end of P1f3, the salinity of the lake decreased, and the water body became anoxic and weakly alkaline. Furthermore, the research on TOC and sedimentary parameters confirmed that the deposition of P1f organic matter is affected by multiple types of factors. A relatively warm climate, lack of oxygen, fresh water–brackish water, suitable debris flow, and high primary productivity conditions promoted organic matter deposition. [ABSTRACT FROM AUTHOR]

A total of 70 shale samples from seven core holes, namely, N203, N208, N209, N210, N211, N215 and N217, in Changning area, south Sichuan, were analyzed for major and trace elements. Based on an analysis of longitudinal variation characteristics of major and trace elements, the REDOX conditions, paleoproductivity, relative sea level height and terrigenous detrital injection amount of gas-bearing shale in Wufeng Formation–Submember Long-11 in the Changning area were studied. And, the main controlling factors of organic matter enrichment in Wufeng Formation–Submember Long-11 were discussed. The results show the following: (1) Trace elements Mo, U, Ba and Ni are relatively enriched in the shale of Wufeng Formation–Submember Long-11 in the Changning area, while the Co element is deficient. The flat distribution pattern of rare earth in Wufeng Formation–Submember Long-11 shows that the shale deposits are weakly influenced by terrigenous materials. The amount of terrigenous injection in Wufeng Formation is higher than that in Submember Long-11, and the amount of terrigenous injection increased gradually from the lower part of Bed Long-111 to the lower part of Bed Long-114. (2) The REDOX environment discrimination indexes such as U/Th, Ni/Co, V/Sc and δU show that the water REDOX conditions of Wufeng Formation–Submember Long-11 in the Changning area have changed many times during the shale sedimentary period, and the overall REDOX conditions are dominated by an anaerobic environment. The REDOX indexes at the boundary between Long-111 and Wufeng Formation are relatively small and in the oxidizing environment range. The REDOX index from Bed Long-111 to Bed Long-114 decreases gradually, and the anaerobic environment of Bed Long-111 and Bed Long-112 gradually changes to the oxygen-poor environment of Bed Long-113; and, the REDOX index of Bed Long-114 is the lowest, which is generally oxygen rich. (3) Organic matter enrichment in Wufeng Formation Submember Long11 in the Changning area is mainly controlled by REDOX conditions. Low terrigenous injection and a relatively high sea level have positive effects on organic matter enrichment, but there is no obvious relationship between the paleoproductivity and organic matter enrichment. [ABSTRACT FROM AUTHOR]

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The organic-rich shale of the Permian Taiyuan Formation (TYF) and Shanxi Formation (SXF) in the Southern North China Basin (SNCB) is considered a potential shale gas source. The shale was formed in a marine-continental transitional sedimentary environment, which has rarely been studied, with the enrichment mechanisms of organic matter (OM) remaining unclear. This study investigated the controlling factors and enrichment mechanisms of OM by analyzing the total organic carbon (TOC) content, paleoclimate, paleoproductivity, sedimentation rate, redox, and paleosalinity. The TOC of the TYF ranged from 0.92 to 7.43 wt.%, with an average of 2.48 wt.%, which was higher than that of the SXF (TOC = 0.36–5.1 wt.%, average of 1.68 wt.%). These geochemical indices suggest that both the TYF and SXF were deposited in warm and humid paleoclimates, with relatively high biological productivity and sedimentation rates. During the deposition process, the TYF experienced frequent transgression and regression events, leading to an enhancement of water reducibility, a relatively high sedimentation rate, reduced OM oxidation, and rapid deposition of OM, which were conducive to the preservation of OM. Moreover, a high biological productivity increased respiratory oxygen consumption in the water column, which could lead to OM accumulation. However, the regression event experienced by the SXF reduced the paleoproductivity and sedimentation rate and increased water oxidation, leading to a decrease in OM. The main controlling factors for the enrichment of OM in the TYF and SXF were the sedimentation rate, paleoproductivity, and redox conditions, thus establishing the enrichment models for OM in the TYF and SXF. This study is conducive to understanding shale enrichment mechanisms and guiding shale gas exploration. [ABSTRACT FROM AUTHOR]

The lacustrine shale, represented by the Lianggaoshan Formation, is widely distributed in oil and gas basins of China and will be a key target for unconventional hydrocarbon exploration in the future. Due to the complexity of geological conditions, the distribution of lacustrine shale and the mechanism of organic matter (OM) enrichment show significant differences between different basins. In this study, seismic interpretation, core observation, high-frequency geochemical analysis and other methods are integrated to reveal factors controlling lacustrine shale distribution and OM accumulation in lacustrine shale. The results suggest that six bottom-to-top organic-rich shale intervals are identified within the Lianggaoshan Formation due to lake-basin migration. The migration process of depocenters controls the planar distribution of lacustrine organic-rich shale. The organic-rich lacustrine shale within 1st Member and 2nd Member is characterized by relatively high paleoproductivity and dysoxic condition. The lacustrine organic-rich shale of the upper to the top of 3rd Member is characterized by relatively low paleoproductivity, relatively high terrestrial input, and dysoxic condition. Paleoproductivity and preservation condition caused by lake-level rise are generally the major influencing factor of organic matter accumulation in 1st Member and 2nd Member organic-rich shale. The input of terrestrial OM, and the condition of preservation caused by rapid deposition are the major factors controlling OM accumulation in 3rd Member of Lianggaoshan Formation.

