Intramolecular carbon isotopic fractionation of propane via thermochemical sulfate reduction (TSR) in natural gas reservoirs

Abstract

Thermochemical sulfate reduction (TSR) is an important organic–inorganic reaction that occurs within sedimentary basins and alters the original chemical compositions and isotopic structures of hydrocarbons in natural gases. We used the GC-Py-GC-IRMS method to study TSR and obtained a novel finding related to intramolecular carbon isotope fractionation in natural propane. The results show that the Δ_C-T (δ¹³C_central-δ¹³C_terminal) and δ¹³C_central values significantly increased to 44.7 ‰ and 11.9 ‰, respectively, with increasing TSR alteration. In contrast, the δ¹³C_terminal values of propane remained largely unaltered by the TSR reaction. This difference in position-specific isotope fractionation can be attributed to the central carbon’s reactivity being higher than that of terminal carbon during TSR. In sum, the results indicate that the δ¹³C_terminal values of propane can serve as robust indicators for source rock identification of natural gas altered by post-generation reactions such as TSR and anaerobic microbial oxidation.