Hydrocarbon enrichment patterns and exploration targets in the Paleogene Liushagang Formation, Wushi Sag, Beibuwan Basin, China

The Paleogene Liushagang Formation in the Wushi Sag of the Beibuwan Basin is characterized by dispersed hydrocarbon distribution, small-scale residual exploration targets and large burial depth. Based on data from drilling, laboratory experiments, and geophysic analysis, this study systematically investigates the hydrocarbon accumulation conditions and enrichment patterns in the Liushagang Formation. The key findings are obtained in five aspects. First, the structural evolution of the sag involved three distinct stages: early faulting, mid-stage detachment deformation and late adjustment, governed by an “extension–detachment–strike-slip” composite fault system that controlled basin subsidence, depocenter migration and sedimentary environment evolution. Second, three principal source rock intervals in the Eocene Liushagang Formation, concentrated in the southern East Sub-sag under the control of the No. 7 Fault Zone, are characterized by considerable thickness and high quality, with the oil shale in the lower part of the second member of Liushagang Formation (lower Liu-2 Member) being the most prolific, providing a robust resource foundation in the sag. Third, four reservoir–seal assemblages are identified, corresponding to three hydrocarbon migration systems: direct source–reservoir contact, fault–sandbody coupling, and fault–structural ridge–sandbody stepwise composite networks. Fourth, three accumulation models are established: “young source, old reservoir” with lateral stepwise migration, “self-sourced and self-stored” intra-source enrichment, and “lower source–upper reservoir” with vertical migration. Fifth, exploration priorities are further delineated, highlighting deep fault-block traps in the central zone of the East Sub-sag, intrasag lithologic traps, and bedrock buried-hill targets with direct source–reservoir connectivity, all demonstrating significant resource potential.