To clarify the organic matter (OM) enrichment of the Lishui Sag, the factors influencing the variable abundance of OM in the Lingfeng Formation are studied using organic geochemical data. The source rocks of the Lingfeng Formation have medium–high total organic carbon (TOC) values (0.53–3.56%). The main type of kerogen is II2-III. Compared to the shallow marine subfacies source rocks, the TOC of the delta front subfacies source rocks is higher. The distribution of biomarkers shows that the redox environment of the delta front subfacies source rock is the sub-oxidizing and oxic environment, and the source rock is mainly supplied by terrigenous higher plants; the redox environment of shallow marine subfacies source rocks is a sub-reducing and suboxic environment, and the OM mainly comes from algae. The link between OM input and OM abundance demonstrates that terrigenous OM (TOM) input has a considerable influence on OM abundance. However, there is no obvious relationship between preservation and OM abundance, which suggests that preservation is not the determining element in OM enrichment. The strong sediment flux decreases the amount of time that OM is exposed to oxygen. As a result, delta front subfacies with large TOM input have a huge number of excellent source rocks. This paper proposes a "delta front-OM input model" for excellent source rocks. [ABSTRACT FROM AUTHOR]

The Upper Ordovician-Lower Silurian Wufeng-Longmaxi shales in the Middle Yangtze area have strong vertical heterogeneity, affecting the gas-bearing capacity of shale reservoirs and the fracturing ability.This study integrates wire-line logs, drilling cores, thin section observations, total organic carbon content, X-ray diffraction mineralogy measurement, and major, trace elements measurement.Under the stratigraphic sequence framework in the study area, the vertical variation of TOC content, mineralogical composition and shale lithofacies were studied.Combined with geochemical proxies for paleoclimate and paleoenvironmental conditions, the main factors controlling vertical heterogeneity were discussed.The results show that the Wufeng-Longmaxi shales have two 3rd-order sequences.The Wufeng Formation is divided into the transgressive systems tract(TST1) and high stand systems tract(HST), and the Longmaxi Formation is divided into the transgressive systems tract(TST2) and the early high stand systems tract(EHST).During the transgression period, the shale has high silica content, low clay content, and high organic carbon content, and mainly developed siliceous shale facies and clay-bearing siliceous shale facies.In the early high-stand systems tract period, shale developed silty bands and lenticular bedding, with high clay content, and low organic carbon content, and mainly developed siliceous-clay mixed shale lithofacies.The vertical changes of paleoclimate indicators(CIA, Rb/Sr), paleo-productivity indicators(Cu/Al, P/Al), redox environmental indicators(MoEF, UEF), and terrestrial input(Ti, Zr) indicators indicate a high productivity, anoxic sedimentary environment, low terrestrial input and relatively dry and cold climate conditions for the transgression period, while during the high-stand systems tract deposition, the study area has low productivity, oxic-dysoxic environment, high terrestrial input and relatively warm and humid climateconditions.The paleo climate, paleo productivity, redox conditions, volcanic activity, terrestrial input and bottom currents are the main factors controlling the stratigraphic heterogeneity of the Wufeng-Longmaxi shales in the study area.Finally, by comparing with some parameters of the shale in the Upper Yangtze region, it is found that the study area shows the differences in deeper burial depth, thinner organic shale, lower gas content and more brittle mineral content.

The Cretaceous greenhouse represents the largest global warming event since the Phanerozoic, significantly affecting the provenance, weathering, regional climate, and paleoenvironment of lake basins globally. During the period of global warming, the control of regional paleoenvironment over organic matter enrichment is still unclear. The Lower Cretaceous Prosopis Formation in the Bongor Basin features thick lacustrine organic-rich mudstones as major effective hydrocarbon source rocks with excellent hydrocarbon potential in the basin. However, the provenance, paleoclimate, paleoenvironment and mechanism of organic matter accumulation in this location have not yet been systematically studied. In this study, 22 dark gray mudstone samples were collected from three cored wells. Clay mineralogy, along with organic and inorganic elemental geochemical analyses were jointly carried out to determine the characteristics of the provenance, paleoclimate and paleoenvironment as well as the major controlling factors for organic matter enrichment. The results imply that the Prosopis Formation mudstones are mainly composed of clay minerals such as illite, kaolinite, chlorite, and illite/smectite mixed layers (I/S). Organic geochemistry indicated that all the studied mudstones reached the standard of good to excellent source rocks (the total organic carbon content ranged from 1.27% to 7.41%). Inorganic elemental geochemistry is characterized by high enrichment of Fe, Mg, P, Mn, Ti, Na, Ca, V, Co, Cu, Sr, Ba and B relative to UCC and PAAS, whereas rare earth elements (REEs) show enrichment of light rare earth elements (LREEs) and a deficiency of heavy rare earth elements (HREEs) relative to chondrite. The source composition of the mudstones is predominantly intermediate-felsic igneous rocks that have not undergone sediment recycling. The parent rocks experienced weak to moderate weathering, corresponding to semiarid paleoclimatic conditions of transitional nature. The organic-rich mudstones were deposited in fresh to brackish waters with paleoredox conditions ranging from weakly oxic to anoxic semi-deep to deep lake environments having relatively low initial productivity. Comprehensive study indicates that paleoredox and paleoclimate are important parameters affecting the enrichment of organic matter in lake basins during global warming. The aridification of climate is one of the most important reasons for the high input and efficient preservation of organic matter. The relatively semiarid climate both increased the supply of terrigenous nutrients to the lake basins and provided a driving force for the increase in salinity, thereby providing better sources and preservation conditions for organic matter. • The provenance is intermediate-felsic igneous rocks, which has undergone weak-moderate weathering in a semi-arid climate. • Organic-rich mudstones were deposited in fresh-brackish waters with weakly oxic-anoxic environments and low productivity. • Redox and climate are important parameters affecting the enrichment of organic matter in lake basins during global warming. • Climate aridification is a causal mechanism for high input and preservation of organic matter in Cretaceous lake basins. [ABSTRACT FROM AUTHOR]

