Overpressure Genesis and Its Controlling Factors in the AX Gas Field, Western Xihu Slope, East China Sea

The Pinghu area in the Xihu Sag of the East China Sea Shelf Basin is an important offshore hydrocarbon exploration region in China. However, the genetic mechanisms of overpressure widely developed in the Pinghu Formation remain unclear, and there has long been a scientific debate between undercompaction-dominated and hydrocarbon-generation-dominated pressurization, which constrains the understanding of hydrocarbon migration patterns, trap effectiveness evaluation, and drilling safety in this area. To address this issue, this study systematically identifies overpressure origins using multiple methods, including well log combination analysis, the Bowers method, sonic velocity-density crossplots, porosity comparison, and comprehensive genetic analysis. On this basis, using drilling, logging, and seismic data from the Pinghu area, basin modeling technology is employed to reconstruct the burial, thermal, and hydrocarbon generation histories of key wells, and to quantitatively simulate pore pressure evolution. The results show that overpressure in the AX area is mainly developed in the middle to lower sections of the Pinghu Formation (below 3500 m), with a maximum pressure coefficient of 1.8. The dominant overpressure mechanism is hydrocarbon-generation pressurization, especially fluid volume expansion caused by late-stage kerogen cracking (liquid hydrocarbons and cracked gas), whereas undercompaction contributes little or not at all. The overall pressure distribution in the middle to lower Pinghu Formation shows a pattern of stronger overpressure in the northeast and weaker overpressure in the southwest.