Empirical and Theoretical Evidence for the Role of MgSO4 Contact Ion-Pairs in Thermochemical Sulfate Reduction

While the process of thermochemical sulfate reduction (TSR) has been recognized by geochemists for nearly fifty years, it has proven extremely difficult to simulate in the laboratory under conditions resembling those encountered in nature. Published estimates of the kinetic parameters that describe the rate of the TSR reaction vary widely and are inconsistent with geologic observations. Consequently, the prediction of the hydrogen sulfide (H2S) generation potential of a reservoir prior to drilling remains a major challenge for the oil industry. New experimental and theoretical evidence indicate that magnesium plays a significant role in controlling the rate of TSR in petroleum reservoirs. A novel reaction pathway for TSR is proposed that involves the reduction of sulfate as aqueous MgSO4 contact ion-pairs prior to the H2S-catalyzed TSR mechanism that is generally accepted. Ab initio quantum chemical calculations have been applied to this model in order to locate a potential transition state and to determine the activation energy for the contact ion- pair reaction (56 kcal/mol). Detailed experimental work shows that the rate of TSR increases significantly with increasing concentration of H2S, which may help to explain why previous estimates of TSR activation energies were so divergent. Preliminary experimental evidence indicates that H2S catalysis of TSR is a multi-step process, involving the formation of labile organic sulfur compounds that, in turn, generate sulfur radicals upon thermal decomposition. A new conceptual model for understanding the process of TSR in geologic environments has been developed that involves an H2S-threshold concentration required to sustain rapid sulfate reduction rates. Although this approach appears to be more consistent with field observations than previous mechanisms, validation of this model requires detailed integration with other geologic data in basin models. These findings may explain the common association of H2S-rich petroleum accumulations with dolomitic rocks (CaMgCO3), have important implications for the maximum possible depth at which oil can exist within TSR-prone sedimentary basins, and will aid in estimating global petroleum reserves.