The sulfur-isotopic compositions of benzothiophenes and dibenzothiophenes as a proxy for thermochemical sulfate reduction

Compound-specific analyses of the 34S/32S isotope ratios of individual organosulfur compounds in Upper Jurassic oil and condensate samples from the Smackover Fm. reveal differences of up to ∼50‰ between compounds. There is a clear distinction between oils altered by thermochemical sulfate reduction (TSR) versus those that are not. Oils that did experience TSR exhibit significant 34S enrichment of benzothiophenes (BTs) compared to dibenzothiophenes (DBTs), while in unaltered oils these compounds have similar isotopic compositions. The δ34S values of BTs are close to those of sulfate-bearing evaporites of the Smackover Fm., whereas the δ34S values of DBTs are spread over a wider range and gradually approach those of the BTs.Gold-tube hydrous pyrolysis experiments using three representative oils show that isotopic alteration readily occurs under TSR conditions and can significantly affect the δ34S values of individual compounds. Our results indicate that BTs can be a sensitive tracer for TSR as they form readily under TSR conditions, with large 34S enrichments relative to the bulk oil. In contrast, DBTs exhibit relatively small changes in δ34S, preserving their original δ34S values longer than do BTs because of their greater thermal stability and slow rate of formation. We propose that comparison of the δ34S values of BT and DBT can be used to detect TSR alteration of oils from the very early stages up to highly altered oils. The approach should find numerous uses in petroleum exploration, as well as for understanding the basic reaction mechanisms and kinetics of thermochemical sulfate reduction and secondary sulfur incorporation into oils.