Overpressure origin and prediction in deep strata of Shawan Sag and its surrounding areas in Junggar Basin

Drilling results from the Shawan Sag in the Junggar Basin revealed widespread overpressure in the deep Permian-Triassic strata, yet research on its distribution and generating mechanism remains relatively insufficient. To clarify the origin, evolution, and distribution characteristics of overpressure in this area and to support hydrocarbon accumulation research, the study investigated the overpressure-generating mechanism using drilling data, well logs, measured formation pressures, and seismic velocities, combined with analyses of overpressure response characteristics and acoustic velocity-density crossplots. Multiple pressure-prediction methods were adopted to delineate the planar and vertical distribution characteristics of formation pressure. Basin numerical simulations were used to reconstruct the pressure evolution process. These analyses clarified the generating mechanism, distribution characteristics, and evolution features of overpressure in the deep Permian-Triassic strata of the Shawan Sag and its surrounding areas. A hydrocarbon generation pressurization model was established for the study area. The results showed that the maximum pressure coefficient in the Shawan Sag reached 2.04. The predicted pore pressures of the Permian and Triassic strata ranged from approximately 95 to 146 MPa, corresponding to pressure coefficients between 1.60 and 2.05. The top interface of the overpressure zone lay at a burial depth of 5 to 6 km. The pressure evolution process was divided into three pressurization stages and two pressure relief stages, and a pressure evolution model dominated by hydrocarbon generation pressurization was established. The study reached the following conclusions.Widespread overpressure existed in the Permian and Triassic strata of the Shawan Sag, with the top interface of the overpressure zone located in the Jurassic. The overpressure origins in the deep Permian and Triassic strata of this area were basically consistent, mainly from hydrocarbon generation rather than compaction disequilibrium. Formation pressures were predicted from multiple aspects using the Eaton method and the Fillippone equation, combined with logging data and 3D seismic velocity spectra. The obtained results were basically consistent with the measured formation pressures.