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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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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]

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]

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]

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]

Organic matter enrichment in shale is one of the key factors that control shale oil resources; however, studies of the mechanism of organic matter enrichment in shale in saline lacustrine rift basins are still lacking. In this study, analyses of total organic content (TOC), stable isotopes of carbon and oxygen for carbonate, major and trace element, rare earth element, biomarker composition, and field emission-scanning electron microscopy analyses were performed on 31 core samples from the source rocks of the middle interval of the third member of the Shahejie Formation (Es3M). The aims of these analyses were (1) to investigate the controlling effects of paleoenvironment, paleosalinity, redox conditions, dilution, sediment accumulation rate, terrestrial input, and paleoproductivity on organic matter enrichment, (2) to reconstruct the paleoenvironment, and (3) to propose a model of differential accumulation mechanism for organic matter in saline lacustrine source rocks in rift basins. Results show that obvious differences exist in the paleoenvironment of the Es3M between the southern and northern regions in the Dongpu Depression. The TOC of the northern source rocks is generally greater than that of the southern source rocks. The southern source rocks were mainly deposited in a closed, warm, and humid environment with medium chemical weathering [the chemical index of alteration (CIA) values and Mg/Ca ratios range from 57.9 to 75.8 and 0.08 to 0.61, with averages of 70.93 and 0.19], with fresh–brackish water, weak water stratification, reducing conditions, moderate paleoproductivity (the P/Ti and P/Al ratios range from 0.13 × 10−4 to 0.26 × 10−4 (mean 0.18 × 10−4) and 5.11 × 10−3 to 11.31 × 10−3 (mean 7.62 × 10−3), respectively). The northern source rocks were mainly deposited in an open, cold, and arid environment with a weak chemical weathering (CIA values and Mg/Ca ratios range from 57.9 to 75.8 and 0.08 to 0.61, with averages of 70.93 and 0.19), with hypersaline water, strong water stratification, strong reducing condition, moderate paleoproductivity [P/Ti and P/Al ratios range from 0.13 × 10−4 to 0.26 × 10−4 (mean 0.18 × 10−4) and 5.11 × 10−3 to 11.31 × 10−3 (mean 7.62 × 10−3)]. The paleosalinity, redox condition, paleoproductivity, and terrestrial input controlled the differential enrichment of organic matter in saline lacustrine source rocks. For the southern region, organic matter enrichment increased with increasing paleosalinity, reduction degree, and terrestrial input. Organic matter enrichment of the northern source rocks increased with increasing paleoproductivity, which was mainly due to high water salinity, stable deep paleowater depth, and strong reducing condition. [ABSTRACT FROM AUTHOR]

The Cretaceous period is a typical representative of a 'greenhouse climate' in geological periods, and is also an important period for the formation of hydrocarbon source rocks, which has become a hot spot in the study of palaeoclimate and organic matter enrichment. In order to further explore the mechanism of action between paleoclimate and organic matter enrichment, the response mechanism of organic matter enrichment to palaeotemperature and palaeorainfall is finely characterised by synthesising the analysis of sporopollen, biomarkers and n-alkane carbon isotopes in this study. The sporopollen indicates that the palaeoclimate is generally temperate, semiarid to semihumid, and the n-alkane and n-alkane carbon isotopes indicate that the palaeoclimate and palaeorainfall have obvious fluctuations and synchronous characteristics of rainfall and heat. Based on the differences in palaeotemperature and palaeorainfall, the palaeoclimate can be further divided into three stages: cool-temperate and semiarid, warm and semihumid, and cool-temperate and semiarid. The trend of palaeoatmospheric pCO 2 recovered from the carbon isotopes of normal alkanes C 27 , C 29 , and C 31 is roughly the same as that of palaeotemperature, indicating that pCO 2 is important in regulating palaeotemperature. According to the distribution pattern of organic carbon, different layers are divided into three sedimentary Units: I, II and III. Different sedimentary Units have good corresponding relationships with different palaeoclimatic stages. Under the cool-temperate and semiarid climate, lower palaeotemperature and higher palaeosalinity are not conducive to the prosperity of lake organisms. The formation of light grey mudstone, silty mudstone and carbonate has lower TOC, and the organic matter types are mostly II 1 and II. Under a warm and semihumid climate, higher palaeotemperature and moderate palaeorainfall are conducive to the prosperity of lake organisms. The formation of oil shale and shale has higher TOC, and the organic matter types are mostly I and II 1. However, excessive palaeorainfall brings much terrigenous debris, leading to the organic matter dilution. Therefore, palaeoclimate and palaeorainfall not only affect the abundance of organic matter, but also affect the types of organic matter. In the paper, the mechanism of organic matter enrichment is improved by delicately characterising the palaeotemperature and palaeorainfall. • Reconstruction of the early Cretaceous paleoclimate using n-alkanes and sporopollen in terrestrial sediments. • High precision recovery of atmospheric pCO 2 in the Early Cretaceous using carbon isotopic of n-alkanes. • Revealed the impact of paleoclimate on organic matter enrichment. [ABSTRACT FROM AUTHOR]

