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Abstract A series of thermochemical sulfate reduction (TSR) simulation experiments were carried out involving different organic matter (crude oil, solid bitumen, type II kerogen, type III kerogen) and different sulfate species (anhydrite and MgSO 4) to address the chemical and carbon isotopic variations of the hydrocarbon, H 2 S and CO 2. The increase of main peak carbons of residual saturated hydrocarbon and the decrease of gaseous hydrocarbons from control treatment to corresponding sulfate treatments suggest that TSR promote the consumption of both liquid and gaseous hydrocarbons. The δ13C 1 -3 values generally shift positively from control treatment to sulfate treatments and the (δ13C ethane -δ13C methane) values in sulfate treatments are higher than that in control treatments. The variation of δ13C 1 -3 in TSR is controlled by the isotope fractionation during the generation and consumption of C 1 -3 in TSR. MgSO 4 is more reactive than anhydrite in TSR. A certain amount of H 2 S is incorporated into solid bitumen as the result of secondary alteration. We believe that the reactivity order of different organic matter in TSR is crude oil > solid bitumen > type II kerogen > type III kerogen. The reactivity of organic matter in TSR depends on the hydrocarbon generation kinetics of each organic matter. The dissolution/decomposition and precipitation of carbonate control the yield of CO 2 in sour reservoirs. The negative shift of δ13CO 2 with increasing TSR extent is mainly due to the inheritance effect of carbon isotope from hydrocarbons. Inorganic CO 2 sourced from the thermal decomposition or acid dissolution of carbonate mineral impose significant influence on δ13CO 2. The H 2 S yields decrease with CH 4 yields and increase with δ13CH 4 value, the δ13CH 4 values increase with residual amount of gaseous alkane (1- H 2 S/(residual alkane + H 2 S)), suggesting that methane acted as reactant in TSR. The (δ13CO 2 -δ13CH 4) values decrease significantly with increasing temperature, and the δ13CO 2 is even more negative than δ13CH 4 in 450 °C MgSO 4 treatments involving type II and type Ⅲ kerogen. Accordingly, methane acted as a predominant reactant in 450 °C sulfate treatments in our experiment. It is possible for natural gas with high gas dryness to experience methane-dominated TSR in geological reservoirs. Highlights • Organic matter type imposes significant influence on TSR. • Dissolution and precipitation of carbonate control CO 2 yield in TSR. • δ 13CO 2 shift negatively with increasing TSR extent due to the inheritance of 13C/12C. • Methane can act as predominant reactant in TSR. [ABSTRACT FROM AUTHOR]

Variations in the stable carbon isotope signature of marine carbonate rocks (δ13C carb) are utilized as an indicator for changes in depositional environment and tracers of diagenetic alteration. Various petrologic (textural changes) and geochemical indicators (trace elements) have been used to evaluate the effect of diagenetic alteration on the δ13C carb signature. However, how diagenetic processes involving interactions between organic matter and minerals impact the δ13C carb records of marine carbonate rocks has not been investigated thoroughly. In this study, systematic stable carbon (δ13C carb) and oxygen (δ18O carb) isotope analyses were conducted on marine carbonates from the Middle Ordovician Majiagou Formation in the Ordos Basin, China to evaluate secondary overprint of the primary δ13C carb record. 496 carbonate samples from the gas reservoir (potential gas source rock) were selected from one well located in the natural gas exploration target—mid-east part of the Ordos Basin. A depleted δ13C carb excursion (i.e., a shift to more negative values) occurred in the 100 m-thick anhydrite-bearing carbonate sequence of this formation. The δ13C carb and δ18O carb values for the whole rock vary from −10.9‰ to 1.8‰ and −11.5‰ to −4.0‰, respectively. The calcite veins and vug-filling calcites also have depleted carbon isotope signature ranging between −21.1‰ and −4.4‰. The secondary calcites observed under the microscope were generated by the conversion from isotopically depleted organic matter and resulted in the depleted δ13C carb signature. The commonly occurring thermochemical sulfate reduction in the study area and the degradation of potentially enriched carboxylic acid salts in anhydrite-bearing carbonate setting are major factors contributing to the depleted δ13C carb values. The conversion from organic carbon to inorganic carbon through organic matter-involved diagenetic processes plays a significant role in explaining the genesis of depleted δ13C carb excursions in deeply buried marine carbonate sequences. [Display omitted] • High-amplitude depleted δ13C carb signatures were first found to occur in the Middle Ordovician carbonate sequence. • Diagenetic processes involving organic matter impacted the original δ13C carb record. • Organic-origin calcites generated by TSR resulted in the depleted δ13C carb signatures. [ABSTRACT FROM AUTHOR]

