Influence of sulfate on the generation of bitumen components from kerogen decomposition during catagenesis

Abstract

High-quality source rocks in saline lacustrine or marine sedimentary environments often show early peak petroleum generation and enhanced hydrocarbon yields, which have conventionally been attributed to organosulfur-enhanced thermal decomposition of kerogen. However, there is increasing awareness that the coexisting inorganic salts, particularly sulfates, might also contribute to the acceleration of petroleum generation. In this study, we investigated the influence of sulfates on the thermal decomposition of kerogen sampled from the Pingliang Formation in the Ordos Basin. Our results demonstrate that the kerogen samples mixed with sulfate generate more hydrocarbons with a lower peak production temperature than their sulfate-free counterparts. Detailed chemical analysis revealed that the presence of sulfates significantly facilitated the generation of resins and asphaltenes at temperatures below 350 °C, corresponding in our simulations to the early stage of petroleum generation (Easy%Ro < 1.0%). We hypothesize that sulfates can promote the fragmentation of kerogen via direct thermochemical oxidation into a complex mixture of bitumen components that consist mostly of compounds typically encountered in the classical resin and asphaltene fractions. The findings reveal a new reaction paradigm between sulfates and organic macromolecules that could offer useful guidance in the exploration for low-maturity oils.