Geochemistry of petroleum systems in the eastern Pearl River Mouth Basin: evidence for mixed oils

Detailed analyses of crude oils and source rock extracts indicate the presence of two effective petroleum source rocks in the eastern Pearl River Mouth Basin of the South China Sea region. Oils derived from the lacustrine source rocks in the Eocene Wenchang Formation are characterized by abundant C30 4-methyl steranes with relatively few C19 tricyclic terpane and bicadinanes. In contrast, oils from the Eocene-Oligocene Enping Formation source typically contain significant amounts of C19 tricyclic terpane and bicadinanes, with relatively few 4-methyl steranes and high pristane/phytane ratios. However, the chemical compositions of a large number of oils do not conform to those of the unknown source rocks, consistent with mixing of the two sources. Mixed oils occur mainly in the Huizhou Sag and along its south margin, and the highest proportion of the Enping Formation source in the reservoired mixed oils is estimated to be around 80%. Laboratory mixing experiments using selected end-member oils indicate that, even with 50–80% contribution from the Enping Formation source, the mixtures still display sterane biomarker ratio signatures diagnostic of the Wenchang Formation source. Therefore, the presence of abundant 4-methyl steranes is a necessary but not sufficient indicator for the Wenchang Formation source in the eastern Pearl River Mouth Basin. Although our results support the Wenchang Formation as a major source to the known oil discoveries in the study area, the contribution of the Enping Formation source to traps in the Huizhou Sag and surrounding area cannot be ignored. There is clear evidence for the recharge of Enping Formation derived oils into early-biodegraded oil accumulations to form light oil accumulations.