The vertical geochemical variations in total organic carbon (TOC) content and major and trace elements of the Lower Cambrian Shuijingtuo Formation from the Yichang Slope in the Upper Yangtze were investigated to assess the environmental conditions (redox conditions, water mass restriction, terrigenous input, relative sea-level terrestrial, and paleoproductivity) and to determine the primary controlling factors for organic matter enrichment. The Shuijingtuo Shale is divided into three intervals. The lithofacies of interval I are mainly black siliceous shale with high TOC content, and interval II is mainly black siliceous shale with moderate TOC content. Interval III consists of black, clay-rich siliceous shale and dark-gray calcareous shale and is characterized by a low TOC content. The effects of diagenesis and hydrothermal activity on the elements were evaluated prior to analyzing the environmental condition. There are good positive correlations between TOC and U/Al or Mo/Al ratios, suggesting that major/trace elements still retain the geochemical signature of the sedimentary environment. Meanwhile, the geochemical proxies consisting of Al, Fe, Mn, and Ti indicate that the study area did not experience hydrothermal deposits. The redox proxies (U/Th, Corg:Ptot, and MoEF–UEF) indicate that the interval I samples were formed under a strong reducing condition. The diagram of TOC vs. Mo indicates that the water mass was moderately restricted during the deposition of interval I shales. Proxies of terrigenous input and relative sea-level (Zr/Al and Zr/Rb) suggest that the organic-rich shales at the bottom were deposited under a relatively high sea-level and experienced minimal input of terrigenous debris from the source area. Additionally, the paleoproductivity indicated by Sibio and (Ni + Cu + Zn)/Al was high for interval I samples. During the interval II period, the relative sea-level began to decline, the seawater still remained in reducing conditions, and there was no change in the productivity, but the input of terrigenous debris increased significantly. In the interval III depositional period, the relative sea-level continued to decrease, the seawater shifted to a dysoxic condition, and the paleoproductivity was also at a lower level. The evolution of the sedimentary environment indicates that the high TOC content in the interval I samples is mainly attributed to the strong reducing condition, the preservation condition and debris dilution together control the organic matter content within the interval II samples, and the low TOC content within the interval III samples is constrained by a combination of the poor preservation conditions and lower paleoproductivity.

Most scholars believe that transgression events can contribute positively to organic matter enrichment and shale gas sweet spot development, while whether or not regression events are conducive to shale gas sweet spot development remains to be further discussed. Variations in organic carbon content (TOC), and major and trace elements at the Rhuddanian–Aeronian stage in the Luzhou area, the southern margin of Sichuan Basin, were analyzed in this paper. We discuss differences in paleoenvironment organic matter enrichment and shale sweet spot development during transgression and regression. A transgressive system tract (TST) occurred during the early Rhuddanian stage, while early highstand system tracts (Ehst-1 and Ehst-2) occurred during the late Rhuddanian stage and Aeronian stage, and a late highstand system tract (LHST) developed during the late Aeronian stage. A rise in sea level during the TST in the upper Yangtze resulted in an anoxic environment, where a continuous upwelling current brought about a large number of nutrients in the seawater, significantly increasing paleoproductivity. Strong tectonic subsidence, weak chemical weathering, and a rising sea level together led to a low terrigenous debris supply in the catchment area. Therefore, paleoproductivity and redox conditions were the primary controlling factors of organic matter enrichment at the TST stage, with a clastic supply of secondary importance. With the advance of the Guangxi orogeny, the organic matter enrichment at the EHST-1, EHST-2, and LHST stages was mainly controlled by redox conditions and debris supply. A comparison of the key physical parameters and geochemical indicators of shale reservoirs from these four system tracts suggests that two shale sweet spot types (type I and II) were developed during the Rhuddanian–Aeronian stage, occurring in the TST and EHST-2 stages. High TOC and high microcrystalline quartz content are key to developing type I sweet spots, while enhanced anoxic conditions in the bottom water caused by ephemeral, small-scale sea level rises are the main determinant of class II sweet spots in the later EHST stage.

The Youganwo Formation oil shale located in the Maoming Basin represents a large commercially valuable lacustrine oil shale resource and a potential bio-shale gas reservoir in South China. With the aim of deepening the understanding of factors that influence organic matter enrichment, this research conducted a geochemical investigation to reconstruct the depositional paleoenvironment of bioproductivity, preservation and dilution. Youganwo Formation oil shale is mainly deposited in semi-deep to deep-lake environments with relatively warm and humid paleoclimate in the subtropical-temperate zone. The total organic carbon (TOC) content (1.46–11.85%), S2 values (4.79–115.80 mg HC/mg rock) and HI (328–1040 mg HC/mg TOC) indicate that the oil shale has a good oil source rock potential. TOC content, (S1 + S2) values and vitrinite reflectance values show that its marginally mature organic matter (OM) belongs to kerogen type I-III with good oil-generating potential.A 3rd order sequence was identified in the Yougnwo formation. Subsequently, the multiple factors including bioproductivity, preservation and dilution that control the OM enrichment of oil shale within system tracts were discussed. Moderate-quality oil shales (Oy-1) were developed in the transgressive systems tract (TST) in an oxidizing condition with abundant detrital input. High-quality oil shales (Oy-2) were deposited during the high-stand systems tract (HST) with increased accommodation space, improved preservation conditions, warm and humid climate, higher water bioproductivity and minimum detrital matter input. During the regressive systems tract (RST, Oy-3), higher detrital matter input and fresher water led to lower TOC values. Among these multiple factors, dilution condition was the major one that influences OM abundance and variation on the basis of sufficient organic matter input. Thus, OM enrichment models of Oy-1, Oy-2 and Oy-3 sub-members were established. The OM enrichment and quality in oil shale were controlled by the combined effect of bioproductivity, preservation, and dilution.

