Effects of inorganic sulfur species on hydrocarbon conversion and 34S isotope fractionation during thermal maturation of Type II kerogen. (June 2022)

• Record Type: Journal Article
• Authors: He, Kun; Zhang, Shuichang; Wang, Xiaomei; Ma, Qisheng; Li, Zhen; Canfield, Donald E.; Mi, Jingkui; Tang, Yongchun; Guo, Jinhao
• Is Part Of: Organic geochemistry. Volume 168(2022)
• Publication Date: 2022-06
• Subjects: Sulfur isotope fractionation -- Kinetic isotope effect -- Equilibrium isotope effect -- Sulfate -- Pyrite; Organic geochemistry -- Periodicals; Biogeochemistry -- Periodicals; Géochimie organique -- Périodiques; 553.205
• DOI: 10.1016/j.orggeochem.2022.104420
• Languages: English
• ISSNs: 0146-6380

Abstract

Highlights: TSR accelerated oil and hydrocarbon gas decompositions in the pyrolysis of OM. Pyrite decomposition was responsible for the 34 S-depletion of H2 S at 330–450 °C. TSR led to the enrichment in 34 S of H2 S and oils in thermal maturation of OM. Two parts of 34 S fractionation by KIE and 34 S exchange were observed for TSR. Abstract: Inorganic sulfur (S) species including pyrite (FeS2 ) and sulfates may co-exist with organic matter (OM) in source rocks. Their inter-related effects on hydrocarbon generation and decomposition, and the 34 S isotope fractionation during thermal maturation remain unclear. In this study, four groups of hydrothermal experiments (kerogen with pyrite, kerogen with pyrite and gypsum, kerogen with pyrite removal, and kerogen with pyrite removal and gypsum) were conducted at 330–450 °C and 50 MPa using a gold-tube system. These experiments showed that pyrite and gypsum had limited effect on the determined vitrinite reflectance (%Ro) and H/C of kerogens under hydrothermal conditions. However, the presence of gypsum led to the occurrence of TSR accelerating the decompositions of oil and hydrocarbon gases. TSR also resulted in the apparent increase of gas dryness and sourness, and the enrichment in 13 C and 2 H of methane. Experimental data confirmed that an equilibrium isotope effect (EIE) was responsible for the small 34 S fractionation between H2 S and its precursor OM-S during thermal cracking of OM-S. The higher yields and more negative 34 S isotopic ratios (δ 34 S) of H2 S in the pyrolysis of kerogen with FeS2 revealed that the decomposition of 34 S-depleted FeS2 contributed to H2 S generation at elevated temperatures. Additionally, δ 34 S of pyrolysis products (i.e., oil, H2 S and residual kerogens) become much more enriched with TSR. Mass balance calculations suggested that the evolution of δ 34 S of H2 S from TSR in closed systems proceeded in two stages: the kinetic isotope effect (KIE) dominates the 34 S fractionation in the early stage of TSR; and 34 S exchange between sulfate and H2 S is more influential in the latter/higher T stage of TSR. These conclusions may provide additional insights for understanding of 34 S isotope fractionation both in hydrothermal settings and in organic-rich shale with multiple S sources.