Origin and occurrence of crude oils in the Zhu1 sub-basin, Pearl River Mouth Basin, China

• The influence of thermal maturity on C19/C23 TT and C24 Tet/αβC30 hopane is minor.
• E2w and E3e are proved to be the prime source rocks.
• Three oil classes have been identified based on four effective biomarkers.
• Oils occurrence is controlled by source rocks distribution and migration pathways.

The origin of the seventeen major oil fields in the Zhu1 sub-basin, Pearl River Mouth Basin (PRMB) was studied based on the results of Rock–Eval pyrolysis on more than 370 samples and biomarker analysis on 31 source rock samples and 63 oil samples. The two possible source rock intervals have different biomarker assemblages and were deposited in different environments. The Wenchang Formation (E2w, 56.5–32 Ma) is characterized mainly by low C19/C23 tricyclic terpane (<2.0), low C24 tetracyclic terpane/αβC30 hopane (<0.06), low bicadinane-T/αβC30 hopane (<2.0) and high 4-methyl steranes/∑C29 steranes (most >0.4) ratios, and were deposited in anoxic to suboxic environments with important contribution from Pediastrum and Dinoflagellates. The Enping Formation (E3e, 32–30 Ma) has high C19/C23 tricyclic terpane, high C24 tetracyclic terpane/αβC30 hopane, widely variable yet overally high bicadinane-T/αβC30 hopane and low 4-methyl steranes/∑C29 steranes ratios, and were deposited in freshwater lacustrine to swamp conditions with significant terrigenous organic matter input. According to oil-source correlation, three oil classes can be identified in the Zhu1 sub-basin. Class 1 oil is E2w-derived and occurs widely. Class 2 oil is E3e-derived and refers to oils from F field in the north of the Huizhou depression. Class 3 oil is a mixture of oils generated from E2w and E3e, only distributed in the Huizhou depression and on its southern margin. The petroleum distribution pattern is mainly controlled by the distribution of source rocks and the migration pathways of oils. This research has important implications for future exploration.