Abnormal pressure and hydrocarbon reservoiring pattern of the Zhanhua sag in Jiyang Depression

In Zhanhua sag of Jiyang Depression in Bohai Bay Basin,structural traps are predominant in the second member of the Shahejie Formation and the strata above it,while subtle traps such as lithologic and structural -lithologic traps dominate the third member of the Shahejie Formation and the strata below it.There is a certain correlation between formation pressure distribution and hydrocarbon distribution in the area,i.e.,structural reservoirs occurring in areas with normal pressure while lithologic reservoirs being developed in areas with abnormal high pressure.Abnormal high pressure provides impetus for the migration of hydrocarbons in deep lithologic reservoirs.When the pressure coefficient curve presents a 3-shape,reservoirs with relatively low-pressure have higher oil potentials.Based on the relationship between formation pressure and hydrocarbon accumulation, we set up two accumulation models:external-sourced hydrocarbons accumulating via carrier and internal-sourced hydrocarbon accumulating via high-pressure injection.Shallow external-sourced hydrocarbon accumulations with normal pressure are mainly controlled by source rock-rooted faults.For example,the hydrocarbon generated from the lower and mid parts of the third member of the Shahejie Formation migrates vertically along the faults and accumulates in the structural traps such as fault block and rolling anticlines in the second member of the Shahejie Formaiton and strata above it,or migrates laterally to and accumulates in structural traps.The controlling factor for the second pooling model is abnormal high pressure.Hydrocarbons generated from the lower and mid parts of the third member of the Shahejie Formation may be charged into adjacent sandbodies and structural-lithologic traps under the drive of the abnormal high pressure.