• The deposition age of the Middle Dingqinghu Formation oil shale was about 18–16 Ma. • The formation of the lacustrine oil shale may have close relationship with the MMCO. • Warm-humid climate and favorable preservation condition control the OM enrichment. • The formation model of the Middle Dingqinghu Formation oil shale is established. The Lunpola oil shale is the largest lacustrine oil shale deposit in Tibet (SW China), which deposited in a Cenozoic lacustrine rift basin and contains abundant organic matter (OM), whereas the crucial factors that controlled the OM enrichment remain elusive. Here, we present new datasets of total organic carbon (TOC) contents, clay minerals and whole-rock geochemistry of the early-middle Miocene Middle Dingqinghu Formation oil shale from the Lunpola basin, central Tibet, in order to elucidate the paleoclimate, paleo-environment condition and the controlling factors on the OM enrichment of the lacustrine oil shale. Oil shales from the Lunpola East section have high TOC contents (avg. 9.60%), while the contiguous mudstones contain low TOC contents (avg. 0.44%). During the oil shale deposition, high smectite and kaolinite contents, low illite contents and low Sr/Cu ratios suggest strong chemical weathering and hence warm and humid climate, which may probably have close relationship with the mid-Miocene climatic optimum (MMCO). However, during the mudstone deposition, the relatively high illite and low smectite contents and higher Sr/Cu ratios suggest weak chemical weathering, indicating relatively arid climatic condition. The paleo-environment indexes (C org /P, Mo EF and U EF) and Mo EF -U EF covariation reflect an anoxic bottom water condition during the oil shale deposition, while the mudstone formed in oxic-dysoxic water environment. The weak and moderate positive relationship between bio-productivity proxies (P/Ti and Cu/Ti ratios) and TOC contents reveal that the bio-productivity condition is not the crucial factor for the OM enrichment. Moreover, the water column environment was brackish and stratified during the oil shale deposition, and the sedimentation rate was moderate, which were also conducive to the OM preservation. The positive correlations between the TOC and the paleoclimate and paleoenvironment proxies demonstrate that warm and humid climate during the MMCO and anoxic water conditions play predominant roles in OM enrichment of the Lunpola lacustrine oil shale. Besides, modest sedimentation rate can prevent the dilution of OM, which can also promote the OM enrichment. This study can not only deepen the understanding of OM enrichment mechanism in lacustrine rift basin but also provide important information for predicting the distribution of the oil shale in the Lunpola Basin. [ABSTRACT FROM AUTHOR]

