Geochemical characteristics and possible sources of crude oils in the Baiyun deep-water area of the Pearl River Mouth Basin, South China Sea

Two classification methods, the distribution of biomarkers and the alternating least squares (ALS) chemometric analysis were used to investigate the genetic characteristics of oils collected from the Baiyun deep-water area of the Pearl River Mouth Basin in the South China Sea. Results of the two methods were generally consistent that most of the oils were originated from the shallow lacustrine source rocks, but oils from the West Sub-Sag (WSS) and the South Sub-Sag (SSS) have a distinct geochemical characteristic with low abundances of bicadinanes and oleanane, and relatively lighter delta C-13 value (<-28.0 parts per thousand). The ALS chemometric analysis can quantitatively determine the sources of the oils. It shows that mixing has commonly occurred between the oils, and three end-member oils were identified (i.e., EM1, EM2, and EM3), which were derived from the semideep-deep Wenchang Formation source rocks that are characterized by a predominance of C-27 steranes and low concentrations of bicadinanes and oleanane, the shallow lacustrine Enping Formation source rocks in the Main Sub-Sag (MSS) that are rich in bicadinanes but depleted in oleanane, and the shallow lacustrine Wenchang and/or Enping formations source rocks in the East Sub-Sag (ESS) that have a relatively high abundance of bicadinanes, a moderate content of oleanane, and a predominance of C-29 steranes, respectively. The oils from the Panyu Low Uplift (PLU) are dominated by 83% of EM2 oil. Oils from the SSS and WSS are dominated by EM1 oil (68.0%-93.1%). Oils from the ESS and eastern part of the MSS (E-MSS) are mixed oils consisting mainly of EM2 and EM3 oil, with an increasing proportion of EM3 moving from the E-MSS to central ESS. The maturity of the oils was assessed based on diamondoid indices, and oils from the PLU have the highest maturities (1.43%-1.59% Rc), whereas oils from the SSS have the lowest maturities (1.23%-1.25% Rc), corresponding to the different maturities of their source rocks. The semideep-deep lacustrine oils discovered in the WSS and SSS highlight the hydrocarbon generation potential of these types of source rocks in the Wenchang Formation, where further oil exploration should be focused.