The total organic carbon of carbonate sediments includes a 5%–45% contribution from acid-soluble organic matter, particularly carboxylic acid salts. However, there is little information available on hydrocarbon generation from carboxylic acid salts. The objectives of this study were to investigate the extent and timing of hydrocarbon generation from calcium stearate. Thus, gold-tube nonhydrous pyrolysis experiments of calcium stearate were conducted from 250 to 600 °C at 50 MPa. Infrared spectroscopy and δ13C testing were also performed on solid pyrolysate and gaseous products, respectively, to trace changes of functional groups and stable carbon isotopes. Pyrolysis of calcium stearate produced hydrocarbons, CO 2 , CaO, CaCO 3 , coke, and other inorganic compounds of low molecular weight. A Van Krevelen diagram indicated that calcium stearate falls in the same region as hydrogen-rich type I kerogen. The maximum yield of methane generated from calcium stearate was as high as that from North Sea oil. Hydrocarbon generation from calcium stearate was related to cracking of the alkyl moiety and similar to that occurring in pyrolysis of n- C 18. As a consequence of kinetic carbon isotope fractionation, with increasing temperature CO 2 becomes enriched in 13C and CaCO 3 becomes enriched in 12C. Notably, CaCO 3 with very low δ13C values is probably an effective proxy indicating hydrocarbon generation from carboxylic acid salts, if no other CaCO 3 is produced via thermochemical sulphate reduction processes. The optimized kinetic parameters demonstrate that calcium stearate had higher methane generation activation energies than those of n -C 18 and type I kerogen. Hence, methane generation from calcium stearate is delayed when extrapolated to geological conditions. This retarded methane generation illustrates the vital role of carboxylic acid salts in the generation of deep oil and gas. Future research should concern the abundance of carboxylic acid salts in carbonate formations and the depositional environments that facilitate the formation of carboxylic acid salts. • Closed gold-tube pyrolysis of calcium stearate was conducted. • Calcium stearate had gas potential as high as crude oil. • Negative δ13C CaCO3 may indicate hydrocarbon generation from carboxylic acid salts. • Methane generation from calcium stearate was behind that of n- C 18. [ABSTRACT FROM AUTHOR]

Helium is an important, scarce, and strategic resource that is critical to national security and advanced technology development. Although helium originates from mantle degassing and the radioactive decay of uranium (U) and thorium (Th) in minerals, commercial helium resources are primarily derived from natural gas reservoirs with He concentrations higher than 1000 ppm. The gas geochemistry characteristics of 920 natural gas samples from nine petroliferous basins in China were used to classify and discuss the enrichment mechanisms of helium. Helium-bearing natural gas reservoirs can be divided into the following three types: basement source-migration-rich accumulation, self-generating and self-preserving, and crust–mantle complex types. The basement source-migration-rich accumulation, which is the most common type of proven helium-rich natural gas reservoir, occurs primarily in inherited anticlines and dome traps located on basement uplifts. Ancient shale systems demonstrate the potential to form helium-rich shale gas that belongs to the self-generating and self-preserving type. Crust–mantle complex helium-rich natural gas reservoirs develop in petroliferous basins in active magmatic–tectonic areas. The origins of helium and hydrocarbon gas are different, but the entrainment of helium during hydrocarbon gas accumulation is a key factor influencing the formation of helium-rich natural gas reservoirs. The nitrogen concentration in helium-rich gas samples is often extremely high, indicating that the nitrogen and the helium are derived from the same source rock or both are extracted from water by migrating gas. • Helium-rich gas includes basement source-migration-rich accumulation, self-generating and self-preserving, and crust–mantle complex types. • Exsoluion gathering of underground dissolved helium is the key mechanism of helium enrichment. • High-quality matching between helium and natural gas is crucial for the helium-rich gas accumulation. [ABSTRACT FROM AUTHOR]

