The occurrence and distribution of rearranged hopanes in crude oils from the Lishu Depression, Songliao Basin, China

Abstract

Four series of rearranged hopanes were detected in 21 crude oils from the Lishu Depression, Songliao Basin. Relative abundances of rearranged hopanes from the same series have positive correlations, and the abundances of the four series of rearranged hopanes relative to hopanes are also positively correlated with each other. Investigation of rearranged hopanes and other aliphatic and aromatic parameters indicate that the relative abundance of rearranged hopanes within the study area are controlled by redox conditions of the depositional environment, water salinity, and thermal maturity. Generally, a suboxic environment with brackish water contributes to the formation and enrichment of rearranged hopanes in the Lishu Depression. The relative abundance of rearranged hopanes in crude oils increases with increasing thermal maturity, which may be due to either rearranged hopanes being preferentially generated during the heating of kerogen, and/or the diagenetic rearrangement of regular hopanes when exposed to higher temperatures. Source-related biomarkers suggest the principal origin of rearranged hopanes may be related to algae rather than terrigenous land plants. In addition, a suite of extended tricyclic terpanes in the C26–C35 range were identified in the crude oils based on GC–MS-MS analyses, and these extended tricyclic terpanes have a close relationship with the rearranged hopanes. This phenomenon may be associated with both source origin and organofacies, rather than thermal maturity.