Main Mechanism for Generating Overpressure in the Paleogene Source Rock Series of the Chezhen Depression, Bohai Bay Basin, China

The Chezhen depression, located in the south of Bohai Bay Basin, is an oil-producing basin in China. The third and fourth members of the Shahejie Formation (Es-3 and Es-4) are the main source rock series in the Chezhen depression. Widespread overpressures occurred in the Es-3 and Es-4 from the depths of approximately 2 000 to 4 600 m, with the maximum pressure coefficient of 1.98 from drillstem tests (DST). Among the sonic, resistivity and density logs, sonic-log is the only reliable pressure indicator and can be used to predict the pore pressure with Eaton's method. All the overpressured mudstones in the source rock series have higher acoustic traveltimes compared with normally pressured mudstones at a given depth. The overpressured mudstones in the Es-3 and Es-4 units are characterized by a normal geothermal gradient, high average density values up to 2.5 g/cm(3), strong present-day hydrocarbon generation capability, abundant mature organic matter and high contents of residual hydrocarbons estimated by the Rock-Eval S-1 values and chloroform-soluble bitumen "A" values. All suggest that the dominant mechanism for overpressure in the mudstones of source rock series in the Chezhen depression is hydrocarbon generation. A comparison between the matrix porosity of the normally pressured sandstones and overpressured sandstones, the quantitative evaluation of porosity loss caused by compaction and the conventional thin section inspection demonstrate that the sandstones in the Chezhen depression were normally compacted. The high contents of hydrocarbons in the overpressured reservoirs prove that the overpressure in the sandstones of the source rock series was caused by pressure transmission from the source rocks.