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The Upper Silesian Coal Basin (USCB), located in southern Poland, is the major coal basin in Poland, and all technological types of hard coal, including coking coal, are exploited. It is also an area of high potential for coal-bed methane (CBM). Despite the increasing availability of alternative energy sources globally, it is a fact that the use of fossil fuels will remain necessary for the next few decades. Therefore, research on coal-bearing formations using modern research methods is still very important. The application of geochemistry and chemostratigraphy in reservoir characterization has become increasingly common in recent years. This paper presents the possibility of applying chemostratigraphic techniques to the study of the Carboniferous coal-bearing succession of the Upper Silesian Coal Basin. The material studied comes from 121 core samples (depth 481–1298 m), representing the Mudstone Series (Westphalian A, B). Major oxide concentrations of Al2O3, SiO2, Fe2O3, P2O5, K2O, MgO, CaO, Na2O, K2O, MnO, TiO2, and Cr2O3 were obtained using X-ray fluorescence (XRF) spectrometry. Trace elements were analyzed using inductively coupled plasma mass spectrometry (ICP/MS). The geochemical record from the Mudstone Series shows changes in the concentration of major elements and selected trace elements, leading to the identification of four chemostratigraphic units. These units differ primarily in the content of Fe, Ca, Mg, Mn, and P as well as the concentration of Zr, Hf, Nb, Ta, and Ti. The study also discusses quartz origin (based on SiO2 and TiO2), sediment provenance and source-area rock compositions (based on Al2O3/ TiO2, TiO2/Zr, and La/Th), and paleoredox conditions (based on V/Cr, Ni/Co, U/Th, (Cu+Mo)/Zn, and Sr/Ba) for the chemostratigraphic units. Chemostratigraphy was used for the first time in the study of the Carboniferous coal-bearing series of the USCB, concluding that it can be used as an effective stratigraphic tool and provide new information on the possibility of correlating barren sequences of the coal-bearing succession. [ABSTRACT FROM AUTHOR]

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]

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 dark mudstone of Taiyuan Formation and Shanxi Formation of Upper Paleozoic in Luyi Sag shows high abundance of organic matter and is an important shale gas exploration horizon. In order to further explore the sedimentary environment, the element geochemical analysis of LY1 well recently operated was carried out, and the elements or combinations sensitive to the sedimentary envi-ronment were selected, combined with the petrological characteristics, to study the paleoclinkate, paleosalinity, paleoredox conditions, relative deposition rate and organic matter preservation conditions. The results show that the index of alteration, ratio between SiO2/Al2O3 and Sr/Cu indicate that the sedimentary period of Taiyuan Formation and Shanxi Formation has a warm and humid paleoclimate background. The changes of V/( V + Ni), V/Cr, Ni/Co and Cu/Zn ratios and petrological characteristics indicate that the water body in the sedimentary period of Taiyuan Formation-lower Shanxi Formation belongs to a reduction-weak reduction environment, and the up-per Shanxi formation is a transition environment of weak oxidation-weak reduction. The variation characteristics of Li, Ni, Ga and Sr contents, Sr/Ba ratio and TOC/TS ratio show that the water body in the sedimentary period of Taiyuan Formation and Shanxi Formation belongs to brackish water-freshwater sedimentary environment. From bottom to top, the injection amount of fresh water gradually increa-ses and the salinity decreases in a oscillatory manner. Combined with Rb/Zr ratio, it is speculated that Taiyuan Formation and Shanxi Formation are shallow water sedimentary environments, and the depth gradually becomes shallow from bottom to top. The variation characteristics of LREE/HREE and LaN,/YbN, reflect that the deposition rate of Taiyuan Formation is relatively stable, while that of Shanxi Formation fluctuates greatly, which is slightly higher than that of Taiyuan Formation. The sedimentary environment in the lower part of Taiyuan Formation and Shanxi Formation is more conducive to the preservation of organic matter than that in the upper part of Shanxi Formation. [ABSTRACT FROM AUTHOR]

