Hydrocarbon accumulation in Sanhecun Sag,Jiyang Depression,Bohai Bay Basin

The migrated oil and gas from the Bonan Sag accumulating in the Sanhecun Sag of Bonan Depression of Bohai Bay Basin was studied using fluid inclusion and biomarker compound analyses, combined with the burial history modelling of typical wells and the evolution history of boundary faults. A new understanding was proposed from two aspects including oil and gas accumulation time and migration path evolution. Three hydrocarbon accumulation processes occurred in the Sanhecun Sag during the early Dongying, Guantao and late Minghuazhen periods according to fluid inclusion homogenization temperatures and biomarkers which reflect maturity.The early oil and gas mainly derived from the upper part of the fourth member of Shahejie Formation, and then from the third and first members of Shahejie Formation in the Bonan Sag, forming a complete hydrocarbon accumulation sequence in the Sanhecun area. During the accumulation period of Dongying Formation, boundary faults failed to stagger delta sand body. As a result, the Bonan oil and gas directly migrated along the contiguous sand and accumulated in Sanhecun area. Faults, sand bodies and unconformities provided a good migration channel for late oil and gas migration.It is confirmed that oil and gas migrated through "the early unified sand body" and "fault-sand body-unconformity composite" to Sanhecun Sag and the southern slope and then accumulated, forming a variety of oil and gas accumulation types.