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Abstract: To understand reaction pathways and isotope systematics during mineral-catalyzed abiotic synthesis of hydrocarbons under hydrothermal conditions, experiments involving magnetite and CO2 and H2-bearing aqueous fluids were conducted at 400°C and 500 bars. A robust technique for sample storage and transfer from experimental apparatus to stable isotope mass spectrometer provides a methodology for integration of both carbon and hydrogen isotope characterization of reactants and products generated during abiogenic synthesis experiments. Experiments were performed with and without pretreatment of magnetite to remove background carbon associated with the mineral catalyst. Prior to experiments, the abundance and carbon isotope composition of all carbon-bearing components were determined. Time-series samples of the fluid from all experiments indicated significant concentrations of dissolved CO and C1–C3 hydrocarbons and relatively large changes in dissolved CO2 and H2 concentrations, consistent with formation of additional hydrocarbon components beyond C3. The existence of relatively high dissolved alkanes in the experiment involving non-pretreated magnetite in particular, suggests a complex catalytic process, likely involving reinforcing effects of mineral-derived carbon with newly synthesized hydrocarbons at the magnetite surface. Similar reactions may be important mechanisms for carbon reduction in chemically complex natural hydrothermal systems. In spite of evidence supporting abiotic hydrocarbon formation in all experiments, an “isotopic reversal” trend was not observed for 13C values of dissolved alkanes with increasing carbon number. This may relate to the specific mechanism of carbon reduction and hydrocarbon chain growth under hydrothermal conditions at elevated temperatures and pressures. Over time, significant 13C depletion in CH4 suggests either depolymerization reactions occurring in addition to synthesis, or reactions between the C1–C3 hydrocarbons and carbon species absorbed on mineral surfaces and in solution. [Copyright &y& Elsevier]

Characteristics of redox-sensitive elements and organic carbon isotopes of a drilling section covering the Ediacaran-Cambrian (E-C) boundary at Anhui Province, South China were studied. Total organic carbon (TOC), organic carbon isotope values of organic compounds, abundances of Mo, V, U, and Ni, and ratios of V/(V + Ni) and Th/U co-vary across the section, suggesting that the redox condition is the main factor controlling carbon isotope values of organic matter, and sequestration of trace elements. The E-C boundary is characterized by the lowest organic carbon isotope and the highest ratios of Mo/TOC, V/TOC, and U/TOC in the section, which are correlated with reported carbon isotope excursions in other regions. This is the first time that signatures of Mo, V, and U in the Hetang Formation at the Lower Yangtze Platform are used for stratigraphic correlations. The results highlight abnormally high ratios of Mo/TOC, V/TOC, and U/TOC near the E-C boundary. Petrological and geochemical characteristics in this section suggests that intense upwelling activities during the E-C transition may be the main contributor to 13 C-depleted CO 2 and enrichment of trace metals. The co-existing pervasive transgression led to ocean anoxia and facilitated the preservation of 13 C-depleted organic matter and trace metal sequestration. Furthermore, oxidation of the bottom ocean water may have completed during the early Cambrian. A combination of these factors may be among the reasons for significant evolutionary changes during the E-C transition. [ABSTRACT FROM AUTHOR]

