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The mechanism of hydrogen sulfide (HS) generation plays a key role in the exploration and development of marine high-sulfur natural gas, of which the major targets are the composition and isotope characteristics of sulfur-containing compounds. Hydrocarbon source rocks, reservoir rocks, natural gases and water-soluble gases from Sichuan Basin have been analyzed with an online method for the content of HS and isotopic composition of different sulfur-containing compounds. The results of comparative analysis show that the sulfur-containing compounds in the source rocks are mainly formed by bacterial sulfate reduction (BSR), and the sulfur compounds in natural gas, water and reservoir are mainly formed by thermal sulfate reduction (TSR). Moreover, it has been shown that the isotopically reversion for methane and ethane in high sulfur content gas is caused by TSR. The sulfur isotopic composition of HS in natural gas is inherited from the gypsum or brine of the same or adjacent layer, indicating that the generation and accumulation of HS have the characteristics of either a self-generated source or a near-source. [ABSTRACT FROM AUTHOR]

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]

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]