The Niutitang Formation in the Upper Yangtze Region comprises a set of the Early Cambrian black shales containing abundant organic matter. We investigated the relationship between the depositional environment and organic matter enrichment based on core description, slice observation, and geochemical analysis of the main trace elements. Additionally, we reconstructed the depositional environment of the Lower Cambrian Niutitang Formation shales and analyzed the main factors controlling the organic matter enrichment of shales during different deposition periods. According to the division of lithofacies, seven types of lithofacies have been identified in the Niutitang Formation. It is believed that the Niutitang Formation in northern Guizhou comprises a set of silicon-rich shales deposited on the continental shelf slope and formed under the control of different hydrodynamic forces. The main factors controlling its formation mechanism changed many times, from the initial hydrothermal deposition and enrichment controlled by oxygen deficiency to the enrichment accelerated by the high productivity at the surface of the water body. Subsequently, the change in sea level caused the change in the sea depth, which made it difficult for lower productivity to promote organic matter enrichment. Multiple main factors cooperated with the evolution of the depositional environment and jointly controlled the formation and distribution of organic-rich shales.

Copyright of Journal of Northeast Petroleum University is the property of Journal of Northeast Petroleum University Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A total of 70 shale samples from seven core holes, namely, N203, N208, N209, N210, N211, N215 and N217, in Changning area, south Sichuan, were analyzed for major and trace elements. Based on an analysis of longitudinal variation characteristics of major and trace elements, the REDOX conditions, paleoproductivity, relative sea level height and terrigenous detrital injection amount of gas-bearing shale in Wufeng Formation–Submember Long-11 in the Changning area were studied. And, the main controlling factors of organic matter enrichment in Wufeng Formation–Submember Long-11 were discussed. The results show the following: (1) Trace elements Mo, U, Ba and Ni are relatively enriched in the shale of Wufeng Formation–Submember Long-11 in the Changning area, while the Co element is deficient. The flat distribution pattern of rare earth in Wufeng Formation–Submember Long-11 shows that the shale deposits are weakly influenced by terrigenous materials. The amount of terrigenous injection in Wufeng Formation is higher than that in Submember Long-11, and the amount of terrigenous injection increased gradually from the lower part of Bed Long-111 to the lower part of Bed Long-114. (2) The REDOX environment discrimination indexes such as U/Th, Ni/Co, V/Sc and δU show that the water REDOX conditions of Wufeng Formation–Submember Long-11 in the Changning area have changed many times during the shale sedimentary period, and the overall REDOX conditions are dominated by an anaerobic environment. The REDOX indexes at the boundary between Long-111 and Wufeng Formation are relatively small and in the oxidizing environment range. The REDOX index from Bed Long-111 to Bed Long-114 decreases gradually, and the anaerobic environment of Bed Long-111 and Bed Long-112 gradually changes to the oxygen-poor environment of Bed Long-113; and, the REDOX index of Bed Long-114 is the lowest, which is generally oxygen rich. (3) Organic matter enrichment in Wufeng Formation Submember Long11 in the Changning area is mainly controlled by REDOX conditions. Low terrigenous injection and a relatively high sea level have positive effects on organic matter enrichment, but there is no obvious relationship between the paleoproductivity and organic matter enrichment.

The Jinyang Basin is a typical volcanic-sedimentary basin, located in the southern peripheral area of the Songliao Basin. Hydrothermal activity is often closely related to the intrusion or eruption of magma. It was recently suggested that dolomite in the Jurassic Beipiao Formation was formed under the influence of magmatic-hydrothermal activity and magmatic-hydrothermal activity might have impacts on the organic matter of the source rocks. No investigation has been aimed at the effect of magmatic-hydrothermal activities on the accumulation of organic matter in the Beipiao Formation source rocks and a comprehensive study is urgent, which would be indicative in unravelling the accumulation mechanism of organic matter and useful in further petroleum exploration. To provide important insights into these issues, we carried out a detailed investigation of geological and geochemical analysis for Wolong (WL) and Dongkuntouyingzi (DK) outcrop shales from the Lower Jurassic Beipiao Formation in the Jinyang Basin. The hydrothermal indicator discrimination diagram (Zn-Ni-Co triangular plot) and rare earth element anomalies (δEu and δCe) indicate that the formation of WL samples is associated with hydrothermal activity, but DK is not. The TOC values suggest that most of the WL and DK samples are good to very good and fair to good source rocks, respectively. The Ro values suggest that both WL (Ro = 1.17%) and DK (Ro = 1.01%) samples have entered the oil-generating stage, and WL samples were influenced by the magmatic activity with higher maturity. The biomarkers such as high steranes/hopanes, high 4-methyl steranes/C29 steranes, low Pr/Ph values and high gammacerane index suggest that WL samples were deposited in an anoxic-prone saline environment with significant contributions of algal sources. Contrarily, the DK samples were deposited in oxic-prone and freshwater paleolake with significant contributions of terrigenous organic matter. The magmatic-hydrothermal activities in the Wolong area brought numerous nutrients to the lake basin, which may facilitated the reproduction of aquatic organisms. At the same time, the magmatic-hydrothermal activities increased the salinity of water and promoted the formation of a water reducing environment, which provided an excellent environment for the preservation and enrichment of organic matter. Therefore, the magmatic-hydrothermal activities in the Wolong area promoted the formation of organic-rich source rocks and the hydrocarbon generation process. [ABSTRACT FROM AUTHOR]