• 9-Phenylphenanthrene is the more enriched isomer under mild hydrothermal conditions. • Preferential bond positions susceptible to hydrothermal attack suggested for PAHs. • Phenyl-derivative compounds are mainly hydrothermally governed in the Zn/Pb deposit. • The Jinding oil and solid bitumen result from rapid hydrothermal generation. • Hydrothermal catalytic conversion results in organics in metastable equilibrium. Hydrothermal organic reactions in the presence of mineral catalysts have been well established by laboratory experiments. However, little is documented about hydrothermal catalytic conversion (HTCC) and the metastable existence of complex molecules in continental hydrothermal systems. Five freshly mined ores containing oil and solid bitumen and associated unmineralised host sediment were sampled from the Beichang mine, Jinding Zn/Pb ore deposit, Lanping, near the Indo-Eurasian collisional margin, southeastern Tibetan Plateau. Here we show evidence for a wide range of mineral (ZnS/PbS) catalysis of polycyclic aromatic compounds in the natural subsurface. An interpretation of hydrothermal alteration (non-biodegradation) was made for the unresolved complex mixture. From unmineralised host sediment through oils to the solid bitumen, a stepwise interaction of organic compounds with metal-bearing hydrothermal fluids was demonstrated by the transformation of polycyclic aromatic hydrocarbons (PAHs) and phenyl/sulfur/oxygen-containing compounds that are thought to be generated through HTCC. Thermodynamically-unstable phenyl derivatives (ortho - or α-substituted) resulted from hydrothermal alteration (ring opening) of the middle and naphthenic (low energy) rings of peri-condensed parent PAHs, as supported by the dominance of 1-phenyldibenzothiophene, 1-phenyldibenzofuran, triphenyleno[1,12- bcd ]thiophene, triphenyleno[1,12- bcd ]furan, 9-phenylphenanthrene, 1,1′:2′,1′′:2′′,1′′′-quaterphenyl, and the corresponding thiophenes or furans over other isomers. Heteroatom addition to phenyl-derivatives generates abundant heterocyclic compounds, which can undergo further dehydrocyclisation. In the presence of ZnS/PbS, the abundance of three compound classes (unsubstituted PAHs, 'aryl-aryl' linked phenyl-derivatives, and S/O atom-containing compounds) is mainly hydrothermally controlled, rather than thermodynamically controlled. Various phenyl/sulfur/oxygen-containing compounds are the metastable equilibrium products of a series of complex redox reactions under specific hydrothermal conditions. We hypothesise that the ubiquitous triphenyleno[1,12- bcd ]furan is an important marker for hydrothermal organic–inorganic interactions in the Zn/Pb deposit. The relative abundance of 9-phenylphenanthrene provides a measure of the degree of hydrothermal alteration and HTCC. Our data is consistent with a close interaction of ascending metalliferous hydrothermal fluids with indigenous organic matter/compounds. Hydrothermal fluids, in the presence of sphalerite (and/or galena), are highly effective for activation of the carbon–carbon bonds in PAHs. [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]

Abstract The genetic identification of different types of natural gas is notably important for assessment of its sources and exploration potential. The chemical and isotopic (C and H, in particular) compositions of natural gas vary significantly due to the complexity of its generation, migration, and accumulation processes. The "coal-type" gas generated from humic matter is generally enriched in 13C as compared to "oil-type" gas generated from sapropelic organic matter. However, gas originating from fresh-brackish water environments is depleted in 13C whereas gas from saline environments is enriched in 13C. Notwithstanding organic precusors and sedimentary environments, both isotope compositions of alkanes tend to become enriched both in 13C and 2H with prograde thermal evolution. Therefore, in addition to thermal maturity, source material is the major controlling factor of carbon isotope compositions, whereas sedimentary environment is predominant in governing hydrogen isotopes. Secondary processes, including thermochemical sulfate reduction (TSR) and diffusion, result in an enrichment of the gases in 13C and 2H due to mass-dependent kinetic isotope effect. Microbial degradation causes a decrease in propane content and an enrichment in 12C and 2H of the residual propane. The abiogenic gases may include methane from deep mantle and high molecular weight hydrocarbons through Fischer-Tropsch type (FTT) synthesis. Methane of mantle origin possesses a narrow range of isotope compositions, although it is still a tall task to determine the exact values. In contrast, isotopes of alkane gases synthesized from FTT processes are in a wide range. In sedimentary basins, the mixing of gases from multiple sources and/or through different secondary processes may pose a challenage to identification of their origins. The detailed assessment is provided here with case studies from major oil and gas basins in China. This review provides identification of misconceptions in genetic types of natural gas using carbon and hydrogen isotopes of alkanes, and sheds insights into using isotope geochemistry as an important diagnostic tool for energy exploration as well. Highlights Classification of natural gas and traditional geochemical methods for genetic identification of natural gas are reviewed. The effect of geological background and secondary alteration on δ13C and δ2H of light alkanes is assessed. New diagnostic tools for genetic classification of natural gas are suggested. [ABSTRACT FROM AUTHOR]

