Distribution, origin and evolution of the Upper Triassic overpressures in the central portion of the Sichuan Basin, SW China

The Sichuan Basin is a prime petroliferous basin in China, and multiple overpressure systems exist vertically in the central portion of the basin. Understanding the mechanisms and evolution of an overpressure system is significant for petroleum migration and accumulation. DSTs and well logs data reveal that the Upper Triassic overpressures start at a depth of about 1300 m, and the pressure coefficient (r) is greater in the northwestern part of the study area (r> 1.8) and relatively lower in the south (r approximate to 1.3). Sonic transit times of shales in the Upper Triassic Formation are abnormally high, and velocity-density plots deviate from the normal compaction trend, which demonstrate that disequilibrium compaction should be the primary mechanism of overpressures. Evolution history of overpressure was reconstructed based on numerical modeling method. The results from 1D and 2D modeling indicate that the Upper Triassic overpressures began developing at 195 Ma because of rapid sedimentation in this period. The maximum overpressure appeared at 90 Ma, which is 38 MPa in the Bajiaochang field and 50 MPa in the Anyue field, respectively. After that, pore pressure reduced to current value due to uplift and erosion that began in the Late Cretaceous. Overpressures are higher in the greater depth area and relatively lower at the structural crest. Gas generation from the Upper Triassic source rocks occurred during 160-90 Ma which is coincident with disequilibrium compaction. The contribution of gas generation to the Upper Triassic overpressures is not important, because gases just created small volume increase and likely to be expelled out by the high disequilibrium compaction overpressure in the corresponding period. Permeabilities of 10(-22) -10(-23) m(2) are required for barriers to maintain the overpressure in geological timescale. Overpressure systems in the central portion of the Sichuan Basin appear to be explained by static paradigms. For that, temperature has significant impact on the sealed systems. Greater pressures are preserved in the area with less temperature reduction. (C) 2016 Elsevier B.V. All rights reserved.