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

Junli Zhang1, Sheng He1, Yuqin Wang2, et al.
1. Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China;2. Research Institute of Exploration and Development, Shengli Oilfield Company, SINOPEC, Dongying 257000, China;3. Wuhan Center of China Geological Survey, Wuhan 430205, China;4. Wuhan Zondy Cyber-Tech Co., Ltd, Wuhan 430074, China

ABSTRACT: 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 (Es3 and Es4) are the main source rock series in the Chezhen Depression. Widespread overpressures occurred in the Es3 and Es4 in depths from approximately 2000 to 4600 m, with the maximum pressure coefficient of 1.98 from drillstem tests (DST). Among the sonic, resistivity and density logs, only the sonic-log is a 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 Es3 and Es4 units are characterized by a normal geothermal gradient, high average density values up to 2.5 g/cm3, strong present-day hydrocarbon generation capability, abundant mature organic matter and high contents of residual hydrocarbons estimated by the Rock-Eval S1 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 by 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.

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Keywords:Overpressure-generating mechanism; source rock series; the Es3 and Es4; the Chezhen Depression

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