Lower Paleozoic shale is an important source rock and reservoir for gas in the northwestern Upper Yangtze Plate. The paleoenvironment, and its impact on organic matter enrichment of the shale, plays a vital role in the exploration and development of the resource. Based on the analysis of the results of total organic carbon (TOC), maceral examination, bitumen reflectance (Rb), major elements, trace elements, rare earth elements (REEs), and X-ray diffraction (XRD) of shale samples from the Niutitang and Longmaxi formations, the origin of silica, the paleoenvironment, and the mode of organic matter enrichment of the shale were examined. The shales of the Niutitang and Longmaxi formations have high TOC abundance, with Type I organic matter and high maturity. Quartz and clay minerals dominate the mineral constituents, leading to characterization as siliceous shale. The average SiO 2 content of the shale in the Niutitang and Longmaxi formations is above 70 wt%. Al 2 O 3 and TFe 2 O 3 (total iron) are the next-most abundant major elements, with average values of 12.80 wt% and 1.95 wt% in the Niutitang Formation, and average values of 11.44 wt% and 4.09 wt% in the Longmaxi Formation. Compared with the Upper Continental Crust (UCC), V, Mo and U are the most enriched elements. The average ∑LREE/∑HREE of the shale samples is very low, and the Chondrite-normalized REE distribution pattern shows sloping LREE trends and flat HREE trends towards the right. The total rare earth elements of the shale vary within the range of 147.85–219.60 ppm and 159.90–222.02 ppm in both the Niutitang and Longmaxi formations, suggesting that the shale has not been subject to obvious hydrothermal fluid activities. The correlation diagrams of SiO 2 Al 2 O 3 , SiO 2 -TOC and Si excess -TOC, the diagrams of hydrothermal origins, and the elemental parameters indicate that the silica of the shales from the Niutitang and Longmaxi formations are mainly of biogenic origins, with a lesser contribution from terrigenous detrital sources. The ratios of MnO 2 /TiO 2 and Al 2 O 3 /(Al 2 O 3 +TFe 2 O 3) of the shale samples all show limited variability, illustrating that the shales are mainly deposited on the continental margin. The average chemical alteration index (CIA) values of the Niutitang and Longmaxi formations are 71.25 and 72.77 with little variation, and the average Sr/Cu ratios are 4.38 and 2.22, reflecting a warm and humid paleoclimate for both formations. The average Ni/Co, Th/U, and δU readings of the shale samples of the formations are 10.91, 0.97 and 1.51, and 12.69, 0.43 and 1.75, indicating that the redox conditions during the sedimentation of the formations were anoxic. The positive correlations between TOC and paleoenvironmental proxies reveal a warm and humid climate, and that anoxia had a positive effect on organic matter enrichment. These conditions have laid a geological foundation for the enrichment of shale gas, and that the Upper Yangtze Plate is favorable for future shale gas exploration. • The shales of Niutitang Formation and Longmaxi Formation are both good source rocks. • The silica origin of the shales is primarily biogenic. • The shales were deposited in warm and humid climate with anoxic condition. • The paleoclimate and anoxic conditions had a positive effect on OM accumulation. [ABSTRACT FROM AUTHOR]

Knowledge of the paleoenvironment is crucial to understand organic matter enrichment and shale gas development. Like organic matter, siliceous microfossils such as radiolarians and sponge spicules are widely distributed in Late Ordovician to early Silurian shales. However, there are few characterizations of the depositional conditions of siliceous microfossil-enriched layers. Here siliceous microfossil-rich shales from the Ordovician–Silurian transition in South China were selected for this study. We used major/trace element geochemistry to determine the sedimentary environment of siliceous microfossil-rich shale and to strengthen the understanding of the effect of siliceous microorganisms on organic matter enrichment. Our results show that most siliceous microfossils were deposited in anoxic reducing conditions with the presence of free hydrogen sulfide and accompanied by a moderately to weakly restricted seawater circulation. In addition, the nutrient-rich water bodies could upwell from the deep sea into the Yangtze Sea, and siliceous microfossil-rich shale was deposited during a period of high primary productivity. The shale layers of siliceous microfossils generally correspond to high gas-producing intervals and high organic matter abundance. The accumulation and preservation of organic matter in siliceous microfossil-rich shales is attributed to anoxic benthic conditions, high levels of primary productivity and flourishing of siliceous microorganisms. This study addresses the knowledge gap of the depositional conditions in siliceous microfossil-rich shale, and can help better understand the history of organic matter accumulation. • Siliceous microfossils were mostly deposited in an anoxic environment with H 2 S. • Siliceous microfossil-rich shale was deposited under high levels of productivity. • Organic-enrichment is controlled by depositional conditions and Si-microorganisms. [ABSTRACT FROM AUTHOR]

