Sulfur isotope exchange between thiophenes and inorganic sulfur compounds under hydrous pyrolysis conditions

Sulfur isotope fractionation during thermal degradation of aromatic organic sulfur compounds was studied utilizing quartz tube-hydrous pyrolysis at 360 °C. BT (benzothiophene), DBT (dibenzothiophene), 3-PT (3-phenylthiophene) and 2-OT (2-octylthiophene) were used to represent organic sulfur compounds with a range of thermal stabilities. These compounds were heated for 40 h with and without the addition of reduced inorganic sulfur species: elemental S, sodium sulfide (Na₂S∗9H₂O) and sodium sulfite (Na₂SO₃). The extent of degradation of each compound showed good correlations with its aromatic stabilization and strongly depended on the added inorganic sulfur species, with DBT being the most stable and 2-OT the least stable. Sulfur isotope enrichments up to 2.2‰ resulted from the thermal degradation of thiophenes. A significant decrease in stability of thiophenes was observed in the presence of reduced sulfur species. In contrast, BT and DBT did not show any exchange of S isotopes with external reduced S pool, while 3-PT did experience S isotope exchange. The results of the present study suggest that the main controls on the δ³⁴S values of BT and DBT during TSR are the relative rates of their degradation and their formation from the reaction between TSR-H₂S and petroleum hydrocarbons. This finding improves our confidence in using the δ³⁴S of individual thiophenes as reliable proxies for the occurrence and extent of thermal maturation and thermochemical sulfate reduction.