The Bohai Bay Basin, located in eastern China, is a significant petroliferous, non-marine rift basin, underwent intricate tectonic activities throughout the Cenozoic. The Nanpu Depression in the Huanghua sub-basin in the northern part of the basin was controlled by Paleogene rifting and increased subsidence. These geological processes resulted in the deposition of the Shahejie and Dongying formations, which exhibited varying quantities and qualities of organic matter. Eighty drill cuttings samples of the Paleogene First Member (Es 1) and the overlying Third (Ed 3) and Second (Ed 2) members of the Shahejie and Dongying formations, respectively, were investigated. An integrated approach including organic and inorganic geochemistry, palynology, and mineralogy was employed to assess the paleoredox and marine incursion conditions, source rock quality and levels of thermal maturity, and factors that governed organic matter accumulation. Results indicated that the Es1 Member was deposited under anoxic conditions, resulting in the accumulation of organic matter-rich mudstones in the Nanpu Depression. Abundant pollen grains and acritarchs were found in the Es1 Member yielded along with minor occurrences of dinoflagellate cysts, freshwater algae, and spores. Terrestrial palynomorph composition provided evidence of increased terrestrial runoff, while the high abundance of acritarchs, accompanied by some dinoflagellate cysts, confirmed episodes of marine incursion and deposition under hypersaline conditions in the lacustrine Bohai Bay Basin. The Ed 2 Member exhibited poor to fair amounts of organic matter and a limited potential of hydrocarbon generation with a variety of kerogen Types, including IV, III, II/III, and II. The Ed 3 Member shows fair to good organic matter richness and hydrocarbon potential of kerogen Types III, II/III, and II. The Es 1 Member emerged as the most promising source rock within the studied interval, boasting good to excellent organic matter richness, and fair to good hydrocarbon generation potential of kerogen Types III and II/III. Evaluating the thermal maturity of organic matter, based on T max and VRo-Eq% and related cross-plots, revealed that the Shahejie and Dongying formations are still in the immature to the early stage of the oil window. The mineralogical composition of the Es 1 Member primarily consists of clay and quartz with some carbonate and accessory minerals, such as epigenetic halite, dolomite, and plagioclase. Weak correlations observed between the total organic carbon (TOC) and mineral components indicated that the availability of organic matter was primarily controlled by enhanced production and preservation rather than limited dilution under saline lacustrine, oxygen-deprived, and well-stratified water column conditions. Regional investigations of the Es 1 Member confirmed the widespread anoxia and deposition of organic-rich source rocks in the Bohai Bay Basin during the Oligocene. • Prevalent anoxia and high TOC preservation occurred during the Es 1 Member deposition. • Terrestrial palynomorphs from the Es 1 Member indicated enhanced freshwater runoff. • Dinoflagellate cysts and acritarchs indicated episodes of marine incursion and hypersaline conditions. • The Es 1 Member is one of the best source rock intervals in the Bohai Bay Basin. • Organic matter was controlled by production and preservation versus reduced dilution. [ABSTRACT FROM AUTHOR]

Lacustrine organic-rich shales are widespread in the Upper Triassic Yanchang Formation Chang 7 Member of the Ordos Basin. However, information on the short-term climate evolution within the Chang 7 Member is still lacking, and the mechanism of organic accumulation in the shales remains controversial. To address these issues, this study analyzes the palaeoenvironment and depositional setting of different intervals in the Chang 7 Member of the southwestern Ordos Basin and constrains the main control factors on accumulation and preservation of organic matter based on organic, major, and trace element data. The lower Chang 7 3 shale, which contains a high abundance of thermally mature Type I and II kerogen, has excellent original hydrocarbon generative potential. The lower Chang 7 3 interval has the most humid climatic condition, the strongest weathering condition, a euxinic bottom water condition, a high paleo-productivity, and a low terrigenous input. During the period of upper Chang 7 3 and Chang 7 2 , the climate became somewhat drier and hotter, which was marked by relatively weak weathering conditions, suboxic to anoxic conditions, low paleoproductivity, and high terrigenous input. The Chang 7 1 was characterized by a moderately humid climate, moderate weathering conditions, paleoproductivity, and terrigenous input. The relationship among the primary productivity, redox conditions, clastic influx indicators and TOC content suggests that primary productivity was the most critical factor controlling the enrichment of organic matter. The high primary productivity during the early Chang 7 3 interval was probably driven by the deposition of volcanic ash and was influenced by marine incursion events. A warm and humid paleoclimate, combined with input of volcanic ash and seawater, resulted in eutrophic, nutrient-rich, and stratified surface water during the early Chang 7 3 period, which led to algal blooms with high primary productivity. A stratified water column with high primary productivity, together with oxygen-deficit bottom water conditions and low terrigenous input, resulted in the greatest enrichment of organic matter in the lower part of Chang 7 3 shale. • The abundance of organic matter in the Chang 7 Member shows strong heterogeneity. • Short-term climate changes have occurred during the Chang 7 period. • Primary productivity was the most critical factor for organic matter enrichment. • High primary productivity was driven by volcanic ash and seawater incursion. • The lower Chang 7 3 interval has a warm and humid paleoclimate. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The mean size of pyrite framboids is related to the paleoredox conditions. • Redox-sensitive trace elements show strongly correlation with mean size of framboids. • The size distribution of pyrite framboids is related to relative sedimentation rate. • A favourable sedimentation rate is beneficial to organic matter enrichment. Abstract Pyrite framboids are spherical or ellipsoidal compact aggregates of submicron pyrite microcrystals. The various lithologies in the Chang 7 Member of the Upper Triassic Yanchang Formation contain abundant pyrite framboids, which provide an opportunity to understand the fluctuation in paleoredox conditions and relative sedimentation rates during deposition. The measured diameters of the pyrite framboids in the Chang 7 Member are characterized by large mean diameters (9.7–16.6 μm) with a wide distribution range that plots in the area indicative of dysoxic to oxic paleoredox conditions during deposition of the Chang 7 sediments. Based on correlation analysis, the strong correlation between paleoredox-sensitive trace elements ratios (V/Cr, U/Th, and Cu/Zn) and the mean size of framboids indicate that the latter can serve as a reliable indicator of paleoredox conditions. The results of the framboid size distribution can also be interpreted through Crystal Size Distribution Theory (CSDT). According to CSDT, the relative sedimentation rate of the Chang 7 Member fluctuated during deposition in parallel with fluctuation in lacustrine levels. Furthermore, the strongly positive correlation between total organic carbon (TOC) and paleoredox conditions indicates that although relatively high sedimentation rate may have contributed to the dysoxic-oxic conditions that were harmful to the preservation of organic matter, but it may also have brought nutrients to increase productivity. Similarly, the correlation between TOC and relative sedimentation rates suggests that the relative sedimentation rates can affect organic matter enrichment in several ways. A high sedimentation rate is favourable for the preservation of organic matter, which can be prevented from oxidation, but large amounts of sediments can dilute the organic matter, which reduces the organic matter abundance in a potential source rock. [ABSTRACT FROM AUTHOR]

