Effect of thermal maturation on the isotopic compositions of volatile organic sulfur compounds released from a kerogen by hydrous and anhydrous pyrolysis

Volatile organic sulfur compounds (VOSCs) may provide an alternative and improved approach in the evaluation of source and thermal maturity of natural gas. However, the role of water in the generation and decomposition of VOSCs is still ambiguous. Hydrous pyrolysis experiments were conducted on the isolated type II-S kerogen of a source rock collected from the Ghareb Formation, Israel at 200-360 degrees C for 72 h (Easy%Ro = 0.32-1.20). The evolved gases were quantified for C1-C5 hydrocarbons, CO2, H2S and VOSCs. Compound specific isotope analysis were conducted both on the VOSCs and H2S (delta 34S), and also on the C1-C5 hydrocarbons and CO2 (delta 13C, delta 2H). The results show that thiols concentration increased from 200 degrees C to 300 degrees C and then decreased from 300 degrees C to 360 degrees C. The formed thiols isotopically co-vary with H2S from 200 degrees C to 340 degrees C and present the opposite trend with H2S from 340 degrees C to 360 degrees C. The delta 34S values of thiols are usually 3-7%o lighter than the parent kerogen and 0.5-4%o heavier than H2S. The concentrations of thiophenes generally increased with thermal stress with peak generation at 360 degrees C, the delta 34S values of thiophenes are mostly < 2-4%o lighter than the parent kerogen and 2-8%o heavier than H2S through the experiments. The delta 13C and delta 2H values of gaseous hydrocarbons are gradually enriched in heavy isotopes with increased thermal maturity. VOSCs dominated by thiols and thiophenes are generated by two ways: 1. the interactions between H2S and hydrocarbon molecules (synthetic pathway), 2. thermal cracking of OMs including kerogen and associated bitumen primary cracking and hydrocarbon secondary cracking (thermal cracking pathway). The synthetic pathway played the major role in the generation of thiols at the immature and early mature stages (200-300 degrees C, Easy%Ro = 0.32-0.71), the cracking pathway has the dominant effect on the occurrence of thiols at higher thermal maturity levels (300-360 degrees C, Easy%Ro = 0.71-1.20). The main pathway of thiophenes formation throughout the experiments is by the thermal cracking pathway. In the hydrous experiments, water promotes the synthetic pathway that generated thiols and retarded the thermal cracking pathway that forms thiophenes, but had no significant impact on delta 34S values of individual VOSCs, delta 13C and delta 2H values of gaseous hydrocarbons. Therefore, combining with carbon and hydrogen isotopic compositions of gaseous hydrocarbons, molecular and sulfur isotopic compositions of individual VOSCs in the gas phase can provide valuable clues for petroleum systems in sedimentary basins including gas sources, generation pathways, thermal maturity levels and petroleum secondary transformations, such as thermochemical sulfate reduction in the subsurface reservoirs.

Automated summary

Volatile organic sulfur compounds (VOSCs) can be used as an alternative and improved method for evaluating the source and thermal maturity of natural gas. The role of water in the generation and decomposition of VOSCs is still unclear. Hydrous pyrolysis experiments were conducted on isolated kerogen from a source rock to study the formation of VOSCs at different temperatures. The results show that VOSCs are formed through both synthetic and thermal cracking pathways, and water promotes the synthetic pathway for the generation of thiols.