Precambrian temperature and pressure system of Gaoshiti-Moxi block in the central paleo-uplift of Sichuan Basin, southwest China

Recently, trillions of cubic meters gas resources have been found in Sinian reservoirs of Gaoshiti-Moxi region in the central uplift of Sichuan Basin, which is the first commercial discovery of primary gas field in Precambrian anywhere in the world. To obtain a great breakthrough in Precambrian hydrocarbon exploration, it is necessary to find out the evolution process of temperature and pressure. Reconstructing the thermal history and pore pressure evolution in carbonate reservoirs are challenging problems, especially in deep-old layers without available samples and methods. To study the temperature and pressure system, we conducted a comprehensive analysis combining the paleo-thermal indicators and inclusions Pressure-Volume-Temperature (PVT) simulation with basin modeling. The results showed that the thermal history of the Gaoshiti-Moxi area could be divided into three stages. From the Late Sinian to Late Paleozoic, the thermal regime was stable with low heat flow value (<65 mW/m(2)). During the Early Paleozoic, the heat flow increased rapidly to the peak value (75-100 mW/m(2)). From Mesozoic to present, the thermal subsidence occurred in the basin with the heat flow decreasing to the present value (60-70 mW/m(2)). With the boundary condition of above thermal history, the simulated pressure of Dengying Formation in Gaoshiti-Moxi area could be divided into four stages. The excess pressure began to form during the Triassic. From the end of Triassic to Middle Jurassic, the excess pressure increased steadily and slowly. From Middle Jurassic to the end of Early Cretaceous, the strata subsided rapidly while the excess pressure increased significantly. Since the Late Cretaceous, the Dengying Formation suffered from rapid uplifting and gas lateral migrating with the excess pressure decreasing, until the Late Neogene, the excess pressure in reservoirs declined to zero.