Lithology of Guantao Formation in the Eastern Bohai Sea and Its Bearing on Oil and Gas Accumulation

Detailed lithological study has been made for the Guantao Formation, the main target layer for oil exploration in the eastern Bohai Sea. Based on the paleontological and cores data from drilling wells, combined with sequence stratigraphic analysis and various well log data, the Guantao Formation is divided vertically into three members consisting respectively of coarse, fine and coarse clastic deposits, indicating a cycle of water regression, transgression and regression. A braided river delta, a meandering river delta, and a shallow water delta occur in a vertical succession and form three reservoir-seal assemblages. The sand body of the lower member consists of high content of sand and thus good for lateral hydrocarbon migration. Specific oil and gas accumulation models for the three members are established by combination of migration faults and reservoirs. Advantageous accumulation assemblage is quantitatively predicted with the newly invented Index C which is established on fault activity, regional seal rock thickness, relative sand percentage of the Guantao and Minghuazheng Formations. The upper and middle members of the Guantao Formation will be the main target layers if the Index C is bigger than 0.4, while the upper member of the Guantao Formation and the Minghuazhen Formation will be the main target layers if the Index C is smaller than 0.4.