Organic-rich shales are generally characterized by high uranium (U) contents. The effects of U irradiation on evolution and hydrocarbon generation of organic matter (OM) have attracted substantial attention. However, details on the generation mechanisms of products and the identification of dominant reactions occurring during OM radiolysis remain understudied. In this study, γ ray was used to irradiate immature kerogen and bitumen, and electron beam was for decane. The contents and carbon (C) isotopic compositions of gaseous products, the liquid products of decane, and the functional groups alterations of kerogens and bitumen were detected. The results showed that, the product generation corresponded well to the structural alteration of OM during radiolysis. C–H and R–CH 3 rupture contributed to H 2 and CH 4 generation, respectively. O-containing group cleavage caused CO 2 generation, and O 2 presence could enhance the oxidation of OM by joining in the structure of OM or by facilitating the formation of new oxides. H 2 O addition could not only provide O, enhancing the oxidation of OM, but could also provide H, thus promoting H 2 generation. We identified cracking, cross-linking, and oxidation reactions during the OM radiolysis. C isotopic composition changes in solid OM could facilitate identification of dominant reactions. Non-closed geological environment of shales promoted the occurrence of cracking reaction. The substantial radiation energy generated from U decay in U-rich shales was significant for OM evolution via decrease in H/C, increasing O/C, and via enhancement of dealkylation, aromatization and unsaturation, which were similar to the thermal effects. Moreover, U irradiation could also promote conventional gas and immature gas generation and could provide exogenous H for hydrocarbon generation of OM, especially for OM in an over mature stage. It is essential to consider the effects of U irradiation on the evolution, hydrocarbon generation, and resources assessment of U-rich shales. • C–H, R–CH 3 and O-containing bonds rupture contribute to H 2 , CH 4 and CO 2 generation. • H 2 O can provide exogenous H for H 2 and hydrocarbons generation. • C isotopic composition changes of OM can be an indicator of dominant reaction. • Radiation like thermal effect, promotes evolution and hydrocarbon generation of OM. [ABSTRACT FROM AUTHOR]

A series of hydrous pyrolysis experiments were conducted to assess the role of sulfate minerals on the chemical and isotopic compositions of light hydrocarbons generated during thermal degradation of kerogen. The shale source rock with low maturity (T max = 425 °C) from Eagle Ford formation in southeast Texas was used in the experiments at 330 °C for the duration of 72 h, while the abundance of gypsum in each experiment varies from 0, 0.5, to 1 wt%. Under experimental conditions, the positive effect of gypsum on the yields of gaseous alkanes (C 1 - C 4) and CO 2 has been explicitly observed, with an exponential relationship between the yield of each gas and the amount of gypsum. While the relative abundance of C 2 - C 4 alkanes has shown an increasing trend with gypsum, the gas dryness (C 1 /∑C 1-4), however, decreased from 73.1% with no gypsum to 49.6% with gypsum of 1 wt%. It was attributed to lower overall yields and corresponding higher increased generation rates of C 2 - C 4 alkanes than methane through thermal cracking of kerogen and other organic compounds. The carbon isotope value of each alkane reached the highest in the experiment with 1 wt% gypsum. Compared to the experiment without gypsum, there was an increase of 11.5‰ for the δ13C value of methane with 1 wt% gypsum. The extent of this 13C enrichment between experiments with different amount of gypsum became less for C 2 to C 4. The δ13C value of CO 2 was the highest among generated gases, ranging from −4.7 to −2.1‰. Combined with chemical compositions of gases, it suggested that enhanced thermal cracking reactions were predominant in generation of light alkanes with the presence of gypsum, whereas dissolution of carbonates was the main source of CO 2. The reactions involving thermochemical sulfate reduction (TSR) were minor (if present) under experimental conditions, most likely due to the small quantity of gypsum (up to 1 wt%) and neutralization of acidity by carbonates. Nonetheless, the changes in chemical and isotopic compositions of light alkanes, which were ultimately caused by the presence of gypsum, need to be incorporated into geochemical proxies for assessing source rocks (maturation in particular) and their generation potential. • Hydrous pyrolysis experiments were conducted to assess the role of gypsum (0-1 wt%) on light hydrocarbon generation. • There is a positive effect on the yield of C 1 - C 4 gaseous alkanes, due to enhanced thermal cracking reactions with gypsum. • Each alkane gas becomes enriched in 13C with gypsum, with the extent of this 13C enrichment being prominent for methane. • The effect of gypsum on chemical and isotopic compositions of light hydrocarbons is important for source rock evaluation. [ABSTRACT FROM AUTHOR]

An intramolecular isotopic study was conducted on natural gases from the Turpan-Hami Basin of China. The set of natural gases exhibits larger δ13C values of the terminal position compared with those at the central position of propane, with Δ C-T (δ13C central -δ13C terminal) values ranging from −3.8‰ to −0.7‰. During the thermal cracking of kerogen, the primary kinetic isotope effect is on the terminal carbon position to generate n -propyl radicals and on the central carbon position to produce i -propyl radicals. Therefore, distinct reaction pathways significantly influence the position-specific carbon isotope distribution in thermogenic propane. In the early stage of maturation, propane is preferentially generated from the i -propyl radicals by capturing hydrogen atoms owing to the lower dissociation energies of i -propyl radicals cleavage from kerogen compared with those of n -propyl radicals, resulting in larger δ13C values in the terminal position compared with those at the central position of generated propane. The position-specific 13C isotope composition of propane could provide new insights into revealing the generation mechanism of natural gas in the geological period at the atomic level. • Propane with early maturation exhibits larger δ13C values in the terminal position than those at the central position. • Propane can be preferentially generated from the i -propyl radical reaction at the early maturation stage. • Decompositions of the branched aliphatic chains structure contribute to the low-mature gas generation in the Turpan-Hami Basin. [ABSTRACT FROM AUTHOR]

