Amrani, Alon and Deev, Andrei and Sessions, Alex L. and Tang, Yongchun and Adkins, Jess F. and Hill, Ronald J. and Moldowan, J. Michael and Wei, Zhibin (2012) The sulfur-isotopic compositions of benzothiophenes and dibenzothiophenes as a proxy for thermochemical sulfate reduction. Geochimica et Cosmochimica Acta, 84 . pp. 152-164. ISSN 0016-7037 http://resolver.caltech.edu/CaltechAUTHORS:20120509-102449517
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Compound-specific analyses of the ^(34)S/^(32)S 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 ^(34)S enrichment of benzothiophenes (BTs) compared to dibenzothiophenes (DBTs), while in unaltered oils these compounds have similar isotopic compositions. The δ^(34)S values of BTs are close to those of sulfate-bearing evaporites of the Smackover Fm., whereas the δ^(34)S 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^(34)S 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 ^(34)S enrichments relative to the bulk oil. In contrast, DBTs exhibit relatively small changes in δ^(34)S, preserving their original δ^(34)S values longer than do BTs because of their greater thermal stability and slow rate of formation. We propose that comparison of the δ^(34)S 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.
|Additional Information:||© 2012 Elsevier Ltd. Received 31 May 2011; accepted in revised form 20 December 2011; available online 28 January 2012. Associate editor: Josef P. Werne. 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.|
|Official Citation:||Alon Amrani, Andrei Deev, Alex L. Sessions, Yongchun Tang, Jess F. Adkins, Ronald J. Hill, J.Michael Moldowan, Zhibin Wei, The sulfur-isotopic compositions of benzothiophenes and dibenzothiophenes as a proxy for thermochemical sulfate reduction, Geochimica et Cosmochimica Acta, Volume 84, 1 May 2012, Pages 152-164, ISSN 0016-7037, 10.1016/j.gca.2012.01.023. (http://www.sciencedirect.com/science/article/pii/S0016703712000464)|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||09 May 2012 18:04|
|Last Modified:||09 May 2012 18:04|
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