The impact of temperature on overpressure unloading in the central Sichuan Basin, southwest China

Abstract

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.