Determination of the total organic carbon (TOC) from well logs is an important step in formation evaluation of shale reservoirs. The ΔlogR model is one of the most commonly used methods in determining the TOC of the source rocks. Wang et al. (2016) made three revisions on ΔlogR model to enhance the prediction accuracy, one of which is to use a changeable slope of log 10 R t versus porosity logs to replace a fixed value. Based on the original ΔlogR model and Wang's revision, we propose an improved model for a better TOC estimation in this paper. Specifically, the approximate linear baseline is replaced by a theoretical one. Under this condition, both the slope and resistivity and porosity log values of the baselined rocks vary from depth to depth. The determination of theoretical baseline is presented, and the varied baseline log values are obtained by a simple mathematical technique. In addition to the shale play of Western Canada Sedimentary Basin, two study areas of Sichuan Basin and Ordos Basin in China, were used to assess the applications of the proposed model and its improvement over the original model. The analysis and discussions presented in this paper indicate that the improved model offers more reliable results in shale plays with large lithological variations. Also, the TOC content of the three study areas was estimated using the proposed model. The results showed that the TOC values estimated from well logs and core analysis are in good agreement. [ABSTRACT FROM AUTHOR]

Analysis of redox conditions of lake water is of great significance to the reconstruction of sedimentary environments. However, previously established threshold values of redox-sensitive element proxies may not be directly applicable to various study areas even with some degree of calibration. The present study of the Permian Lucaogou Formation (P 2 l), Santanghu Basin, re-evaluates and calibrates commonly used redox proxies, including bimetal ratios, the C–S–Fe–P system, and proxies based on U abundances. Specifically, we rely on the compound covariation theory of proxies proposed by previous researchers within the redox framework according to three key redox thresholds (T1: suboxidized/subreduced boundary, T2: middle of subreduced zone, and T3: suboxic/euxinic boundary) and organic/inorganic geochemical data. The results show that the fixed thresholds of widely used redox proxies, including bimetal ratios, are not fully applicable to the studied succession. Reassessment of the use of bimetal ratios based on covariation theory suggests that the V/Cr, V/(V + Ni), TOC/P, and Fe/Al ratios are especially useful to redox analysis of the P 2 l succession whereas the Ni/Co, Ni/V, (Cu + Mo)/Zn, U/Th, and U auth proxies yield spurious results. It is important to point out, however, that the threshold values of those proxies that have yielded useful results cannot be used without calibration. Therefore, those redox proxies used in our investigation of the P 2 l succession have been re-calibrated based on the proxy covariation theory. For example, V/(V + Ni) values of T1, T2, and T3 thresholds of approximately 0.78 differ from the fixed threshold values established by previous investigations, suggesting that avoiding erroneous redox interpretations will require calibration of the thresholds. Other redox proxies display a similar need for calibration. Deposits of the P 2 l, by virtue of small variations among the three thresholds, record a complex history of rapid transitions from suboxic to euxinic conditions. • Redox conditions of the Permian Lucaogou Formation (P 2 l) were revealed in detail. • The effectiveness of redox proxies commonly used in the P 2 l was reassessed. • Specific threshold values of redox proxies for the P 2 l were established. • Redox proxy with fixed threshold cannot be applied to other area without calibration. [ABSTRACT FROM AUTHOR]

