Pore pressure control on multi-scale reservoir properties in deep and ultra-deep sandstones of the Cretaceous Bashijiqike Formation, Kuqa Depression, Tarim Basin, NW China

Understanding the effects of pore pressure on multiscale reservoir properties in deep and ultra-deep sandstones is important for many geological processes. This study integrated core analysis, computed tomography (CT) scanning, petrography, and well logs to quantify the effects of pore pressure on multiscale reservoir properties of the Cretaceous Bashijiqike Formation in the Kuqa Depression, Tarim Basin, NW China. Pore pressure was measured from Modular Formation Dynamics Tester (MDT), and then Eaton’s method, which incorporates sonic transit time and vertical effective stress, was used for pore pressure prediction. Image logs were used for fracture characterization including identification of fracture appearances and calculation of fracture aperture. The crossplot between pore pressure and core-measured porosity reveals that elevated pore pressure mechanically inhibits compaction-driven porosity loss and effectively maintains porosity during deep burial. A pore pressure coefficient higher than 1.75 is an important factor in maintaining matrix porosity as high as 6.0% at depths exceeding 6000 m. Correlation analysis of measured pore pressure coefficient and mechanical fracture aperture demonstrates that elevated pore pressure promotes the opening of natural fractures, and mechanical fracture aperture can reach 4.7 mm when the pore pressure coefficient is higher than 1.75. In addition, pore pressure has positive effects on microscopic pore structure, and increasing the pore pressure coefficient contributes to a higher pore throat radius. Coarse-grained and well-sorted sandstones with pore pressure coefficient higher than 1.75 contribute to a maximum pore throat radius (r_max) higher than 1.0 μm. A structural model synthesizing these findings reveals that overpressure compartments in the fore-thrust and back-thrust of fold-and-thrust belts are favorable for the openness of fractures and preservation of matrix porosity and pore structure. The quantitative relationships between pore pressure and multiscale reservoir properties provide a new and robust analog framework for reservoir quality prediction in other deep compressional basins.