Molecular fingerprints in shales from the Sanhu biogenic gas fields in eastern Qaidam Basin, NW China: Evidence of biodegradation of shale organic matter

Abstract

This study provides evidence for hopane and sterane biodegradation in shale organic matter (OM) via geochemical characterization of shale extracts from the biogenic gas fields of the Sanhu Depression in eastern Qaidam Basin, NW China. Nineteen immature source rock cores of the upper Pliocene–Quaternary from the study area are divided into two groups based on distinct results of Rock-Eval analysis and molecular fingerprints. Organic-rich group A shales are dominated by the ααα 20R configurations of steranes, diagenetic hopanoids and are lean in the αββ 20S configurations of steranes, C27–C29 diasteranes, C21–C22 pregnanes and C19–C29 tricyclic terpanes. In contrast, organic-poor and sandy shales of group B have much higher hopane and sterane isomerization ratios, higher C21–C22 pregnanes, C27–C29 diasteranes and C19–C29 tricyclic terpanes but diminished diagenetic hopanoids. C29 5α-sterane and C31 αβ-hopane isomer concentrations and isomerization ratios reveal a depth-controlled diagenetic profile in group A samples whereas they are randomly or inversely distributed in group B. Moreover, the C27–C30 25-norhopane series has been identified in group B core extracts but are absent in group A. These differences are mainly ascribed to biodegradation of group B core OM. Regardless of microbial alteration of hopanes and steranes in the shale extracts, no significant biodegradation of n-alkanes has been observed in the extracts of both shale groups. However, instead of reflecting preferential degradation of biomarkers, this difference is caused by lower microbial availability of n-alkanes than the biomarkers in the special pore structure of fine-grained sediments and dilution of biodegraded n-alkane patterns by the newly generated n-alkanes from the shale OM. The rare occurrence of hopane and sterane biodegradation in fine sediments in the Sanhu Depression is ascribed to the low hydrocarbon yields from organic lean group B cores that failed to alter the biodegradation signature, since no similar biodegradation case has been reported in organic-rich biogenic gas source rock systems worldwide. Due to a self-sourced biogenic gas system consisting of frequently interbedded shales and silts/sands, the Sanhu biogenic gases form predominantly in the more porous and permeable rocks represented by group B but with most energy sources for microbial metabolism diffused from adjacent, organic-rich shales represented by group A.