Theoretical study on the reactivity of sulfate species with hydrocarbons

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 SO₄ ^{2 -} 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.