The early Carboniferous experienced profound climate cooling that drove the Earth's climate from a mid-Paleozoic greenhouse into the Late Paleozoic Ice Age. Several climate cooling events have been reported, including the Tournaisian and Visean (early to mid-Early Carboniferous). In this study, we perform multi-proxy analyses (organic carbon isotopes, nitrogen isotopes, major and trace elements and organic petrology/geochemistry) of samples from an early Carboniferous section in Nandan region (Guangxi, China). Our goal is to investigate the global carbon‑nitrogen cycle, and associated oceanic productivity and redox perturbations, during a key climatic transition interval, as well as the controls on organic matter enrichment in the sediments. The carbon and nitrogen isotope profiles of the Nandan section record major perturbations during the mid-Tournaisian and early Visean. The mid-Tournaisian carbon isotope excursion (TICE) is marked by a positive δ13C org shift of 1.8 ‰ (from −27.7 ‰ to −25.9 ‰), correlating with a positive δ15N shift. The early Visean carbon isotope excursion (VICE) is characterized by a positive shift in δ13C org (from −28.2 to −25.5 ‰, with an excursion magnitude of 2.7 ‰), but associated with a negative shift in δ15N (from +5 ‰ to +4 ‰). The positive δ13C org excursions during these events most likely reflect enhanced organic matter burial with expansion of anoxic seafloor in the global ocean. The drop in nitrogen isotope values in the early Visean is interpreted to be linked with less denitrification under more oxic conditions. The decrease of organic matter contents up section is consistent with the shift to more oxic conditions and increased sedimentary dilution caused by sea-level fall, which is ultimately controlled by orogenic events and climate cooling. • The C and N cycle during the mid-Tournaisian and early Visean were studied. • The positive δ13C org excursions may reflect enhanced organic carbon burial. • The drop of δ15N in the early Visean may be linked with reduced denitrification. • Organic matter enrichment in the Luzhai Formation linked with climate cooling. [ABSTRACT FROM AUTHOR]

The connections between the depositional environment, sediment chemistry, and pore structure of deep-water mudstones are still not completely understood. This study: 1) investigates the sediment chemistry and the depositional conditions of the Pennsylvanian Cline Shale chemofacies in the center of the Midland Basin, 2) characterizes the conditions favorable to organic matter accumulation of each mudstone chemofacies, and 3) examines the connection between the pore structure and the mudstones' paleo-environmental and paleo-geochemical context and processes. This study uses X-ray fluorescence (XRF) and statistical methods (Isolation Forest, Principal Component Analysis, and K-means) to identify four chemofacies of 127 m of Cline Shale core from the Matthew 'D' 0906 well located in the Midland Basin depocenter in Glasscock County, Texas. In addition, petrographic and scanning electron microscopy and measurements by dual-energy computed tomography, X-ray diffraction, Rock-Eval pyrolysis, N 2 isothermal adsorption tests, and mudrock reservoir properties were conducted on thin-sections, core slabs, and samples to understand the bulk geochemistry and petrophysical properties of the mudstones. Four chemofacies are identified: (1) Siliceous Mudstone with high Zr and low Ca and P, (2) Calcareous-argillaceous Mudstone I with high Ca and Zr, and low P, (3) Argillaceous Mudstone with high P and low Ca and Zr, and (4) Calcareous-argillaceous Mudstone II with high Ca and P, and low Zr. Based on elemental distribution, the first two and last two chemofacies are interpreted as low sea-level deposits and high sea-level deposits, respectively. Water stratification and P regeneration accounted for the highest total organic carbon (TOC) content, which resulted in the high porosity and pore volume of the low sea-level Siliceous Mudstone chemofacies, especially visible in the 8 nm–186 nm organic matter pores. Carbonate input hinders the development of the inorganic pore system, especially for organic lean Calcareous-argillaceous Mudstones. Nine sea-level cycles were observed in the cores and correlated to nine US mid-continent cycles attributed to the 405-kyr Milankovitch parameter (long orbital eccentricity) during the late Pennsylvanian. We hypothesize that late Pennsylvanian climate change, paced by orbital cycles, controlled the distribution and stacking pattern of mudstone chemofacies, which also controlled source rock quality and consequent pore system development. • Sediment chemistry of late Pennsylvanian mudstones from the central Midland Basin reveals four chemofacies. • Chemofacies correlate to nine cycles of sea level change and follow long eccentricity (405-kyr) orbital cycles. • The highest total organic carbon (TOC) contents occur during low sea level when the basin was partially restricted. • Sediment and TOC accumulation rates match those of organic-rich sediments of the Black Sea. • Organic porosity correlates with depositional environment; carbonate input hinders development of inorganic pore systems. [ABSTRACT FROM AUTHOR]

Depositional events have a significant impact on the terrestrial redox conditions and provide evidence for studying the organic matter enrichment. The objective of this study was to evaluate the influence of volcanic and turbidity current events on benthic redox conditions during the Upper Triassic Chang 7 member (Ch7) of the Ordos Basin. To address these issues, the varying sedimentological settings, paleoredox conditions, and relationships between the redox environment and depositional events were investigated via sedimentary analysis of the profiles, microscopic analysis, organic geochemical analysis, and elemental geochemical data. The fine-grained sediments in the Chang 7 3 submember (Ch7 3) consist of abundant organic matter, collophanite, and framboidal pyrite. However, in the upper part of the Ch7 member, there was a decrease in organic matter, a decrease in the number of microorganisms, and an increase in pyrite size, indicating that the oxic environment is not favorable for organic matter enrichment. The element and geochemical proxies show similar vertical variations and redox changes. Volcanic activity can bring substantial amounts of material or elements to the basin. The enrichment of Hg and S exhibited considerable variation, ranging from 8.7 to 274.4 ppb and from 0.16 to 4.53 wt%, respectively, which influenced the organic matter accumulation through the flourishing and death of microorganisms and redox changes in the benthic environment of the terrestrial basin. As the lake basin shrinks, the Ti and Al contents increase with increasing frequency of turbidity current events, and the terrestrial debris transported to the lake basin gradually increases while carrying large amounts of oxygen and affecting sedimentation rates, contributing to the destruction of the reducing conditions of the benthic environment, subsequently, influencing organic matter accumulation. These results will be helpful in understanding the effect of multiple depositional events on organic matter enrichment in lacustrine basins. • Geochemical and sedimentary results suggest altered redox conditions. • Volcanic activity enhanced the redox conditions. • Turbidity current events influence dilution and biodegradation efficiency. [ABSTRACT FROM AUTHOR]

