ABSTRACT

Crude oil obtained from the Southwest Depression of the Tarim Basin contains high concentrations of rearranged hopanes. However, the geochemical factors controlling this phenomenon, and the correlation between the rearranged hopanes and other biomarkers remain unclear. In this study, nine crude oil samples were collected from this region and analyzed using the gas chromatography-mass spectrometry in terms of hopane compositions, by combining the Pearson correlation analysis (PCA) to understand the cause of the unusually high abundance of rearranged hopanes. Three series of rearranged hopanes were detected in the samples: 17α(H)-diahopanes (C₂₉ and C₃₀ rearranged hopanes), 18α(H)-neohopane (Ts and C₂₉Ts), and C₃₀ early eluting rearranged hopanes (30E). The high content of these rearranged hopanes relative to hopanes correlated well with each other, indicating that they were derived from the same source. The related ratios of the rearranged hopanes and other saturated biomarkers suggest that the unusually high abundance of rearranged hopanes in the crude oil samples are closely related to redox conditions, whereas other biomarkers, which represent water salinity condition, thermal maturity, and biological sources are weakly correlated with the rearranged hopane series relative to hopanes. The PCA confirmed that rearranged hopanes are readily present in crude oils and are developed in an anoxic and fresh lacustrine source rock depositional environment.