Characteristics, evolution, and formation of pressure in the Sinian-Cambrian gas reservoirs of the Anyue gas field, Sichuan Basin, China

The Anyue gas field, with reserves of 1 trillion m3, was discov-ered in the Central Sichuan Basin, China. It includes three main production layers from the bottom to the top: the Deng-2 and Deng-4 Members of the Sinian Dengying Formation and the lower Cambrian Longwangmiao Formation. The gas reservoirs in the Deng-2 and Deng-4 Members have normal pressure, whereas the Longwangmiao Formation is overpressured. In this study, we used PVTsim software to calculate the minimum cap-ture pressure produced during the accumulation period, and reconstructed the burial, thermal, hydrocarbon generation, and pressure evolution histories using PetroMod software. The results show that there were three stages in the evolution of pore pressure in the Dengying Formation: a hydrostatic-pressure stage, an abnormal-pressure formation stage, and an abnormal-pressure release stage.The formation pressure in the Longwangmiao Formation also occurred in three stages: a hydrostatic-pressure stage, an abnormal-pressure formation stage, and an abnormal-pressure maintenance stage. The hydrocarbon generation history shows that the oil and gas generated from kerogen and the gas generated through crude oil cracking are the main factors for the development of abnor-mally high pressure in this area. Also, we established pressure com-partment models to explain the present pressure-generating mechanism of the gas reservoirs. The Longwangmiao gas reservoir was a closed pressure compartment. The fluid volume that increased due to the cracking of a large amount of liquid hydrocar-bons could not penetrate the top, bottom, and lateral seals. The pressure could not be released, gradually forming abnormally high pressures. These pressures have been preserved, resulting in the present-day abnormally high-pressure gas reservoir. The gas reser-voir in the Dengying Formation is a semiclosed pressure compart-ment. Natural gas preferentially accumulated in high-quality reservoirs in the platform margin strata, where abnormally high pressures gradually formed. When the pressures exceeded the breakthrough pressures of the tight rocks between the reservoirs, natural gas migrated from the platform margin strata into the plat-form strata. Subsequently, the gas-bearing area and gas reservoir volume expanded, and the gas-water interface decreased. These processes formed the present-day normally pressured gas reser-voirs. This study improves our understanding of the formation mechanisms of pore pressures and the processes of gas accumula-tion of ancient carbonate rocks.