The organic-rich shale of the Permian Taiyuan Formation (TYF) and Shanxi Formation (SXF) in the Southern North China Basin (SNCB) is considered a potential shale gas source. The shale was formed in a marine-continental transitional sedimentary environment, which has rarely been studied, with the enrichment mechanisms of organic matter (OM) remaining unclear. This study investigated the controlling factors and enrichment mechanisms of OM by analyzing the total organic carbon (TOC) content, paleoclimate, paleoproductivity, sedimentation rate, redox, and paleosalinity. The TOC of the TYF ranged from 0.92 to 7.43 wt.%, with an average of 2.48 wt.%, which was higher than that of the SXF (TOC = 0.36–5.1 wt.%, average of 1.68 wt.%). These geochemical indices suggest that both the TYF and SXF were deposited in warm and humid paleoclimates, with relatively high biological productivity and sedimentation rates. During the deposition process, the TYF experienced frequent transgression and regression events, leading to an enhancement of water reducibility, a relatively high sedimentation rate, reduced OM oxidation, and rapid deposition of OM, which were conducive to the preservation of OM. Moreover, a high biological productivity increased respiratory oxygen consumption in the water column, which could lead to OM accumulation. However, the regression event experienced by the SXF reduced the paleoproductivity and sedimentation rate and increased water oxidation, leading to a decrease in OM. The main controlling factors for the enrichment of OM in the TYF and SXF were the sedimentation rate, paleoproductivity, and redox conditions, thus establishing the enrichment models for OM in the TYF and SXF. This study is conducive to understanding shale enrichment mechanisms and guiding shale gas exploration.

Early Cambrian time witnessed accumulation of high-quality organic-rich siliceous shale deposits that comprise an especially promising shale gas exploration target, especially in China. The present study of the Cambrian Niutitang Formation of the shelf area of the Upper Yangtze Platform utilizes mineralogical and elemental geochemical data to aide in the paleoenvironment reconstruction of these organic-rich deposits. We are especially interested in elucidating organic carbon and silicon enrichment mechanisms of the Niutitang Formation that will benefit shale gas exploration and development of these deposits. The Niutitang Formation can be subdivided into three intervals based on lithology, gamma ray (GR) signature, and TOC abundance. Enriched TOC (average = 6.7%) and nano-size authigenic microcrystalline quartz (Qn) contents of Interval I deposits likely reflect deposition in an environment that experienced persistent euxinic and elevated organic matter delivery associated with enhanced paleoproductivity driven by hydrothermal activity. Interval II shale is interpreted to have accumulated under euxinic-ferruginous conditions that experienced the effects of robust paleoproductivity and diminished terrigenous input related to persistently warm, humid climatic conditions. Enriched TOC (average = 4.9%) and moderate microcrystalline quartz content of these deposits were likely the result of redox conditions and elevated paleoproductivity. Interval III strata appears to have accumulated under oxygenated bottom conditions that experienced moderate paleoproductivity, brackish water and elevated terrigenous detritus input driven by the establishment of cold, dry climatic conditions. The combined results of these conditions are diminished TOC content (average = 0.7%) and enrichment of the sediment in terrestrial quartz. The elevated content of authigenic quartz appears to have diminished the reservoir capacity of Niutitang Formation shale. Interval II organic-rich siliceous shale deposits of Upper Yangtze Platform shelf areas, which have the comparable authigenic microcrystalline quartz content, reservoir performance, and greater organic matter abundance as the rift trough, appear to comprise the stratigraphic horizon with the greatest exploration and development potential. Results of the present study offer theoretical guidance for future shale gas exploration and development efforts in geologically similar regions. • The environment during the Niutitang shale sedimentation period are reconstructed. • Mechanism of organic matter and silica enrichment of the Niutitang shale are revealed. • Authigenic quartz significantly affects the reservoir capacity of the Niutitang shale. [ABSTRACT FROM AUTHOR]

