Chemical and petrographic evidence for thermal cracking and thermochemical sulfate reduction of paleo-oil accumulations in the NE Sichuan Basin, China

Significant extents of heterogeneity are observed in the optical properties, reflectance values and S/C atomic ratio values of solid bitumens in the carbonate reservoirs of the Lower Triassic Feixianguan and Changxing formations in the Puguang gas field, the largest in marine carbonates in China. The chemical and stable carbon isotope compositions of the gaseous alkanes in the produced gases also depend on field location, with no clear correlation with the hydrogen sulfate content. These data indicate that the gas reservoirs are highly compartmentalized. This manuscript describes an integrated chemical and petrographic approach for distinguishing solid bitumens formed by thermal cracking and by thermochemical sulfate reduction (TSR). The fact that most reservoir bitumens have only been partially affected by TSR suggests that sulfate supply appears to be a major limiting factor for the TSR reactions involving gaseous alkanes.