Fourteen gases and their associated oils were collected at the surface from production wells in the Shahejie Formation, Dongying Formation, and Archean in six zones around the Qingshui Sub-sag of the Liaohe Depression, Bohai Bay Basin. The molecular compositions and carbon and hydrogen isotopic compositions of natural gas were measured. The origin, migration, accumulation, and secondary alteration of natural gas were studied based on the lateral and vertical comparison of natural gas geochemical characteristics. The natural gases include humic-type (δ13C 2 > −29.0‰, δ13C 1 > −43.0‰), sapropelic-type (δ13C 2 < −29.0‰, δ13C 1 > −43.0‰), and secondary microbial (δ13C 1 of −48.5‰, C 1 /(C 2 +C 3) of 46.3, δ13CO 2 of −1.5‰) gases. The humic-type and sapropelic-type gases were derived from the source rocks in the third and fourth members of the Shahejie Formation, respectively. The humic-type gases dominate. The biodegradation level of natural gas in the study area varies, and biodegraded gases are characterized by high Δ13C(C 3 –C 2), low C 3 /C 2 , and low C 3 /iC 4 and are associated with biodegraded oils. The vertical migration of natural gas in the west and east of the Qingshui Sub-sag is limited. There are good additional exploration prospects for humic-type gas, derived from high-maturity source rocks of the third member of the Shahejie Formation in the deep layers (depth >4.5 km) of the Qingshui Sub-sag. Secondary microbial gas is a potential exploration target in the shallow layers (depth <2.5 km). • Humic-type gas is from the source rock of the 3rd member of Shahejie Formation. • Sapropelic-type gas is from the source rock of the 4th member of Shahejie Formation. • Biodegraded gas: 13C-rich propane, depletion of propane and n-butane. • Secondary microbial gas: 13C-rich carbon dioxide and 13C-depleted methane. • Natural gas mainly accumulates near the source rock. [ABSTRACT FROM AUTHOR]

Proterozoic strata have become a new target for hydrocarbon exploration worldwide. They exhibit the potential to become a resource replacement target. To provide a reference for hydrocarbon source rock evaluation, the sedimentary environment and mechanism of organic matter enrichment of dark shales in the Mesoproterozoic Cuizhuang Fm. were investigated for the first time by combining organic and inorganic research concepts. Although the total organic carbon (TOC) content in the Cuizhuang Fm. is relatively low (mean value of 0.59%), it still can be considered as a potential source rock. Thin-section observations, main and trace element contents, and biomarker compounds revealed the following: (1) the Cuizhuang Fm. was deposited in a neritic shelf sedimentary environment at a passive continental margin, and the climate was warm and humid. The hydrocarbon-generating organisms mainly comprised prokaryotes such as cyanobacteria and other bacteria. All these characteristics meet the conditions of source rock formation. (2) The low oxygen level, felsic source provenance, and absence of submarine hydrothermal activity led to low productivity, and the low productivity and low-grade hydrocarbon-generating organisms led to low TOC content in the Cuizhuang Fm. (3) The enrichment of V in the Cuizhuang Fm. is apparent, but does not reach the hyper-enrichment threshold of 500 ppm associated with euxinic conditions, revealing anoxic ferruginous conditions. Moreover, the V content is positively correlated with TOC content, indicating that organic matter enrichment was mainly controlled by the redox conditions of the bottom water. Therefore, the sedimentary areas with high productivity were related to basic source provenance and submarine hydrothermal activity, and anoxic bottom water may have promoted the development of better source rocks. This findings may represent the main direction for identifying effective source rocks in subsequent work. • Sedimentary environments of the dark shales from Cuizhuang Fm. were studied, which satisfied the requirements of source rock formation. • The organic matter enrichment mechanism of the dark shales from Cuizhuang Fm. belongs to "preservation-type". • The sedimentary areas with basic source provenances and active submarine hydrothermal activity may provide high productivity and form more abundant organic matter in the Mesoproterozoic. [ABSTRACT FROM AUTHOR]

Adsorption and desorption processes of shale gas possess particular characteristics compared to conventional natural gas. Previous studies conducted desorption experiments on fresh core samples to investigate the isotope and chemical fractionations during the desorption process. However, complete chemical and isotope fractionations cannot be obtained due to the loss of gases during the coring process. In this study, we utilized a modified experimental apparatus based on the fixed adsorption bed to conduct systematic desorption experiments in confined systems with different proportions of free and adsorbed gases. The results show that different adsorption capacities and non-synchronous desorption for alkane gas components are leading factors influencing chemical fractionation during the desorption process. Meanwhile, C 1 /C 2 and C 1 /(C 1 +C 2) vary with different proportions of adsorbed gases in the same adsorption matrix. Generally, isotope fractionation between the adsorbed and free phases during desorption follows the Rayleigh fractionation model. The linear correlations between proportions of adsorbed CH 4 and isotope fractionation factors can be applied to predict the proportions of adsorbed gases. Calculations show that on average 55% of hydrocarbon gases exist at adsorbed state in shale gas from Yanchang Formation of Ordos Basin. • A modified experimental apparatus was firstly applied to study the chemical and isotope fractionations during desorption. • Isotope fractionation between the adsorbed and free phases during desorption follows the Rayleigh fractionation model. • Established mathematical model can be applied to predict the proportion of adsorbed gas in shale. [ABSTRACT FROM AUTHOR]