Assessing the extent of thermochemical sulfate reduction in deep carbonate formations: Implication for predicting high H2S distribution

Abstract

Thermochemical sulfate reduction (TSR) is a major geochemical alteration processes that can occur in deep petroleum carbonate reservoirs, resulting in the generation of high volumes of hydrogen sulfide (H2S), a toxic and economically-damaging gas that destroys hydrocarbons. Consequently, understanding the extent of TSR is crucial in order to determine H2S distribution, evaluate reservoir fluid properties and hence lower or mitigate the risks during petroleum exploration and production (E&P). Sulfur isotopic composition of hydrogen sulfide and anhydrite as well as the carbon isotopic composition of gases are powerful techniques which can be used to assess the extent of TSR intensity in different intervals. In this study, sulfur and carbon isotopic measurements of two different sour carbonate formations are utilized to: i) assess and compare the relative extent of TSR in the two different carbonate intervals, and ii) better elucidate the effect of TSR on the respective fluid properties. Our results show that the studied carbonate formations have experience different degrees of TSR alteration. Sulfur isotopic values of measured H2S range from 5‰ to 21‰ respectively, suggesting that H2S was generated via TSR processes in both studied reservoirs. H2S concentration for the studied intervals vary significantly, suggesting that the extent of TSR is controlled by multiple factors (e.g., sulfur mineralization, and mixing of gases). In addition, we observed high CO2 concentrations and enrichment of 13C carbon isotope values for measured hydrocarbons, thus indicating destruction of hydrocarbons by TSR. Our results show evidence to suggest that TSR occurs more extensive in a carbonate formation containing liquid hydrocarbons compared to one containing gases. The intensity of TSR reactions can be determined from fractionation values between the sulfur isotopic composition of the produced H2S and anhydrite. We observed that the carbonate formation with small fractionation values between the sulfur isotopic compositions from H2S and anhydrite was influenced by higher TSR levels (i.e., higher TSR rate). We conclude that both sulfur and carbon isotopes can be used as tracers to assess the extent of TSR and high H2S distributions in the petroleum reservoirs.