The deposition of black shale is influenced by several factors, including organic matter input, which is controlled by primary productivity; preservation, which is governed by redox conditions and deposition rates; and dilution, which is controlled by terrigenous input. However, determining the individual contribution of these factors to organic matter enrichment (OME) is complex due to significant variations in palaeodepositional conditions across different regions. In this study, we aimed to investigate the palaeoenvironmental conditions and OME mechanisms in the Wufeng-Longmaxi Formation in western Hubei by integrating data on total organic carbon (TOC) content, mineral compositions, and major and trace elements. We also examined the rapid variations in the palaeoenvironment, which led to the mass extinction in the Late Ordovician. Our findings indicate that dolomite siltstone and silty dolomite are present in the Guanyinqiao Formation (GYQ), which differs from other regions. The silica in the Wufeng-Longmaxi Formation has a mainly nonhydrothermal origin, while the upper Wufeng Formation (WF) has an entirely biogenic source. The organic-rich shales were primarily deposited in a warm and humid climate with low terrigenous input, low salinity, dysoxic conditions, and moderate to high palaeoproductivity due to moderate to strong upwelling. The GYQ serves as a transitional section from the WF to the Longmaxi Formation, which was characterized by a cold climate, oxic conditions, weak upwelling, and deposition in a strongly restricted setting, potentially leading to the mass extinction. Overall, OME was primarily dominated by preservation, with factors that influenced OME changing vertically. Palaeoproductivity, combined palaeoproductivity and palaeoredox conditions, and palaeoredox conditions were the main controlling factors in the WF, the Lower Longmaxi Formation (LLMX), and the Upper Longmaxi Formation (ULMX), respectively. Additionally, the Huangling basement, a submarine high, and the Xuefeng uplift's uplift and compression played important roles in OME. However, shale gas exploration prospects in the study area are limited due to the thinness of the organic-rich shales, oxic to dysoxic conditions, and relatively low palaeoproductivity. • Guanyinqiao Formation exhibits unique characteristics in terms of dolomite siltstone and silty dolomite formation. • Organic matter enrichment in the study area is primarily influenced by preservation conditions. • Shale gas accumulation potential in Huangling submarine high is limited. [ABSTRACT FROM AUTHOR]

Breakthroughs in shale gas exploration in the Upper Ordovician-Lower Silurian strata of the Upper Yangtze Platform have attracted interest in its sedimentary-tectonic evolution, but the tectonic background of the northern margin of the Upper Yangtze Platform remains unclear. In this paper, the Wufeng-Longmaxi formations on the northern margin of the Upper Yangtze Platform were investigated. Based on geochemical and mineralogical analyses of the tuffs/K-bentonites of the Wufeng Formation and the barite in the Longmaxi Formation, as well as previous research results, it was concluded that the northern margin of the Upper Yangtze Platform was in an extensional tectonic background during the Late Ordovician-Early Silurian. Detailed analysis revealed that, (1) the U–Pb zircon age of the tuff in the Bajiaokou section in South Qinling is 443.91 ± 0.92 Ma. The Zr/TiO 2 –Nb/Y diagram of the tuffs/K-bentonites indicates that their protoliths were alkaline-subalkaline basalt and andesite series rock. Based on the Th–Hf/3-Ta, Th–Tb*3-Ta*2, and TiO 2 –Nb/3-Th diagrams, there are undiscovered intraplate tension calc-alkaline basalts in the northern Yangtze Platform or the southern Qinling region, which provided volcanic clastic materials to the Ziyang, Lan'gao, Chengkou, Yichang and other regions. (2) Scanning electron microscopy revealed that the barite crystals in the Longmaxi Formation exhibit dissolution features and have a large particle size. Energy spectrum analysis of these barite crystals revealed that they have C, O, S, and Ba contents of 8.48 wt%, 22.98 wt%, 13.09 wt% and 55.44 wt%, so they are speculated to have been formed via cold methane seep genesis in a weak extensional tectonic setting. The 87Sr/86Sr ratios of the barite revealed that different types of barite were simultaneously formed in this area under the influences of hydrothermal and cold methane seeps. (3) The analysis of the heavy minerals in the Lower Silurian strata in the Bajiaokou section revealed that the provenance in the South Qinling area changed significantly during the late Early Silurian. Based on the above analyses, the northern margin of the Upper Yangtze Platform was in an extensional tectonic setting during the Late Ordovician-Early Silurian. The distribution of the total organic carbon content indicated that the extensional tectonic background provided good conditions for the enrichment and preservation of organic matter. The results of this study provide an understanding of the regional sedimentary-tectonic pattern and evolution of the Yangtze Platform during this period, as well as a reference for future shale gas exploration in this region. • The tectonic background of the Late Ordovician-Early Silurian was studied. • The northern margin of the Upper Yangtze Platform is an extensional background. • The extensional tectonics caused the cold seepage events. • The extensional fault controlled the distribution of volcanic clastic sediments. • Organic matter accumulation was influenced by the extensional fault depression. [ABSTRACT FROM AUTHOR]