With onoing studies on the diversification of animals during the early Cambrian in the Yangtze Block, resource effects associated with environmental changes have started to be elucidated, in which regional sedimentary events might have played an important role. The gas-rich Yichang area in the southern Huangling anticline is a potential area for shale gas development and provides a more suitable target for unravelling the debates on organic matter (OM) accumulation in the Low Cambrian marine black shales. This study integrated elemental geochemical and mineralogy time-series analyses based on core samples drilled in Yichang area on the shallow-water platform to constrain the evolution of the depositional environments and consider the forcing function behind OM enrichment in the Shuijinguo Formation. Distinctive TOC and Cu/Al peaks within the black shale deposited in the Shuijingtuo Member I suggest intensification of bioproductivity level. The chemical index of alteration (CIA), regarded as an indicator of paleoclimatic variation, increased in the upsection, and together with increasing Sr/Cu and quartz/feldspar ratio values, suggest a trend toward warming conditions and a moderate degree of chemical weathering. The geochemical proxies for the degree of water mass restriction (Mo/TOC and Co × Mn) and paleoredox (Mo EF , U EF , U/Th and C org /P) indicate that the shallow-water shelf experienced weakly-to moderately-restricted water conditions under an anoxic/euxinic environment in the Shuijingtuo Member I and transited to a moderately-restricted water mass with suboxic conditions during the late Shuijingtuo and Shipai periods. A positive Eu anomaly within the Shuijingtuo Member I provided robust evidence of hydrothermal activity. We propose here for an opening of deposition conditions on the shallow-water platform which results in enhanced upwelling and hydrothermal activity (Si, Ba, et.), thereby ultimately disrupting the original nutrient cycling for the metabolism and prompting the OM accumulation in the epicontinental sea during the early Cambrian. • An intensification of bioproductivityl was detected in the Shuijingtuo Member I. • Chemical weathering indices suggest a trend toward a warm and humid climate. • Black shale was deposited under weakly-restricted and anoxic water conditions. • Positive Eu anomalies are indicative of hydrothermal activity. • The OM enrichment was jointly affected by hydrothermal and upwelling events. [ABSTRACT FROM AUTHOR]

The origin of the organic-rich shale in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation is complex and controversial. This paper reports the geochemical data of Wufeng-Longmaxi Formations in the Upper Yangtze region to restore the paleoenvironment and explore the accumulation mechanism of organic matter. The total organic carbon (TOC) content of the Wufeng Formation was relatively high, with an average of 2.86%. The Lower Longmaxi Formation showed the highest TOC content, with an average of 3.99%, and the upper part was a continuously low value with an average of 1.22%. The paleoproductivity proxies (Babio, Cu/Al, Ni/Al, Siexcess) showed that in the Katian and Rhuddanian-Aeronian Stages, the Upper Yangtze Sea had high primary productivity, indicating that organic matter accumulation was more affected by terrigenous influx and redox conditions. Al, Zr, and Zr/Al indicated that terrigenous influx was relatively high in the Kaitian-Hirnantian Stages, it was at a constant low in the Rhuddanian Stage, and increased again in the Aeronian Stage. The correlations between redox-sensitive trace elements (Mo, U, V) and TOC indicate that the organic-rich shale of the Wufeng Formation was deposited in the anoxic−euxinic environment. In the Longmaxi Formation, organic-rich shales formed in a more hypoxic environment, and overlying organic-lean shales formed in a suboxic environment. Therefore, the anoxic−euxinic conditions of the Late Ordovician Yangtze Sea were the main reason for the organic matter accumulation, but the high terrigenous influx caused by regression and/or structural controls diluted the organic matter to some extent. For the Early Silurian, a complete transgression−regression cycle changed terrigenous influx and redox conditions, resulting in significant differences in organic matter accumulation.

The Qiangtang Basin is the largest Mesozoic marine basin in the Tibetan Plateau. The Upper Triassic black mudstones are among the most significant hydrocarbon source rocks in this basin. Here, we present geochemical data for the Upper Triassic black mudstones to determine their paleoenvironment conditions, provenance, and tectonic setting. To achieve these, 30 black mudstones formed in various sedimentary environments were collected from the Zangxiahe, Zana, and Bagong formations. The results show that the total REE concentrations of mudstones from these formations range from 169 to 214 ppm, 204 to 220 ppm, and 141 to 194 ppm, respectively. All samples have chondrite-normalized REE patterns with enrichment of LREE, depletion of HREE and negative Eu and Ce anomalies. Specifically, mudstones from the Bagong Formation exhibit higher negative Eu anomalies and lower REE contents than those from the Zangxiahe and Zana formations. Mudstones from the Zangxiahe and Zana formations with low Sr/Ba and Sr/Cu ratios indicate the humid climate, whereas the high Sr/Ba and Sr/Cu ratios of rocks from the Bagong Formation suggest the arid climate. The low U/Th, (Cu + Mo)/Zn, V/Cr and Ni/Co ratios of rocks from the Zangxiahe, Zana, and Bagong formations are indicators of oxidized conditions. The bivariate diagrams (TiO 2 vs. Al 2 O 3 , TiO 2 vs. Zr, La/Th vs. Hf, and Co/Th vs. La/Sc) reveal that mudstones from the Zangxiahe and Zana formations were potentially derived from intermediate igneous rocks, whereas mudstones from the Bagong Formation were probably sourced from felsic igneous rocks. Their source rocks are mostly deposited in the collisional setting. REE of mudstones from the Zangxiahe, Zana, and Bagong formations were possibly originated from terrigenous detritus, with minor non-terrigenous contributions into the Zana samples. The REE contents of these mudstones are controlled mainly by terrigenous detrital minerals, rather than by the paleoclimate, paleoredox conditions, or organic matter. However, calcite minerals could dilute REE. Therefore, the REE contents of the Bagong Formation mudstones are significant lower than those of the Zangxiahe and Zana formations mudstones. [ABSTRACT FROM AUTHOR]

