Geochemical characteristics of inclusion oils and charge history in the Fushan Sag, Beibuwan Basin, South China Sea

The Fluid Inclusion (FI) petrology and biomarker analysis of the inclusion oils were carried out on three sandstone samples from the Liushagang Formation (Els) reservoirs in the Fushan Sag, the Beibuwan Basin, South China Sea. The purpose of this study is to identify the origin of the inclusion oils and understand the charging history and oil accumulation process in the Els reservoirs through the inclusion oil-source correlation, analysis of Homogenization Temperature (Th) and 1D basin modelling. The results show that the maturities of the Els22 inclusion oils are higher than that of the Els1 inclusion oil based on methylphenanthrene index and the inclusion oils were originated from fluvial/delta mudstone. The geochemical correlation between the inclusion oils and source rocks based on the biomarkers clearly shows that the inclusion oils from the Els22 sandstone reservoirs may have sourced from Els22 source rock or mixed source from the Els3 and Els22 mudstones, and the inclusion oil in the Els1 reservoir was generated from the Els1 source rock in the sag. The oil source of inclusion oils from different layers was almost consistent with that of crude oil in reservoirs. Combined with the Th of the FIs and 1D basin modelling, two episodes of oil charge are identified in the center of the Bailian Sub-sag: the first oil charge in the Els22 reservoirs started around 38.5 Ma in the lower petroleum play, which is consistent with the early oil generation time of the Els3 and Els22 source rocks in the Bailian Sub-sag. The second oil charge in the Els1 reservoir was coincided with the early oil generation time (∼13-0 Ma) of Els1 source rock. The oil charge and accumulation occurred in the isolated upper and lower petroleum plays. The oil charge and accumulation in the lower petroleum play from the Els3 and/or Els22 source rocks is almost distributed in the whole sag, and the oil in the upper petroleum play from the Els21 and/or Els1 source rock is mostly distributed in the center of the sag. •The FIs in different layers are distinguished based on petrology and geochemistry of FI.•Oil source of inclusion oils is proposed and consistent with crude oils.•Two episodes of oil charge are determined and the time is at about 38.5 Ma and 13-0 Ma, respectively.•Spatial difference of hydrocarbon generation and accumulation results in complex charging history and distribution.