Late Cretaceous oil shales, located in the Yarmouk sub-basin (North Jordan), were analysed using techniques of organic and inorganic geochemistry, coupled with a microfossil study. Results of this study were used to reconstruct the paleoenvironmental and paleoclimatic conditions during the Late Cretaceous and to investigate their influence on organic matter enrichment of the oil shale deposits. The analysed oil shales have high TOC and S total content values in the ranges of 2.11–12.85 wt.% and 1.83–8.81 wt.%, respectively. The high S total content and its relation with TOC values indicate that the sulfur is associated with organic matter and that the oil shales were deposited in a marine environment under reducing conditions. This is shown by petrographic observation and supported by biomarker distributions. The microscopic investigation indicated that the oil shales contain abundant liptinitic materials (i.e. alginite and amorphous organic matter) and planktonic foraminifera. The presence of these sapropelic materials and planktonic foraminifera assemblages further suggests a marine origin. Biomarker distributions are also known to reflect dominantly marine plankton/algal and microorganisms source input. Water column stratification and low oxygen (reducing) conditions are indicated by narrow Pr/Ph values, relatively high V/Ni elemental ratios and the presence of gammacerane. Reducing conditions contribute to preservation of organic matter. Warm-water periods during the Late Cretaceous are also indicated by dominant occurrences of warm water index microfossil taxa. The warm climate combined with elevated phosphorus (P) are the major causes of excessive algae growth, and led to enhanced biological productivity within the photic zone of the water column during deposition of the oil shale deposits. This increased bio-productivity in combination with good preservation of organic matter are the main mechanisms of organic matter enrichment within the Late Cretaceous oil shales in the north Jordan. [ABSTRACT FROM AUTHOR]

Oil shales are developed in the Shale Member of the Middle Jurassic Shimengou Formation in the Qaidam Basin, China. The oil shales can be classified into three quality groups (low-, medium-, and high-quality oil shales) through a comprehensive analysis protocol that includes Rock-Eval pyrolysis, total organic carbon (TOC) content, proximate analysis, gas chromatography-mass spectrometry (GC-MS), X-ray diffraction (XRD), major and trace element analyses, and maceral analysis. The low-quality oil shales mainly contain type II1 kerogen, the medium-quality oil shales mainly contain type I-II1 kerogen, and the high-quality oil shales mainly contain type I kerogen. All are immature to early thermally mature. The oil yield of the oil shales is directly related to their quality and are positively correlated with TOC content and calorific value. All studied samples were deposited under anaerobic conditions but in different paleoenvironments. The low-quality oil shales were mainly deposited in fresh-water environments, whereas the high-quality oil shales were usually developed in highly saline and reducing environments. Salinity stratification and evidence of algal blooms that are conducive to organic matter enrichment were identified in both medium- and high-quality oil shales, the latter having the highest paleoproductivity and the best preservation conditions. In summary, shale quality is controlled by a combination of factors, including algal abundance, preservation conditions, the existence of algal blooms and salinity stratification, and paleoproductivity. This study reveals how these different factors affect the quality of oil shales, which might provide an in-depth explanation for the formation process of lacustrine oil shales. [ABSTRACT FROM AUTHOR]

