Thermochemical sulfate reduction in sedimentary basins and beyond: A review

Thermochemical sulfate reduction (TSR) is a set of redox reactions between organic matter and sulfates at temperatures >120 °C, and occurs in varied environments including deeply buried carbonate and clastic rocks, Mississippi Valley- and Sedimentary Exhalative type metal deposits, and early oceans on Earth and likely on Mars. TSR may be methane-, and heavy hydrocarbons-dominated ones, and is most likely initiated by H2S with labile organosulfur compounds (OSCs) as an intermediate product to generate CO2 and organic non‑sulfur and sulfur compounds. Of the organic non‑sulfur containing compounds, dealkylated- and carboxylated- polycyclic aromatics are formed due to oxidation, and 1- to 6- cage diamandoids are generated under highly reducing environments. Sulfur in thermally stable dibenzothiophenes and thiadiamondoids of a TSR oil represents cumulative or mixed sulfur from source kerogen decomposition and newly generated sulfur from different TSR stages, whilst sulfur from thiolanes is simultaneously generated from or exchanges isotopically with H2S newly generated during TSR. Thermochemical reduction of dissolved sulfate by organic matter produces significant sulfur isotope fractionation, resulting in extremely heavy residual sulfates and wide ranges of produced sulfide δ34S values in some metal deposits and likely on Mars. In contrast, no significant fractionation occurs in the majority of petroleum reservoirs due to anhydrite dissolution as a rate-limited step. On Mars the presence of kerogen, methane, sulfates, sulfides, OSCs, chlorobenzene and likely sulfonic acids and active magmatic-hydrothermal activities indicates a potential of ongoing and past TSR. Methane-dominated TSR may generate water and carbonates with oxygen contributed from sulfate, and both are in oxygen isotope equilibration. In contrast, no water is formed by TSR of oils and condensates and carbonates newly precipitated are not in oxygen isotope equilibration with formation water.