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 ℃ 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 (δ34S), and also on the C1-C5 hydrocarbons and CO2 (δ13C, δ2H). The results show that thiols concentration increased from 200 ℃ to 300 ℃ and then decreased from 300 ℃ to 360 ℃. The formed thiols isotopically co-vary with H2S from 200 ℃ to 340 ℃ and present the opposite trend with H2S from 340 ℃ to 360 ℃. The δ34S values of thiols are usually 3–7‰ lighter than the parent kerogen and 0.5–4‰ heavier than H2S. The concentrations of thiophenes generally increased with thermal stress with peak generation at 360 ℃, the δ34S values of thiophenes are mostly < 2–4‰ lighter than the parent kerogen and 2–8‰ heavier than H2S through the experiments. The δ13C and δ2H 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 ℃, 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 ℃, 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 δ34S values of individual VOSCs, δ13C and δ2H 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.
Bibliographical noteFunding Information:
We thank Lubna Shawar, Alex Meshoulam, Yoav Rosenberg and Shimon Feinstein for technical assistance in the experiments and for useful discussions. A.A thanks the Israeli Science Foundation grant No. 3195/19 for partial support of this study. Q.X appreciates the grants from National Nature Science Foundation of China (No. 41961144023 , 42073066 , 41673041 ), the State Key Laboratory of Organic Geochemistry , GIGCAS (No. SKLOG202013 ), and the Key Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education (No. PI2021-05 ). We appreciate the valuable comments and suggestions by Dr. Yuhong Liao and an anonymous referee that greatly improved this paper.
© 2023 Elsevier B.V.
- Compound specific sulfur isotope
- Equilibration isotope effect
- Kinetic isotope effect
- Thermal maturation
- Volatile organic sulfur compounds