Controlling factors and geological applications of the drimane-skeleton compound content and composition of crude oils in complex petroleum systems: New insights from the Western Pearl River Mouth Basin, South China Sea

Abstract

Crude oils generally contain drimane-skeleton compounds, but the uncertainty of the main factors controlling drimane distributions limits their effective geological applications, especially in complex petroleum systems. In this study, the drimane-skeleton compound composition and the relative content of each drimane-skeleton compound to C 30 hopane (CDSC) were systematically studied for crude oils in different structural regions of the Western Pearl River Mouth (WPRM) Basin, South China Sea. The results show that the C DSC of crude oil is mainly controlled by its organic sources. Higher C DSC values in crude oil indicates a greater contribution from terrigenous organic matter to their source rocks. For crude oils from source rocks with similar biotic inputs, the C DSC values are controlled mainly by the depositional environment. Higher C DSC values in crude oil correspond to higher concentrations of clay minerals and lower salinities. In addition, the drimane-skeleton compound composition of crude oil is largely controlled by its maturity. The ratios of drimane to homodrimane (Dr/HDr), total rearranged drimane to total drimane and homodrimane (TRDr/TDr), and 8-rearranged drimane to 9-rearranged drimane (8RDr/9RDr) synchronously increase with increasing maturity. These ratios can effectively characterize the maturity of various types of crude oils. The maturity range characterized by the drimane maturity parameters is greater than that characterized by conventional maturity parameters. Correlation diagrams based on drimane parameters and other organic geochemical parameters can be used to classify crude oils and reveal their sources and origins, which has great application potential in complex petroliferous basins, especially in deep and ultra-deep petroleum systems with abundant light oil/condensate