Geochemistry and origins of petroleum in the Neogene reservoirs of the Baiyun Sag, Pearl River Mouth Basin

Abstract A total of 25 light oils/condensates (5 condensates from 2 new exploratory wells) were collected from the Baiyun Sag in the Pearl River Mouth Basin (PRMB) in the South China Sea and investigated to establish their origins and sources. Three oil families were identified among samples following analysis of various geochemical parameters using biomarkers, stable carbon isotope compositions, and Hierarchical Cluster Analysis (HCA). Family-I oils, distributed in the northern structural belts, are characterized by significantly high pristane to phytane (Pr/Ph) ratios (4.77–7.93), moderate oleanane to C 30 hopane (OL/C 30 H) ratios (0.18–0.97) and raised bicadinane-T/C 30 H (T/C 30 H) ratios (1.67–7.67). Family-II oils are distributed along the eastern flank of the Main Sub Sag towards the northeastern tectonic zones, and, have relatively high Pr/Ph ratios (3.14–4.68), elevated OL/C 30 H values (0.73–2.29) and moderate T/C 30 H values (1.64–4.52). The geochemical characteristics of the oils suggest that oils of both Family-I and Family-II are of terrestrial origin, but from two entirely independent sources. Oils of Family-III (a newly discovered oil group) have relatively lower Pr/Ph ratios (0.89–1.99), moderate OL/C 30 H values (0.13–0.82) and lower T/C 30 H values (0.34–1.61), indicating a reduced contribution of higher plant organic matter in related source rocks. Family-III oils are consistent in geochemical characteristics with shallow lacustrine source rocks and their related oils discovered in the Zhu I and III depressions of the PRMB, suggesting that they are likely to be derived from source rocks deposited in a shallow lacustrine environment during the Eocene Wenchang-Enping period. The oils of Family-III mainly occur in the southwest flank of the Baiyun Sag, which implies that this is the preferred direction for migration of lacustrine oils. The discovery of lacustrine oil demonstrates that the Baiyun Sag possesses enormous exploration potential in lacustrine oils and that this may become the next favored petroleum exploration target.