Logging response characteristics and quantitative prediction model of overpressure in deep marine shale of the Lower Cambrian Qiongzhusi Formation, Sichuan Basin

The Qiongzhusi Formation in the Sichuan Basin hosts substantial shale gas resources, which is one of the deepest and the greatest potential for shale gas reservoirs in China. The delineation of high-production sweet spots is positively correlated with formation overpressure intensity, yet the genetic mechanism of overpressure remains unclear. Based on geological and logging data, combined with multiple discrimination methods for overpressure genesis, this study investigates the genetic mechanism of overpressure in shale formations. On this basis, an optimized Bowers method matching the overpressure genesis is established by integrating the equivalent depth method and Bowers’ loading/unloading principles, which is applied to quantitative evaluation of formation pressure in single wells. This method realizes the quantitative classification and prediction of hydrocarbon-generation overpressure and undercompaction overpressure, and also yields the vertical distribution of hydrocarbon generation overpressure increments. Supported by pore pressure data from a large number of single wells, a new 3D pore pressure prediction method is developed by combining the smooth interpolation characteristic of Kriging and the reliability of sequential Gaussian simulation in characterizing the heterogeneous distribution of geological attributes. The results show that the prediction error of the optimized single-well pore pressure method is within 3%, while the integrated single-well/3D approach enabled pre-drill prediction for new wells with errors under 10%. This methodology provides critical support for sweet spot identification, well design, drilling fluid optimization, fracturing design, and production allocation, thereby facilitating safe and economically efficient exploration of deep shale gas resources.