Sepetiba Bay (SB) is one of the Brazilian coastal regions most impacted by anthropogenic activities due to increased population density, the installation of tourist and industrial mega-developments, and essential port, agricultural and fishing activities. Given the great concern about its environmental quality, several studies have focused on it, especially since the 1980s. This study aims to analyze the current ecological risk by potentially toxic elements (PTEs) in the bottom sediments of Sepetiba Bay (SB). The work integrates numerous variables (physicochemical, textural, and geochemical) analyzed at 73 stations in SB and the transition area to Ilha Grande Bay. The results show that the sediments in the eastern-inner region of Sepetiba Bay, along the Itacuruçá coast, and in some areas near the Marambaia Barrier Island (MBI) are severely polluted by Cd (4.47 mg kg−1) > Sn (7.31 mg kg−1) > Zn (824 mg kg−1) > Co (12.50 mg kg−1) > Pb (36.6 mg kg−1) > Cr (65.0 mg kg−1) > Fe (4.98 %) > Ni (21.4 mg kg−1) > V (66.0 mg kg−1), (maximum values), and highly polluted by U (2.8 mg kg−1) > Mn (857 mg kg−1) > Sb (0.32 mg kg−1) > Tl (0.63 ppm) > Hg (170 μg kg−1) > As (11.2 mg kg−1), maximum values. The pollution load index (PLI) and contamination degree (Cdeg) indexes also indicate considerable to very high contamination, indicating severe pollution in that zone. The Potential Ecological Risk Index (PERI) suggests high (160≤RI < 320) and very high (RI ≥ 320) ecological risk in the inner and middle areas of Sepetiba Bay by Cd > Hg > Cu > Co > Pb > Sn > Tl > Ni. Relatively low pH and Eh values were identified in the inner region of Sepetiba Bay, indicating eutrophication processes. A comparison of the concentration factors in the surface sediments of SB in 2009–2011 and 2022 suggests that there has been a general increase in contamination by metals, namely Fe, Mn, Pb, Co, Cr, Ni, U, V, and Hg. Although metals accumulate more pronounced in the inner and the most impacted marginal areas of Sepetiba Bay, their enrichment factors (EF) indicate an increase in contamination in the outer sector. Metals are being imported from the nearby oceanic region, where there is a dredging site, to the outer sector of Sepetiba Bay. Thus, the choice of dredged spoil disposal sites and how contaminated dredged material is deposited needs to be better addressed and is still a challenging task. This aspect often results in problems being transferred from one region to another. The results suggest that mitigation measures must be adopted to prevent the situation from worsening. • Worsening of metal pollution in 2022 compared to 2009–2011 in Sepetiba Bay. • There are two main axes of pollution in Sepetiba Bay: in the inner and outer areas. • 45% of the analyzed sites located in the inner area have high/very high potential ecological risk. • Transferring pollutants from the inner to the outer zone by dredging activities. [ABSTRACT FROM AUTHOR]

• Organic petrology and biomarkers in the Lower Cretaceous Jiufotang Formation from the Kazuo Basin were investigated. • Terrigenous organic matter was diluted by detrital minerals and phytoplankton was the most important organic matter source. • Anoxic bottom water may be necessary for organic matter enrichment in lake systems. • Climate changes controlled the organic matter enrichment model during deposition of the Jiufotang Formation. The Lower Cretaceous Jiufotang Formation, situated in western Liaoning Province, China, has attracted attention for abundant Jehol Biota fossils and the potential to improve understanding of the relationships among climate change, evolution of terrigenous organisms, and organic matter (OM) enrichment in lake systems during the Early Cretaceous. Based on organic petrology and biomarker analyses, this study investigated the depositional environment, OM sources, and enrichment mechanism of Jiufotang mudstone in the Kazuo Basin. Maceral composition shows that the OM source of Jiufotang mudstone shifted from mainly aquatic to terrigenous, and biomarkers such as abundant diterpenoids and retene indicate that terrigenous higher plants were mainly conifers during deposition of the Jiufotang Formation. Pr/Ph ratios (0.07–0.65) show that the studied section was deposited in anoxic bottom water that may be necessary for OM enrichment in lake systems. Gravity flows were accompanied by input of many clastic minerals and oxygenation of bottom waters, which is unfavorable for OM enrichment. Therefore, the best OM enrichment model for lake systems includes anoxic bottom water, stable water column stratification, low sedimentation rates, and high phytoplankton contributions. The climate changed from relatively dry to humid, which drove changes in the OM enrichment model in the lake. More importantly, the climate and terrigenous plant information obtained by biomarkers is useful to reconstruct the ecological environment of the Jehol Biota. [ABSTRACT FROM AUTHOR]

Shale oil is an unconventional oil resource that needs to be developed and utilized urgently. However, the Chang 7 shale in the Ordos Basin, as the most typical continental source rock in China, is limited by the study of organic matter (OM) enrichment factors in continental lacustrine facies, and there are still controversies about the controlling factors, which limit the progress of oil and gas exploration. This paper aims to reconstruct the paleoenvironment of Chang 7 shale in the southern margin of Ordos Basin and reveal the controlling factors of organic rich shale by organic and elemental analysis, X-ray diffraction (XRD) analysis, thin section observation, and scanning electron microscopy-energy dispersive spectrometer (SEM-EDS) analysis. The results show that during the deposition period of Chang 7 shale, the climate was warm and humid, the lake water has strong reducing, low salinity and rapid depth changes. Total organic carbon (TOC) is positively correlated with salinity and hydrothermal action and inversely proportional to terrigenous input. The high productivity, low consumption and low dilution result in high enrichment of shale OM in the southern margin of Ordos Basin. [ABSTRACT FROM AUTHOR]

