Types of subtle buried-hill oil reservoirs and their accumulation simulation in Jizhong Depression, Bohai Bay Basin

The types of subtle buried-hill resevoirs and their accumulation characteristics are analyzed based on the exploration progression of subtle buried-hill reservoirs in the Jizhong Depression in recent years. These reservoirs are classified into buried-hill head reservoirs that are easy to discover, and buried-hill-slope reservoirs and intra-buried-hill reservoirs that are difficult to discover. Effective seal is the key factor for the formation of intra-buried-hill reservoirs, and hill controlling faults are the important hydrocarbon migration pathways. As for buried-hill-slope reservoirs, two sealing conditions must be satisfied, including argillaceous caprocks on top of the unconformities and intra-buried-hill barriers on the flank, as well as unconformities acting as primary migration pathways. Simulation experiments on petroleum migration and accumulation using a 2D experimental model show that the preferential charging location of hydrocarbons is determined by the charging force and the permeability ratio of transport pathways (faults or unconformities) to buried-hill reservoirs. When faults or unconformities are highly efficient transport pathways, hydrocarbons will preferentially charge the top of the buried hills, thus forming buried-hill head reservoirs. If faults or unconformities are poor in permeability or highly heterogeneous, hydrocarbons will charge the internal layers or slopes of buried hills, forming intra-buried-hill reserovirs or buried-hill-slope reservoirs when there are high-permeability reservoir rocks within the internal layers or slopes of the buried hills.