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

Alon Amrani*, Andrei Deev, Alex L. Sessions, Yongchun Tang, Jess F. Adkins, Ronald J. Hill, Michael J. Moldowan, Zhibin Wei

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

98 Scopus citations

Abstract

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 δ 34S values of BTs are close to those of sulfate-bearing evaporites of the Smackover Fm., whereas the δ 34S 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 δ 34S 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 δ 34S, preserving their original δ 34S values longer than do BTs because of their greater thermal stability and slow rate of formation. We propose that comparison of the δ 34S 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.

Original languageAmerican English
Pages (from-to)152-164
Number of pages13
JournalGeochimica et Cosmochimica Acta
Volume84
DOIs
StatePublished - 1 May 2012

Bibliographical note

Funding Information:
We thank Zeev Aizenshtat and Ward Said-Ahamed (Hebrew University, Jerusalem) for the EA-IRMS analysis of bulk S isotopes, and Adam Subhas (California Institute of Technology) for technical assistance with the MC-ICPMS. This study was partially supported by Marathon Oil Company and the Power, Energy and Environmental Research Institute (PEERi). We appreciate the discussions and comments by Tongwei Zhang, Zeev Aizenshtat, Geoffrey Ellis, Clifford Walters, Richard Worden, and an anonymous reviewer that help to improve this manuscript.

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