Pyrite is the most common authigenic mineral preserved in many ancient sedimentary rocks. Pyrite also widely exists in the Longmaxi and Wufeng marine shales in the middle Yangtze area in South China. The Longmaxi and Wufeng shales were mainly discovered with 3 types of pyrites: pyrite framboids, euhedral pyrites and infilled framboids. Euhedral pyrites (Py4) and infilled framboids (Py5) belong to the diagenetic pyrites. Based on the formation mechanism of pyrites, the pyrites could be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. Under a scanning electron microscope (SEM), the syngenetic pyrites are mostly small framboids composed of small microcrystals, but the diagenetic pyrites are variable in shapes and the diagenetic framboids are variable in sizes with large microcrystals. Due to the deep burial stage, the pore space in the sediment was sharply reduced and the diameter of the late diagenetic framboids that formed in the pore space is similar to the diameter of the syngenetic framboids. However, the diameter of the syngenetic framboid microcrystals is suggested to range mainly from 0.3 µm to 0.4 µm, and that of the diagenetic framboid microcrystals is larger than 0.4 µm in the study area. According to the diameter of the pyrite framboids (D) and the diameter of the framboid microcrystals (d), the pyrite framboids could be divided into 3 sizes: syngenetic framboids (Py1, D < 5 µm, d ≤ 0.4 µm), early diagenetic framboids (Py2, D > 5 µm, d > 0.4 µm) and late diagenetic framboids (Py3, D < 5 µm, d > 0.4 µm). Additionally, the mean size and standard deviation/skewness values of the populations of pyrite framboids were used to distinguish the paleoredox conditions during the sedimentary stage. In the study area, most of the pyrite framboids are smaller than 5 µm, indicating the sedimentary water body was a euxinic environment. However, pyrite framboids larger than 5 µm in the shales indicated that the sedimentary water body transformed to an oxic-dysoxic environment with relatively low total organic carbon (TOC: 0.4−0.99%). Furthermore, the size of the framboid microcrystals could be used to estimate the gas content due to thermochemical sulfate reduction (TSR). The process of TSR occurs with oxidation of organic matter (OM) and depletes the H bond of the OM, which will influence the amount of alkane gas produced from the organic matter during the thermal evolution. Thus, syngenetic pyrites (d ranges from 0.35 µm to 0.37 µm) occupy the main proportion of pyrites in the Wufeng shales with high gas content (1.30−2.30 m3/t), but the Longmaxi shales (d ranges from 0.35 µm to 0.72 µm) with a relatively low gas content (0.07−0.93 m3/t) contain diagenetic pyrites. Because of TSR, the increasing size of the microcrystals may result in an increase in the value of δ13C1 and a decrease in the value of δ13C1-δ13C2. Consequently, the size of pyrite framboids and microcrystals could be widely used for rapid evaluation of the paleoredox conditions and the gas content in shales.

Organic matter accumulation exhibits different distribution patterns in time and space and tends to be controlled by many factors. This study investigated the mechanisms that control the accumulation of organic matter in the Xianfeng area of western Hubei Province, South China through sedimentological and geochemical analyses of wellbore samples. The results showed that the total organic carbon (TOC) content, mineral content, and elemental geochemical data of the Wufeng-Longmaxi Formations varied significantly with depth. Based on the sharp changes in the vertical profiles of the TOC content, Gamma-ray, and numerous geochemical parameters, as well as regional stratigraphy, this study divided the section from the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation into six units (W1, W2, GYQ, L1, L2, and L3). The TOC content was positively correlated with redox proxies (DOP T , V EF , and Mo EF) and primary productivity proxies (Cu EF and Ni EF), and negatively correlated with terrigenous input proxies (Al, K, and Ti). The correlation coefficients suggest that the primary factors affecting organic matter accumulation are redox conditions and paleoproductivity, while terrigenous input is a secondary controlling factor. This study proposed an organic matter accumulation model based on the organic matter characteristics, lithofacies, sedimentary environment, and geological setting of the study area. During the deposition of Unit W1, organic matter was well-preserved owing to the anoxic bottom water environment resulting from strong water mass restriction. During the deposition of Unit W2, the rising sea level led to the development of anoxic-euxinic bottom water conditions, and the joint influence of nutrients from upwelling in the open sea and volcanic eruptions enhanced productivity, resulting in the most intense accumulation of organic matter. Unit GYQ had low TOC content due to low productivity despite anoxic conditions. During the deposition of Unit L1, the uplift of the Hunan-Hubei Submarine High caused the depocenter to shift westward and the sea level in this area to drop, resulting in suboxic bottom water and relatively low productivity in the surface water layer. During the deposition of Unit L2, the regional sea level drop resulted in increased input and dilution of terrigenous clasts, thus leading to low TOC content. During the deposition of Unit L3, continuously enhanced terrigenous input due to climate warming and sea level drop became the major factor controlling the organic matter accumulation. In summary, the main factors controlling the organic matter accumulation differed significantly at different evolutionary stages. Compared with the Jiaoshiba area, the Xianfeng area in western Hubei experienced continuously increased terrigenous input, a less reductive water mass, a significant decrease TOC content, and a westward shift of the depocenter since the early stage of the Longmaxi Formation deposition. These changes may be related to the intense uplift of the Hunan-Hubei Submarine High triggered by the Kwangsian Orogeny. These results provide important implications for the understanding of the accumulation of organic matter in the Wufeng-Longmaxi Formations and paleogeographic changes. [ABSTRACT FROM AUTHOR]

