Geochemical and carbon isotopic studies of crude oil destruction, bitumen precipitation, and sulfate reduction in the deep Smackover Formation

Abstract

Crude oil generated by the Lower Smackover source facies migrated to Upper Smackover reservoirs where slow thermal cracking of crude oil resulted in the formation of gas-condensate and late solid bitumen. Ultimately, only pyrobitumen, methane, and nonhydrocarbon gases including hydrogen sulfide persist in the deepest Smackover reservoirs. The carbon isotopic compositions of crude oils became heavier during crude oil destruction. The carbon isotopic compositions of asphaltenes, NSO-compounds, and saturated hydrocarbons in late solid bitumen and the Lower Smackover source facies became isotopically lighter during crude oil destruction. It is suggested that some isotopically-light components from crude oils were incorporated in late solid bitumen by reactions involving thermochemical sulfate reduction. Thermochemical sulfate reduction and crude oil destruction occurred over a long span of geologic time at temperatures in the 120–150°C range.