Lacustrine rift basins in Eastern China possess large amounts of petroleum, especially the shale gas and tight sand gas in deep formations, where "sweet spots" are closely related to organic-rich shale intervals. However, the OM enrichment of deep formation shale in lacustrine rift basins and the main controlling factors have not been well-researched in previous studies. The organic matter (OM) enrichment is mainly controlled by the OM input, preservation, and/or dilution. The shales deposited in deep formations of Songliao Basin, including the Jurassic Huoshiling (J 3 h), Cretaceous Shahezi (K 1 sh), and Yingcheng (K 1 yc) formations, are evaluated as fair to excellent source rocks. Taking the shales as an example, the characters and impact of input, preservation, and dilution on OM enrichment in shales are discussed by utilizing the parameters of element concentrations and biomarkers. Results show that the OM sources are mixed sources of aquatic microorganisms and higher plant materials, and the higher plant materials are dominant. The bio-productivity were moderate to high. The paleoclimate was mainly the cold and semi-humid to semi-arid. The lake salinity was fresh-to brackish-water. The redox conditions were sub-reducing to sub-oxidizing. The dilution degree in Yingcheng was the strongest, followed by the Shahezi and Huoshiling. Consequently, the OM enrichment in the J 3 h shale was called moderate bio-productivity and preservation model; the OM enrichment in the K 1 sh shale was called high bio-productivity and preservation model; the OM enrichment in the K 1 yc shale was named high dilution model. This study not only improves the model of OM enrichment but also provide some information for the research of "Deep time" in the Cretaceous of Songliao Basin, guiding the prediction of organic-rich shale distribution in lacustrine rift basin. • Fair to excellent hydrocarbon generation potential. • Organic matter origins are mainly a mixture of aquatic microorganisms and terrestrial higher plants. • Arid and cold paleoclimate and fresh- and brackish-water. • Reducing to sub-reducing, sub-reducing to oxidizing, and sub-reducing to sub-oxidizing conditions. • Moderate bio-productivity and preservation model, high bio-productivity and preservation model, high dilution model. [ABSTRACT FROM AUTHOR]

The Fushun Basin is a typical continental basin filled with organic-rich fine-grained sedimentary rocks, such as coal, oil shale and greyish green mudstone. Currently, research on this basin predominantly focuses on the enrichment mechanisms of organic matter in coal and oil shale and the controlling factors of coal seam transformation to oil shale. However, during the transformation stage, the deposition of oil shale ceased and was converted to interbeds of grey green mudstone and thin oil shale layers. The control mechanism of this type of sedimentary succession is still not clear. In this study, the thick oil shale in the upper part of the Jijuntun Formation (Unit I) and the high-frequency interbedded mudstone and thin oil shale in the Xiloutian Formation (Units II and III) were analysed as key strata. Organic and inorganic geochemical data were analysed to reconstruct the transformation of the various types and sources of organic matter and controlling factors including the palaeoclimate, palaeoweathering, bioproductivity, terrigenous clastic input, palaeosalinity, and redox conditions. Among these, Unit I was affected by humid climate conditions predominantly. The water depth and volume were large, and the input of terrigenous debris was low. Freshwater algae blooms and anoxic conditions led to a supply of organic matter and that was well preserved. This further resulted in a high abundance of organic matter in the sedimentary rocks. Unit II was affected by semihumid-semiarid and arid climate conditions. The lake became shallower while terrigenous clastic input and lake palaeosalinity increased; these changes represented the transition stage from freshwater to brackish water. The lake still exhibited suboxic/anoxic conditions, but the biological living space decreased, resulting in a limited organic matter supply and a low abundance of organic matter in the sediment. Unit III was affected by semihumid-semiarid climate conditions, and the palaeosalinity continued to increase. The lake had brackish water, with a high bioproductivity, a decreased water volume and a limited supply of organic matter. However, brackish water stratification occurred in the lake, which created a strongly anoxic environment and good preservation conditions, resulting in a secondary high abundance of organic matter. A comprehensive analysis indicates that the environmental change caused by the palaeoclimate was the main factor driving the oil shale deposition to cease and the onset to deposit grey green mudstone and thin oil shale in the Fushun Basin, Northeast Asia. • Different from the previous research, thin layer oil shale layers are also developed in the Xiloutian Formation. • The palaeoclimate, palaeosalinity and bioproductivity showed cycle evolution in the Xiloutian Formation. • High organism supply promoted the formation of thick high-quality oil shale in Jijuntun Formation in anoxic environment. • The excellent preservation condition is the key factor for the formation of oil shale in the Xiloutian Formation. [ABSTRACT FROM AUTHOR]

