Sulfur ions reduce the activation energy of steranes and terpanes under hydrothermal action

Steranes and terpanes, as key biomarkers in oils and source rock extracts, provide valuable information about their biological origins, depositional environment, and thermal maturity. The distribution patterns of steranes and terpanes in ultra-deep oil reservoirs within the Shuntuoguole low uplift are complex. The intensity of hydrothermal activity and the influence of sulfur ions were assessed based on the conversion of “Biphenyl–Dibenzothiophene–Phenanthrene” series compounds. The results reveal that the southern sections of F5, F12, and F4 exhibit relatively strong hydrothermal activity (F number: No. number fault zone). The southern sections of F5, F1, and F4 are more significantly impacted by sulfur ion activity under the hydrothermal action. Studies indicate that sterane and terpene concentrations decrease due to sulfur ion activity under the hydrothermal action. Tricyclic terpanes, cholestanes, C_31-35 homohopanes, C₃₀ hopane, C₂₉ diahopane, and pregnane exhibit low stability under the influence of sulfur ion activity in the hydrothermal action. Notably, although tricyclic terpanes, cholestanes, and pregnane show low stability, they are not preferentially depleted. Combined analysis of the relative Gibbs free energies of C₃₀ diahopane and C₃₀ hopane showed that sulfur ions do not promote pyrolysis by altering the degradation pathways of steranes and terpanes. Instead, sulfur ions likely lower the pyrolysis threshold by reducing the activation energy, preserving the relative stability of steranes and terpanes. This study advances the understanding of organic–inorganic interactions in hydrothermal systems.