Accumulation mechanism of lithologic reservoir with ultra-low permeability in Chang8 oil formation, Zhenjing Oil Field

The Chang8 oil formation in the Zhenjing Oil Field is characterized by low porosity and ultra-low permeability.Based on the studies of reservoir deposition,diagenesis effect and storage characteristics,the generation mechanism of lithologic reservoir with ultra-low permeability in the Chang8 oil formation has been studied from several aspects such as lithologic trapping condition,accumulation dyna-mics,threshold for fluid charging,time for trapping and petroleum charging as well as controlling factor for sweet point.Conclusions have been made as follows.1) The Chang8 oil formation is favorable for the generation of medium-large lithologic reservoir.2) Sedimentary microfacies is the basic controlling factor for the generation of lithologic reservoir.3) Three elements (fluid charging dynamics,threshold of reservoir physical property,and effective trap matching with hydrocarbon charging period) control the formation of Chang8 lithologic reservoir.4) Prolific reservoir is mainly controlled by reservoir sedimentary facies,and also influenced by natural fracture.