Geochemistry, grouping and origins of crude oils in the Western Pearl River Mouth Basin, offshore South China Sea

Thirty-seven crude oils and 20 source rocks were selected for detailed molecular geochemical and isotopic analyses in order to establish the genetic relationships between the discovered oils and various petroleum source facies in the Western Pearl River Basin, offshore South China Sea. Four groups of oils were identified. The Group I oils, distributed in the southwestern WC19-1 field of Wenchang B sag, are characterized by high abundance of 4-methyl steranes relative to regular steranes, low abundance of bicadinanes and high δ13C values, showing good correlation with the medium-deep lacustrine source facies in the Wenchang Formation. The Group II oils represent the majority of the discovered oil reserves in the study area, including those from the Qionghai uplift and the northeast block of the WC19-1 oil field. These oils have moderate concentrations of C30 4-methylsteranes, abundant bicadinanes and low δ13C values, and correlate well with the shallow lake source facies of the Wenchang Formation in the Wenchang B and A sags. The group III oils, found in the Wenchang A sag and surrounding areas, are devoid of C30 4-methylsteranes, with abundant bicadinanes and similar δ13C values to those of the Group II oils. These characteristics show close affinity with the coal-bearing sequence in the Enping Formation, thus suggesting a coaly source for these oils. The Group IV oils, found only in the WC8-3 field, display intermediate chemical compositions between the Group II and Group III oils, most likely from mixed sources in the Wenchang A sag. These oil–source genetic relationships suggest dominantly short distance oil migration and thus strong source facies controls on the geographic distribution of oil and gas fields within the Western Pearl River Mouth Basin.