The origin and distribution of crude oil in Zhu III sub-basin, Pearl River Mouth Basin, China

• The main source rock was identified in Wenchang A and B depression respectively.
• The origin of the crude oil in Zhu III sub-basin was traced.
• An assumption was put forward to explain the distribution of crude oil.

The origin of thirteen major oil fields in Zhu III sub-basin, Pearl River Mouth Basin, was studied by using the results of Rock-Eval pyrolysis on 340 samples and biomarker analysis on 18 source rock samples and 33 oil samples. The two possible source rock intervals have different biomarker assemblages and were deposited in different environments. The Eocene Wenchang Formation (E2w, 49.5–35 Ma) is characterized by high hopane/sterane ratio (>37.84), high 4-methyl sterane index (0.28–1.06), low gammacerane index (<0.09), low extended tricyclic terpane ratio [ETR = (C28 + C29)/(C28 + C29 + Ts), 0.13–0.20] and bicadinane/C30 hopane (<0.02), which was deposited in fresh-brackish water and dominated by algae. On the contrary, Oligocene Eenping Formation (E3e, 35–30 Ma) has high gammacerane index (>0.10), high bicadinane/C30 hopane (up to 73.62) but low hopane/sterane ratio (<12.50), and it was deposited in relative saline water environment with significant high terrestrial plant debris input. Because of complex geologic settings, oils have different distribution. By using oil-source correlation analysis and hierarchical cluster analysis, two source-related oil groups were identified. Oils found in the Wenchang A depression and Yangjiang uplift, which are classified to oil group 1, were derived from E3e. Oils found in the Wenchang B depression and Shenhu uplift, which are classified to oil group 2, were derived from both E2w and E3e source rocks. But the Qionghai uplift received supply from both the Wenchang A and B depression. This distribution pattern is also proven by using a simple three-dimensional model called PathWay™. An explanation was put forward in this paper about the oils in Zhu III sub-basin and four potential petroleum accumulations were roughly predicted where no well drilled.