(¹College of Chemistry and Chemical Engineering, Shaanxi University of Science & Technology, Xi’an 710021, China)

Abstract: The Ordovician Majiagou Formation in the Ordos Basin is characterized by widespread carbonate–evaporite assemblages, in which sulfate-related organic–inorganic interactions may have operated from deposition and early diagenesis to subsequent deep burial and high-maturity evolution. To investigate the evolution of carbon–sulfur coupling and its geochemical responses, inter evaporite carbonates from Well DS1 were analyzed for carbon isotopes, total organic carbon (TOC), and elemental geochemistry, and the results were integrated with previously published subsalt natural gas data from our research group. The inter evaporite carbonates show pronounced whole-rock δ¹³C_carb negative excursions. The main negative excursion samples range from −7.8‰ to −2.2‰ and are concentrated mainly in intervals characterized by frequent alternations of evaporites and carbonates, indicating clear stratigraphic differences. δ¹³C_org values range from −28.6‰ to −26.3‰, whereas Δ¹³C_carb–org varies from 17.9‰ to 25.9‰. Combined with sedimentary associations, TOC characteristics, and elemental geochemical variations, these observations suggest that evaporative restriction promoted differentiation of the local dissolved inorganic carbon (DIC) pool from open-marine waters. Organic-matter degradation and remineralization could introduce δ¹³C depleted carbon into local DIC, while microbial sulfate reduction may have contributed to early organic-matter remineralization under sulfate rich conditions. The resulting local DIC signal was subsequently incorporated into carbonate during deposition and early diagenesis. During later deep burial, hydrocarbons underwent thermochemical sulfate reduction (TSR). Published subsalt gas data show that H₂S contents are as high as 23.58%, accompanied by TSR-related secondary calcite, elemental sulfur, and pyrite. TSR preferentially consumed C_₂–₅ hydrocarbons, increased the dryness of the gas, and significantly modified ethane carbon isotope compositions, whereas methane isotopes were comparatively less affected. These features record a later stage of sulfate-driven organic–inorganic interaction distinct from the early carbon-cycle signal preserved in inter evaporite carbonates. Overall, the Majiagou carbonate–evaporite system records stage-dependent carbon–sulfur coupling: early evolution was dominated by restricted-water DIC differentiation and organic-matter-related carbon cycling, whereas deep burial was characterized by hydrocarbon-involved TSR and consequent modification of subsalt natural gas. This evolutionary linkage provides a framework for connecting early carbonate carbon-isotope signals with later gas geochemical alteration in deeply buried evaporite-bearing systems.

Keywords: Majiagou Formation; carbonate–evaporite assemblage; carbon isotopes; microbial sulfate reduction; thermochemical sulfate reduction; natural gas

Author Profile: Zhang Qian, Female, Lecturer, PhD, mainly engaged in sedimentology and geochemistry research. E-mail: zhangq199416@sina.com.