11874 11 archive 1 disk0/00/01/18/74 2008-10-08 06:38:36 2008-10-08 06:38:36 2008-10-08 06:38:36 article show 0 Ma Qisheng Ma-Q Ellis Gregory S. Ellis-G-S Amrani Alon Amrani-A Zhang Tongwei Zhang-T pub cls restricted Copyright © 2008 Elsevier. Received 14 August 2007; accepted 5 May 2008. Available online 19 June 2008. This work was supported by the Joint Industrial Program on Thermochemical Sulfate Reduction at the Power, Environmental and Energy Research Center at the California Institute of Technology. Industrial sponsors include BP, Chevron, ExxonMobil, SaudiAramco, Shell Oil, Total and ENI. Constructive technical reviews were provided by Michael Lewan, Zeev Aizenshtat, and two anonymous reviewers, and editorial comments were provided by Dick Keefer. The abiotic, thermochemically controlled reduction of sulfate to hydrogen sulfide coupled with the oxidation of hydrocarbons, is termed thermochemical sulfate reduction (TSR), and is an important alteration process that affects petroleum accumulations in nature. Although TSR is commonly observed in high-temperature carbonate reservoirs, it has proven difficult to simulate in the laboratory under conditions resembling nature. The present study was designed to evaluate the relative reactivities of various sulfate species in order to provide greater insight into the mechanism of TSR and potentially to fill the gap between laboratory experimental data and geological observations. Accordingly, quantum mechanics density functional theory (DFT) was used to determine the activation energy required to reach a potential transition state for various aqueous systems involving simple hydrocarbons and different sulfate species. The entire reaction process that results in the reduction of sulfate to sulfide is far too complex to be modeled entirely; therefore, we examined what is believed to be the rate limiting step, namely, the reduction of sulfate S(VI) to sulfite S(IV). The results of the study show that water-solvated sulfate anions View the MathML source are very stable due to their symmetrical molecular structure and spherical electronic distributions. Consequently, in the absence of catalysis, the reactivity of SO4 2- is expected to be extremely low. However, both the protonation of sulfate to form bisulfate anions (HSO4-) and the formation of metal-sulfate contact ion-pairs could effectively destabilize the sulfate molecular structure, thereby making it more reactive. Previous reports of experimental simulations of TSR generally have involved the use of acidic solutions that contain elevated concentrations of HSO4- relative to SO4 2-. However, in formation waters typically encountered in petroleum reservoirs, the concentration of HSO4- is likely to be significantly lower than the levels used in the laboratory, with most of the dissolved sulfate occurring as SO4 2-, aqueous calcium sulfate ([CaSO4](aq)), and aqueous magnesium sulfate ([MgSO4](aq)). Our calculations indicate that TSR reactions that occur in natural environments are most likely to involve bisulfate ions (HSO4-) and/or magnesium sulfate contact ion-pairs ([MgSO4]CIP) rather than ‘free’ sulfate ions (SO4 2-) or solvated sulfate ion-pairs, and that water chemistry likely plays a significant role in controlling the rate of TSR. 2008-09-15 published Geochimica et Cosmochimica Acta 72 18 Elsevier 4565-4576 CaltechAUTHORS:MAQgca08 TRUE 0016-7037 http://resolver.caltech.edu/CaltechAUTHORS:MAQgca08 http://dx.doi.org/10.1016/j.gca.2008.05.061 doi Akilan C., Rohman N., Hefter G. and Buchner R. 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(1996) The effects of thermochemical sulfate reduction upon formation water salinity and oxygen isotopes in carbonate gas reservoirs. Geochim. Cosmochim. Acta 60, 3925–3931. Zhang T., Amrani A., Ellis G. S., Ma Q. and Tang Y. (2008) Experimental investigation on thermochemical sulfate reduction by H2S initiation. Geochim. Cosmochim. Acta. 72, 3518–3530. Zhang T., Ellis G. S., Walters C. C., Kelemen S. R., Wang K. s. and Tang Y. (2008) Geochemical signatures of thermochemical sulfate reduction in controlled hydrous pyrolysis experiments. Org. Geochem. 39, 308–328. You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format. Power, Environmental and Energy Research (PEER) Center, Caltech BP plc Chevron ExxonMobil SaudiAramco Shell Oil Total ENI CaltechAUTHORS 12406 3 11874 1 application/pdf en validuser other

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