The Songliao Basin (SLB) is a large terrestrial petroliferous basin located in northeastern China. The Nenjiang Formation represents excellent hydrocarbon source rocks for the Daqing oil field. Previous studies have indicated that the oil shale intervals from the first (K2n1) and second (K2n2) members of the Nenjiang Formation were formed in different depositional settings. In this study, we provide a new high-resolution (1 m interval) record from SK-1s core and compile three sets of published datasets from two drilling holes (Zk3389 and LY-1) and a composite outcrop section. According to the total organic carbon (TOC) chemostratigraphy, we have divided three variation cycles spanning from K2n1 to K2n2 and detected three potential oil shale intervals in the Nenjiang Formation. Combined with the productivity, salinity, and oxygenation proxies, we discuss the paleolimnological environmental changes during deposition of the Nenjiang Formation. Our new and compiled records support the model that excellent preservation conditions were associated with the formation of organic-rich sediments in the K2n1, while the productivity was the major controlling factor for organic matter enrichment in the K2n2. [ABSTRACT FROM AUTHOR]

Based on the field investigation of oil shales in the southern Ordos Basin, detailed observations, and systematic sample analyses of core samples from a well that penetrated an oil shale, the influence of volcanic and hydrothermal activity on the Chang 8–Chang 7 members of the Yanchang Formation on organic matter (OM) enrichment was geochemically and petrologically analyzed. According to maceral analyses and rock pyrolysis, the kerogen types of the samples in the study area are widely distributed. Those of the oil shale with the highest OM abundance are mostly type II-I, and are immature or in a stage of low-maturity. Six layers of tuff and tuffaceous mudstone were identified by core observation and element characteristics; they are characterized by higher total rare earth element (∑REE) and δCe values and Th contents, and lower (La/Yb) N and δEu values. Mineral assemblages of anhydrite–pyrite–marcasite and relevant diagrams of the elemental geochemistry confirmed the existence of hydrothermal deposits. The high Y n /Ho n and U/Th ratios and total sulfur (TS) contents indicate that the Chang 7 Member experienced four episodes of intense hydrothermal activity. Early in the Chang 73 Member, volcanic ash and hydrothermal fluids brought in a large amount of nutrients, which not only increased primary productivity, but also increased the salinity of the water column, enhanced anoxia in the sedimentary environment of the lake bottom, promoted the preservation of OM, and finally formed the oil shale layer with the richest OM. However, the strong volcanic eruption in the late Chang 73 Member led to changes in the regional climate, causing it to dry and become colder and resulting in a decrease of primary productivity. Earthquakes triggered by volcanism resulted in turbidite deposits, which destroyed the preservation and diluted the enrichment of OM. This study reveals in detail the influence of volcanic and hydrothermal activity on OM enrichment during the evolution of the basin, and thereby deepens our theoretical understanding of OM enrichment. • The influence of volcanic and hydrothermal activities on OM enrichment is revealed. • Volcanic ash and hydrothermal fluids promoted the enrichment of OM. • Strong volcanic eruptions may lead to changes in paleoclimate, which is not conducive to the enrichment of OM. • Sedimentary models of OM in the Chang 8–Chang 7 Members of Yanchang Formation are established. [ABSTRACT FROM AUTHOR]