The Upper Cretaceous carbonate-rich Niobrara Formation is a prominent petroliferous unconventional reservoir that is generally characterized by fine-scale heterogeneities that complicate local and regional correlation of target zones, well placement and completion strategies. Within rhythmically interbedded reservoirs like the Niobrara Formation, the occurrence of thin marl and chalk beds within chalk- or marl-dominated intervals respectively, further impedes identification of marl or chalk benches. This study uses a combination of petrophysical and geochemical data from 6 oil and/or gas wells within the Niobrara interval of the DJ basin to identify and correlate eight (8) chemozones (C1, C2, C3, C4, M1, M2, M3 and M4) and four (4) chemostratigraphic packages (Sharon Springs, Smoky Hill, Fort Hays and Codell Sandstone) that are essential for constructing a robust and dynamic stratigraphic framework. The results showed synchronous variation in petrophysical trends, elemental, mineralogical and organic composition at contacts, ubiquitous cyclicity within, and across the Niobrara Formation in the northern DJ basin. The stratigraphic units are geochemically separated based on TOC, total clay, total carbonate, and quartz + feldspars, and Ca, Sr, Si, Al, Ti, Rb, Zr, Nb, K, and Cr, while the internal stratigraphic units within the Niobrara Formation are defined by integrating gamma-ray (GR), resistivity, carbonate proxy, marl proxy, carbonate/marl proxy and terrigenous input. The chalk and limestone units (C1–C4) have higher Ca, Sr, total carbonate concentrations and relative brittleness index relative to the marl units (M1-M4). This multiproxy approach reveals variable calcareous and terrigenous/detrital input, and paleoredox conditions as the main controlling factors behind the changing lithologies/cyclicity within the Niobrara Formation. Hence, an adequate characterization and correlation of inherent heterogeneities within marls and chalks of the Niobrara Formation may require incorporation of a multiproxy approach at various scales. By integrating color scale, geochemistry, TOC and wireline well log curves responses, higher resolution identification of chalk reservoirs and marl source beds can be achieved. Spatial distribution of chalks and marls consistently show a trend of total carbonate and Ca increasing southwards towards the Tethys Sea with highest average concentrations recorded in Fort Hays Limestone while quartz + feldspars, and total clay concentrations increasing northwards, suggesting a southern and northern provenance, respectively. • Identification of contacts and internal stratigraphy of Niobrara Formation using geochemical proxies. • Reverse spatial variation trend in carbonate and total clay distribution. • Chalk intervals are excellent targets due to higher relative brittle index and oil saturation index. • Strong redox gradation from oxic/suboxic to anoxic/euxinic conditions influenced organic matter preservation. • Porosity influenced by diagenesis. [ABSTRACT FROM AUTHOR]

The Cambrian Drumian carbon isotope excursion (DICE) event is a negative carbon isotope (δ 13 C) excursion. It is associated with the first appearance datum (FAD) of the agnostoid arthropod Ptychagnostus atavus , whose first appearance in the Global boundary Stratotype Section and Point (GSSP) defines the base of the Cambrian Series 3 Drumian Stage. Although this isotopic event has been clearly identified in the Great Basin, USA, it has not been consistently reported in other areas of the world. Here, we report a study integrating isotopic and geochemical data from middle Cambrian (Series 3) sections from the northeastern Georgina Basin, Australia, to investigate the occurrence of the DICE event in Australia and the paleoenvironmental conditions associated with this event. The similarity between the isotopic excursions observed in the Georgina Basin (− 2‰ to − 3.2‰ δ 13 C excursion) and that observed in the Great Basin (− 2‰ to − 3.6‰ δ 13 C excursion) highlighted the presence of the DICE in Australia and emphasized the potential use of this event for refining global isotopic correlations. The multi-proxy correlation used in this study also showed a pulse of euxinia in association with the DICE, suggesting strongly euxinic conditions near the base of the Drumian Stage. [ABSTRACT FROM AUTHOR]

