Geochemical composition and origin of Tertiary oils in the Yinggehai and Qiongdongnan Basins, offshore South China Sea

This study comprehensively characterizes the geochemical compositions of Tertiary crude oils in the Yinggehai and Qiongdongnan Basins, offshore South China Sea, and differentiates their source properties using chemical and isotopic data. The oils in the two basins have distinct source characteristics. The majority of the oils from the Yacheng (YC) field in the Qiongdongnan Basin are commonly marked by high Pr/Ph ratios (mostly from 4.5 to 7.0), unusually abundant angiosperm-derived compounds and a domination of C29 in diasteranes, suggesting a predominantly terrigenous organic source. In contrast, the oils from the Dongfang (DF) and Ledong (LD) fields in the Yinggehai Basin have relatively lower Pr/Ph values (2.5-5.5) and a low amount of bicadinanes. Moreover, some oils in this basin contain abundant 4-methylsteranes and detectable C30 desmethyl steranes. The difference in the molecular composition is also apparent with regard to the aromatic hydrocarbons. The former oils contain more abundant biphenyls, fluorenes and 2-phenylnaphthalene than the latter ones. The oils from the Qiongdongnan Basin are relatively depleted in 13C and have δ13C values of the whole oil in the range of −24.9 to −26.9‰, consistent with the great terrigenous input. In contrast, the Yinggehai oils are isotopically heavier with δ13C values from −21.6 to −23.6‰, typical of a marine isotope signature. Correspondingly, their n-alkane isotopes have different δ13C ranges and exhibit distinct distribution profiles. Based on the oil-source rock correlation, the oils in the YC field were generated from the coastal plain-swamp and shallow marine strata of the Oligocene in the Qiongdongnan Basin, while the Yinggehai oils were sourced from the Miocene shallow marine mudstones in the basin.