• Organic compounds in vent fluids from SI Deep Hole and West Thumb, Yellowstone Lake. • Diverse structural complexities and relative quantities of organic molecules. • Fluid-rock interactions under physicochemical conditions key to organic formation. • Involvement of magmatic degassing and microbial activity for dissolved CO 2 and CH 4. • Interplay of geological, chemical and biological processes in Yellowstone ecosystem. Hydrothermal fluids were collected over a three-year interval (2016–2018) from two sublacustrine vent fields in Yellowstone Lake, Wyoming: Stevenson Island Deep Hole (YL16 and YL17 series in 2016 and 2017, respectively) and West Thumb (YL18 series in 2018). The TOC content in SI Deep Hole vent fluids varies from 2.7 mg/L to 6.3 mg/L in YL16 and 5.0 mg/L to 16.2 mg/L in YL17. Compared to YL16, the average TOC value of YL17 has more than doubled. The range of TOC is from 7.3 mg/L to 13.7 mg/L in the West Thumb samples. A positive correlation between TOC and dissolved CO 2 concentration is observed in most samples, suggesting their close relationship with sublacustrine heat and mass transfer processes. The organic compounds detected at both vent fields are diverse in molecular structure. In the SI Deep Hole, complex cyclic and unsaturated organic compounds are dominant, with cyclic hydrocarbons having the highest abundance, followed by carboxylic acids, PAHs, and alcohols. At West Thumb vents, propanol is most abundant, followed by alcohols, PAHs, alkanes, and carboxylic acids. This difference in structural complexity and relative quantity of organic compounds may be attributed to the involvement of hydrothermal processes. The vapor-dominated hydrothermal system and extensive fluid-rock interactions in SI Deep Hole facilitate alteration/decomposition of organic precursors. While CO 2(aq) is predominant in vent fluids, dissolved CH 4 is much less abundant. In SI Deep Hole, the carbon isotopic composition of CH 4 in most samples ranges from −20.4‰ to −38.0‰, with values of less than −60‰ in three samples, while CO 2 ranges from −4.4‰ to −10.5‰. A temporal variation of δ13C values was obtained for CO 2 , with the average increasing from −9.2‰ (YL16) to −5.2‰ (YL17). In West Thumb, the δ13C value of CO 2 and CH 4 fall in a similar range, with an average of −10.5‰ and −19.0‰, respectively. There are multiple sources for observed carbon species: the interaction of vent fluids with subsurface rocks and lake sediments is the predominant process, followed by thermal decomposition of organic matter. Microbial activity on the lake floor may contribute to methane formation, while magmatic degassing is the major process that produces CO 2. The connection between vapor-dominated hydrothermal alteration, elevated TOC contents and dissolved CO 2 in vent fluids, and enrichment of 13C in CO 2 in YL17 in SI Deep Hole suggests an interplay of geological, chemical, and biological processes and their imprints on vent fluids, all of which make significant contribution to the carbon cycle in the Yellowstone Lake ecosystem. [ABSTRACT FROM AUTHOR]

Black organic-rich shale is a sedimentary product of strongly reducing environmental water bodies, and is the main source of shale oil and gas. During its formation, the presence of sulfate promotes bacterial sulfate reduction (BSR), but little attention has been paid to how BSR affects the preservation of organic matter (OM) in different sedimentary environments. In this study, the Wufeng-Longmaxi Formations from the Sichuan Basin, the Chang-7 Member (the seventh member of the Triassic Yanchang Formation) from the Ordos Basin, the Qingshankou and Nenjiang Formations in the Songliao Basin, as well as the New Albany shale from the Appalachian Basin are selected for investigation. Results show that there is a positive correlation between total sulfur (TS) and total organic carbon (TOC) contents, and the sulfate reduction index (SRI) and TOC show two-stage exponential changes. When the SRI is higher than 1.375, the overall TOC content is low, indicating that a large amount of organic matter is consumed through intense BSR. When the SRI is lower than 1.375, the TOC content is significantly higher, indicating that weak BSR has relatively little effect on organic matter degradation. In the normal marine sedimentary environment for the New Albany shale in North America, the high concentration of sulfate was conducive to BSR, but the increased input of terrestrial OM compensated for the marine OM consumption by BSR, resulting in a high TOC value in the shale. Volcanic activity provides additional sulfates into the water column and triggers BSR occurrence, but the high concentration of nutrients brought by volcanic activity in the shales of the Wufeng-Longmaxi Formations and the Chang-7 Member promoted paleoproductivity, offset the effect of OM consumption by BSR, and resulted in the high TOC values seen in these shales. When a large amount of sulfate was input into terrestrial lakes through volcanic activity or marine transgression, there were significantly different effects on the preservation of organic matter in lacustrine shales. Nutrients from the volcanic activity increased the paleoproductivity of lakes during the Chang-7 period, while decreasing the impact of BSR on OM, resulting in a high TOC value in the Chang-7 Member shale. However, when the lacustrine basin was subjected to marine transgression, paleoproductivity weakly increased and BSR consumed a larger proportion of the OM, reducing the TOC values and forming the relatively low TOC shale of the Qingshankou and Nenjiang Formations in the Songliao Basin. • Strong BSR is a consumption process of OM. • Anoxic conditions caused by BSR promoted OM preservation in shales. • SRI is established to assess consumption and preservation of OM by BSR. • Tuff input to a lake is more beneficial to OM concentration than transgression. [ABSTRACT FROM AUTHOR]