Uranium (U) provides especially useful information for reconstructing paleoredox conditions due to its unique geochemical properties and behavior in supergene environments. Its proxy potential under suboxic –oxic conditions associated with salinization of surface water in closed hydrologic systems, however, has not been adequately explored. This study examined the abundance and distribution of U in a sedimentary sequence of calcareous lacustrine clastics and salt layers from a high quality (938.5 m-long) core drilled in the western Qaidam Basin, NE Tibetan Plateau. This study examined U concentrations in calcareous clastic sediments (including carbonates, acid insoluble fractions, as well as the amounts of iron and manganese oxides/oxyhydroxides) and salt layers. In the calcareous clastic sediments, variations in U concentrations within the acid insoluble fraction follow the fluctuations of reconstructed lake levels based on lithologic investigations, providing a tool for tracing redox conditions in paleolake bottom water. Correlations between the variations in carbonate content with U and Sr concentrations in the carbonate fractions demonstrate that the types and abundance of carbonate minerals control the U distribution in carbonates. In the salt layers, U and Al concentrations correlate closely, implying that both derived mainly from the detrital fraction in the salt layers. However, enrichments of U and Mg are systematically higher in salt minerals formed in more concentrated brine and lower in those formed in less concentrated brine. These results indicate that during the process of carbonate precipitation, carbonate might effectively remove U from lake water. During the subsequent stage of salt mineral precipitation, U behavior is conservative, and U resides for a long period in dissolved forms within the saline water, giving rise to enhanced U enrichment in the more concentrated brines and the associated precipitated salt minerals. Our study suggests, however, that U-related redox proxies should be interpreted with caution in suboxic–oxic environments during the long-term paleolake evolution. [ABSTRACT FROM AUTHOR]

Ocean circulation has undergone significant changes during the glacial and interglacial periods. The redox condition at the sediment-water interface is mainly regulated by export productivity and ventilation of abyssal water mass. The changes in the redox condition associated with bottom water circulation in the Indian Ocean are still unclear. The present study assessed multi redox proxies (Mn/Ti, Mo/Ti, U/Ti, V/Ti, Ni/Ca, Cu/Ca, Mn EF , U EF, and V EF) and C org in the sediment core collected from the Arabian Sea (NW Indian Ocean) to investigate the past variations in bottom water redox condition. The bottom water in the Arabian Sea is presently bathed with the Lower Circumpolar Deep Water (LCDW), consisting of a fraction of both North Atlantic Deep Water (NADW) and Antarctic Bottom Waters (AABW). Our results show significant changes in oxygenation conditions in response to changes in ocean circulation. The present study revealed that bottom water in the northern Indian Ocean was more oxic during LGM than the deglacial period. We interpret the variability in the contribution of NADW and AABW influenced the redox condition at the study location. Comparing the current study results with previously reported Nd isotopes suggests that the contribution of AABW to the Indian Ocean increased during LGM, resulting in a higher oxic condition in LCDW. • We investigated the redox conditions of bottom watermass (> 3800 m), i.e. LCDW. • Mn/Ti, Mo/Ti, U/Ti, V/Ti, Ni/Ca, Cu/Ca, Mn EF , U EF , and V EF proxies were used. • High oxic condition in bottom waters of the Indian Ocean observed during LGM. • The contribution of AABW increased in the Indian Ocean during LGM. [ABSTRACT FROM AUTHOR]

Abstract: This paper is a synthesis of the use of selected trace elements as proxies for reconstruction of paleoproductivity and paleoredox conditions. Many of the trace elements considered here show variations in oxidation state and solubility as a function of the redox status of the depositional environment. Redox-sensitive trace metals tend to be more soluble under oxidizing conditions and less soluble under reducing conditions, resulting in authigenic enrichments in oxygen-depleted sedimentary facies. This behavior makes U, V and Mo, and to a lesser extent certain other trace metals such as Cr and Co, useful as paleoredox proxies. Some redox-sensitive elements are delivered to the sediment mainly in association with organic matter (Ni, Cu, Zn, Cd) and they may be retained within the sediment in association with pyrite, after organic matter decay in reducing sediment. This particularity confers to Ni and Cu a good value as proxies for organic C sinking flux (frequently referred to as productivity). Elements with only one oxidation state such as Ba and P are classically used to assess paleoproductivity levels but they suffer from the fact that they are solubilized under reducing conditions and may be lost from oxygen-deprived sediments. The combined used of U, V and Mo enrichments may allow suboxic environments to be distinguished from anoxic–euxinic ones. Specifically, these elements tend to be much more strongly enriched in anoxic–euxinic environments and to exhibit weaker covariation with TOC than in suboxic environments. [Copyright &y& Elsevier]

The middle–late Miocene siliceous mudstones and siltstones of the Akita and Aomori regions in Northern Honshu, Japan, provide an excellent geological record of the paleoenvironment of the Japan Sea. In this paper, we present results of sedimentological and geochemical analyses of the Akaishi and Odoji Formations from the Tsugaru Basin in the Aomori region and of the Onnagawa Formation from the Sugisawa area in the Akita region. Lithofacies descriptions, geochemical compositions (including major elements, total organic carbon [TOC], total nitrogen [TN], and total sulfur [TS]), and petrographic observations of the framboidal pyrite were used to investigate the origin of the detrital fraction, the paleoproductivity, and the paleoredox conditions of the bottom waters in the Japan Sea. Moreover, through interpretation of these data, the relationship between organic carbon preservation and the paleoenvironmental evolution was clarified. The data indicate that paleoweathering was moderate to high in the detrital fraction source terrain; the detrital fraction originated from a felsic source and was probably transported from Asia to the Japan Sea by monsoons and from the Japanese islands by riverine input; the high concentrations of biogenic SiO 2 indicate high primary productivity in the surface water, probably induced by intense upwelling of nutrients; the Tsugaru Basin area was characterized by oxygenated bottom water, whereas the Sugisawa area was characterized by oxygenated to low-oxygenated bottom water based on the lithofacies variation and the size of the framboidal pyrite; and the organic carbon accumulation in the Odoji and Onnagawa Formations was mainly controlled by paleoproductivity and dilution from the detrital fraction rather than the paleoredox conditions. Alternatively, the organic carbon accumulation in the Akaishi Formation was likely controlled by the paleoredox conditions. • We studied the paleoenvironment of the Japan Sea during the middle-late Miocene. • Warm-humid climate and medium-high chemical weathering prevailed at detrital source. • Basin geometry and paleodepth influenced the paleo-oxygenation of bottom waters. • Asian monsoon and continental input from Japan influenced the detrital fraction. • The detritus enhanced the preservation of TOC in dysoxic intervals. [ABSTRACT FROM AUTHOR]