Lithofacies paleogeography shows that a set of black shale in the foreland basin front zone is mainly developed in the Lower Yangtze region at the turn of Ordovician-Silurian, and the Lower Yangtze region is obviously different with the Middle and Upper Yangtze regions. The study on the sedimentary environment evolution of the black shale is relatively weak, especially the related study on organic matter enrichment. Therefore, it is necessary to clarify the sedimentary environment and organic matter enrichment mechanism of Wufeng Formation-Gaojiabian Formation shale in the Lower Yangtze region, which is of great significance for deepening the understanding of shale gas enrichment law. Based on the continuous underground core data obtained by the latest shale gas geological survey well(WDD1)in the southwest of the Lower Yangtze region, 38 core samples were tested for organic carbon, major and trace elements. The vertical variation characteristics of major and trace elements in the profile and their relationship with the sedimentary environment of the basin were analyzed. The main controlling factors of organic matter enrichment of Ordovician-Silurian shale in the southwest of the Lower Yangtze region were discussed. The results show that Wufeng Formation and the second member of Gaojiabian Formation are enriched in organic matter, and the CIAcorr value indicates that the climate of Jiangnan transitional zone is relatively warm and humid at the turn of Ordovician-Silurian. Contents of Babio, Cu, Zn and other indicators confirm Wufeng-Gaojiabian deposition period has a high level of paleoproductivity; V/Cr, V/(V+Ni)and V/Sc values reveal that the bottom water of Wufeng Formation during the deposition period is an anaerobic sulfide state, and the bottom water of the first member of Gaojiabian Formation during the deposition period is converted into an oxygen-poor oxidation state. Diagram of Mo-TOC and covariation diagram of EFMo-EFU show that Wufeng deposition period is medium-retention sedimentary environment, and Gaojiabian deposition period is medium-weak retention sedimentary environment. TOC shows a weak positive correlation with contents of Al and Ti in Wufeng deposition period, while it turns into a negative correlation in Gaojiabian deposition period. The enrichment mechanism of organic matter in black shale of Wufeng Formation and Gaojiabian Formation is different. The main controlling factor of organic matter enrichment in black shale of Wufeng Formation is the semi-retention environment of reduction-sulfidation, and the nutrients brought by terrigenous clastic input promote to some extent. The differences in the enrichment mechanism between Wufeng Formation and Gaojiabian Formation show that the paleo-ocean environment in the Lower Yangtze region has undergone significant changes at the turn of Ordovician-Silurian. [ABSTRACT FROM AUTHOR]

The study of reservoir space development characteristics and the geochemical parameters of the Permian Longtan Formation in the southeast of the Sichuan Basin at Well X1 was analyzed by using core observations, optical thin-section observations, Ar-ion polishing, scanning electron microscopy, and nitrogen adsorption tests. Major and trace elements of samples, the V/Cr, U/T, enrichment coefficients EF (Mo) and EF (U), chemical alteration indexes CIA and Sr/Cu, and Mo, P, and other index values of different samples were calculated based on the test of organic geochemical parameters. The depositional environment of the Longtan Formation in the study area was systematically analyzed. The Longtan Formation was in a warm and humid climate during the deposition period. The main body of the sedimentary water was in an oxygen-depleted state, and a portion of it was in an oxidized state with high biological productivity. Therefore, it had a small deposition rate. The clay mineral content and organic matter content in the sample played a major role in controlling the development of reservoir space. The content of clay minerals in the sample was high, and it was mainly chlorite and a mixed layer of illite/smectite. Under a microscope, it was observed that the mixed layer of flake illite/smectite and chlorite aggregate mostly developed interlayer pores or microfractures, which played a positive role in the specific surface area and pore volume of the sample.

The fine-grained rocks in the Paleogene Shahejie Formation in Nanpu Sag, Huanghua Depression, Bohai Bay Basin, are extremely important source rocks. These Paleogene rocks are mainly subdivided into organic-rich black shale and gray mudstone. The average total organic carbon contents of the shale and mudstone are 11.5 wt.% and 8.4 wt.%, respectively. The average hydrocarbon (HC)-generating potentials (which is equal to the sum of free hydrocarbons (S1) and potential hydrocarbons (S2)) of the shale and mudstone are 39.3 mg HC/g rock and 28.5 mg HC/g rock, respectively, with mean vitrinite reflectance values of 0.82% and 0.81%, respectively. The higher abundance of organic matter in the shale than in the mudstone is due mainly to paleoenvironmental differences. The chemical index of alteration values and Na/Al ratios reveal a warm and humid climate during shale deposition and a cold and dry climate during mudstone deposition. The biologically derived Ba and Ba/Al ratios indicate high productivity in both the shale and mudstone, with relatively low productivity in the shale. The shale formed in fresh to brackish water, whereas the mudstone was deposited in fresh water, with the former having a higher salinity. Compared with the shale, the mudstone underwent higher detrital input, exhibiting higher Si/Al and Ti/Al ratios. Shale deposition was more dysoxic than mudstone deposition. The organic matter enrichment of the shale sediments was controlled mainly by reducing conditions followed by moderate-to-high productivity, which was promoted by a warm and humid climate and salinity stratification. The organic matter enrichment of the mudstone was less than that of the shale and was controlled by relatively oxic conditions. [ABSTRACT FROM AUTHOR]

A suite of source rock consists of mudstone and shale, with great thickness and continuous deposition was found in the well LK-1 in Lingshan island in Ri-Qing-Wei basin. In order to evaluate the hydrocarbon generation prospects of these source rock and find the mechanism of organic matter enrichment, shale samples were selected from the core for TOC (total organic carbon) and element geochemistry analysis. The results show that organic matter abundance of the source rocks are generally high with average TOC content of 1.26 wt%, suggesting they are good source rocks. The geochemical features show that the sedimentary environment is mostly anoxic brackish water to salt water environment with arid to semiarid climate condition. The enrichment mechanism of organic matter varied with the evolution of the basin, which was divided into three stages according to the sedimentary characteristics. In the initial-middle period of rifting evolution (stage 1 and early stage 2), paleoproductivity is the major factor of OM-enrichment reflecting by high positive correlation between the TOC contents and paleoproductivity proxies. While with the evolution of the rift basin, redox condition and terrigenous clastic input became more and more important until they became the major factor of OM enrichment in the middle stage of rift evolution (stage 2). In the later stage of rift evolution (latest stage 2 and stage 3), besides terrigenous clastic input, the effect of paleoclimate on OM-enrichment increased gradually from a minor factor to a major factor.