Effects of inorganic sulfur species on hydrocarbon conversion and 34S isotope fractionation during thermal maturation of Type II kerogen

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 34S 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 13C and 2H of methane. Experimental data confirmed that an equilibrium isotope effect (EIE) was responsible for the small 34S fractionation between H2S and its precursor OM-S during thermal cracking of OM-S. The higher yields and more negative 34S isotopic ratios (δ34S) of H2S in the pyrolysis of kerogen with FeS2 revealed that the decomposition of 34S-depleted FeS2 contributed to H2S generation at elevated temperatures. Additionally, δ34S of pyrolysis products (i.e., oil, H2S and residual kerogens) become much more enriched with TSR. Mass balance calculations suggested that the evolution of δ34S of H2S from TSR in closed systems proceeded in two stages: the kinetic isotope effect (KIE) dominates the 34S fractionation in the early stage of TSR; and 34S exchange between sulfate and H2S is more influential in the latter/higher T stage of TSR. These conclusions may provide additional insights for understanding of 34S isotope fractionation both in hydrothermal settings and in organic-rich shale with multiple S sources.