The organic-rich lower Cambrian Yurtus Formation is considered to host one of the most important source rocks in the Tarim Basin of China. To investigate the paleoenvironment and mechanism of organic matter accumulation, we conducted petrological and geochemical analyses of the Yurtus Formation using samples obtained from three outcrop sections in the northwestern Tarim Basin. The Yurtus Formation can be divided into five units and contains two sets of shale (upper and lower). Both the lower and upper shales were deposited during transgressive phases and exhibit strong enrichment in total organic carbon (TOC). The mineralogy of the lower shale is dominated by quartz, feldspar, barite, and clay, whereas the upper shale consists mainly of quartz, feldspar, ankerite, and clay, with minor apatite and hematite. Field investigation, trace element enrichment and rare earth elements suggest that the lower shale was influenced by weak hydrothermal activity. The rock assemblage of organic-rich shale, chert, phosphate, and glauconite is well developed in the two transgressive-regressive cycles. Based on Co EF × Mn EF , Co (ppm) × Mn (%), and Cd/Mo, Mo/TOC ratios and enrichment factors of trace elements that are comparable with those of modern upwelling and restricted setting, upwelling is proposed to have been active and widespread during the deposition of the Yurtus Formation. Enrichment factors of redox-sensitive elements (e.g., U, V, and Mo) indicate that the lower shale was deposited in an anoxic conditions with less frequent euxinic conditions, whereas the upper shale was deposited in a suboxic environment. Excess silica and P 2 O 5 contents indicate high productivity throughout the deposition of the lower shale, with lower and decreasing productivity throughout the deposition of the upper shale. During the early Cambrian, a global transgression led to the development of an extensive carbonate platform in the northwestern Tarim Basin, and the climate became warm and humid. During the deposition of the lower shale, widespread silica-rich hydrothermal fluids provided abundant trace elements (including U, V, Mo, Ni, Cu, and Zn) that are beneficial for the prosperity of biomass. Upwelling provided abundant nutrients that further enhanced paleoproductivity and expanded the vertical extent of the oxygen minimum zone, favoring the preservation of organic matter. During the deposition of the upper shale, paleoproductivity decreased owing to the termination of hydrothermal activity and an increase in carbonate content. Organic matter accumulation was possibly controlled mainly by decreased but still high paleoproductivity and oxygen-poor level bottom water. • The lower Cambrian Yurtus Formation shale was divided into two sets. • Hydrothermal activity and upwelling were widely developed. • The lower shale was deposited in an anoxic and high productivity setting. • The upper shale was deposited in a suboxic and low productivity setting. • Organic matter accumulation of two sets of shale was driven by different factors. [ABSTRACT FROM AUTHOR]

The origin of the organic-rich shale in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation is complex and controversial. This paper reports the geochemical data of Wufeng-Longmaxi Formations in the Upper Yangtze region to restore the paleoenvironment and explore the accumulation mechanism of organic matter. The total organic carbon (TOC) content of the Wufeng Formation was relatively high, with an average of 2.86%. The Lower Longmaxi Formation showed the highest TOC content, with an average of 3.99%, and the upper part was a continuously low value with an average of 1.22%. The paleoproductivity proxies (Babio, Cu/Al, Ni/Al, Siexcess) showed that in the Katian and Rhuddanian-Aeronian Stages, the Upper Yangtze Sea had high primary productivity, indicating that organic matter accumulation was more affected by terrigenous influx and redox conditions. Al, Zr, and Zr/Al indicated that terrigenous influx was relatively high in the Kaitian-Hirnantian Stages, it was at a constant low in the Rhuddanian Stage, and increased again in the Aeronian Stage. The correlations between redox-sensitive trace elements (Mo, U, V) and TOC indicate that the organic-rich shale of the Wufeng Formation was deposited in the anoxic–euxinic environment. In the Longmaxi Formation, organic-rich shales formed in a more hypoxic environment, and overlying organic-lean shales formed in a suboxic environment. Therefore, the anoxic–euxinic conditions of the Late Ordovician Yangtze Sea were the main reason for the organic matter accumulation, but the high terrigenous influx caused by regression and/or structural controls diluted the organic matter to some extent. For the Early Silurian, a complete transgression–regression cycle changed terrigenous influx and redox conditions, resulting in significant differences in organic matter accumulation. [ABSTRACT FROM AUTHOR]

