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The Sichuan Basin is overpressured and has experienced a reduction in temperature since the Late Cretaceous due to uplift and denudation. The temperature and pressure of the central Sichuan Basin are investigated utilising borehole data. Pressure positively correlates with temperature in each overpressured system, with a coefficient of 0.851 MPa/°C for the Cambrian pressure system and 1.064 MPa/°C for the Upper Triassic pressure system. Physical simulation experiments carried out with absolute sealing conditions demonstrate that the temperature–pressure relationship in a water-filled system is linear with a gradient of 1.076 MPa/°C, similar to the observed temperature–pressure gradients in the study area. The integration of thermal indicators, including vitrinite reflectance and fluid inclusions, is used to reconstruct the palaeotemperature of the basin. Formation temperatures have decreased since 90 Ma, with a significant decrease since 20 Ma. The amount of cooling varies between hydrocarbon fields as it is dependent on the local extent of exhumation during basin uplift. The Weiyuan field exhibits the greatest amount of cooling, followed by the Moxi field, with the Bajiaochang field experiencing the smallest reduction in temperature. The maximum palaeopressure is calculated to be > 200 MPa in the Cambrian strata and ∼150 MPa in the Upper Triassic strata. The existence of gas within the hydrocarbon fields can change the temperature–pressure relationship, lowering the maximum pressure. However, we propose that temperature reduction is the primary cause of the pressure decrease since 90 Ma and that it controls the present-day overpressure distribution in the basin. [ABSTRACT FROM AUTHOR]

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 ≈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. [ABSTRACT FROM AUTHOR]