The paleoceanic redox conditions and the vertical and spatial distribution of organic-matter-rich lower Cambrian Niutitang shales are varied in the Upper Yangtze region, China. An integrated approach, including organic geochemistry, major and trace elemental geochemistry, and X-ray diffraction (XRD) mineralogy, was used to characterize changes in redox conditions and to determine key controls on organic-matter accumulation during deposition of the lower Cambrian Niutitang shales. A 202-m-long cored well located in Kaiyang County in northern Guizhou, China, was thoroughly characterized by the integrated approach in this study. Vertically, total organic carbon (TOC-) rich siliceous and argillaceous black shales were developed in the Lower and Middle Niutitang Formation, and TOC-lean silty shales were found in the Upper Niutitang Formation. The TOC content in the Lower and Middle Niutitang shales ranges from 0.27% to 6.7%, with an average value of 2.1%, and it decreases from 0.30% to 0.11% in the Upper Niutitang Formation shales. The TOC-rich siliceous shales in the Lower Niutitang Formation were characterized by high total sulfur content, enrichment in redox-sensitive trace elements, high C org :P ratios, and high authigenic Mo U covariation, indicating they were deposited under persistent anoxic conditions. Most organic-rich shales from the Middle Niutitang Formation were deposited under anoxic conditions, whereas a few intervals having low TOC contents were deposited in a predominantly oxic depositional environment. In contrast, the Upper Niutitang Formation were deposited under fully oxic conditions. A combination of high primary productivity and enhanced organic preservation under euxinic water column conditions is the key control on organic-matter accumulation in the Lower Niutitang Formation shales. However, a coupling of redox depositional environment and terrestrial mineral dilution plays an important role in organic-matter accumulation and preservation in the Middle Niutitang Formation. Similar organic-matter abundance distribution and carbon isotope excursion of organic matter in the sediments deposited at different depositional facies on the Yangtze Platform suggest regional isotopically homogenous organic carbon input due to the widespread transgressive event with deep basinal waters being transported into the shallow shelf environments at the base of the lower Cambrian. As a result, δ13C org distribution patterns can be used as a stratigraphic correlation marker in the Upper Yangtze region. • Similar δ13C org patterns as stratigraphic correlation marker in the Yangtze region. • Lower and Middle Niutitang TOC-rich shales were deposited under anoxic conditions. • Mo U covariation indicates an unrestricted marine environment in the early Cambrian. • The controls on organic-matter richness in the Niutitang shales are different. [ABSTRACT FROM AUTHOR]

Pyrite is the most common authigenic mineral preserved in many ancient sedimentary rocks. Pyrite also widely exists in the Longmaxi and Wufeng marine shales in the middle Yangtze area in South China. The Longmaxi and Wufeng shales were mainly discovered with 3 types of pyrites: pyrite framboids, euhedral pyrites and infilled framboids. Euhedral pyrites (Py4) and infilled framboids (Py5) belong to the diagenetic pyrites. Based on the formation mechanism of pyrites, the pyrites could be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. Under a scanning electron microscope (SEM), the syngenetic pyrites are mostly small framboids composed of small microcrystals, but the diagenetic pyrites are variable in shapes and the diagenetic framboids are variable in sizes with large microcrystals. Due to the deep burial stage, the pore space in the sediment was sharply reduced and the diameter of the late diagenetic framboids that formed in the pore space is similar to the diameter of the syngenetic framboids. However, the diameter of the syngenetic framboid microcrystals is suggested to range mainly from 0.3 µm to 0.4 µm, and that of the diagenetic framboid microcrystals is larger than 0.4 µm in the study area. According to the diameter of the pyrite framboids (D) and the diameter of the framboid microcrystals (d), the pyrite framboids could be divided into 3 sizes: syngenetic framboids (Py1, D < 5 µm, d ≤ 0.4 µm), early diagenetic framboids (Py2, D > 5 µm, d > 0.4 µm) and late diagenetic framboids (Py3, D < 5 µm, d > 0.4 µm). Additionally, the mean size and standard deviation/skewness values of the populations of pyrite framboids were used to distinguish the paleoredox conditions during the sedimentary stage. In the study area, most of the pyrite framboids are smaller than 5 µm, indicating the sedimentary water body was a euxinic environment. However, pyrite framboids larger than 5 µm in the shales indicated that the sedimentary water body transformed to an oxic-dysoxic environment with relatively low total organic carbon (TOC: 0.4–0.99%). Furthermore, the size of the framboid microcrystals could be used to estimate the gas content due to thermochemical sulfate reduction (TSR). The process of TSR occurs with oxidation of organic matter (OM) and depletes the H bond of the OM, which will influence the amount of alkane gas produced from the organic matter during the thermal evolution. Thus, syngenetic pyrites (d ranges from 0.35 µm to 0.37 µm) occupy the main proportion of pyrites in the Wufeng shales with high gas content (1.30–2.30 m3/t), but the Longmaxi shales (d ranges from 0.35 µm to 0.72 µm) with a relatively low gas content (0.07–0.93 m3/t) contain diagenetic pyrites. Because of TSR, the increasing size of the microcrystals may result in an increase in the value of δ13C1 and a decrease in the value of δ13C1-δ13C2. Consequently, the size of pyrite framboids and microcrystals could be widely used for rapid evaluation of the paleoredox conditions and the gas content in shales. [ABSTRACT FROM AUTHOR]