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AAPG Bulletin

AAPG Bulletin, Preliminary version published online Ahead of Print 7 February 2022.

DOI:10.1306/02072220030

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

Zeqing Guo1 , Guoqi Wei1 , Long Wen2 , Wuren Xie1 , Zengye Xie1 , Chunlong Yang1 , and Xiaobo Wang1

1 Research Institute of Petroleum Exploration and Development, Petrochina, Guanyang District, Langfang city, Hebei province, 065007, China
2 Exploration and Development Research Institute, PetroChina Southwest Oil & Gasfield Company, Tianfu Avenue, Chengdu city, Sichuan Province, 610041, China

The Anyue gas field, with reserves of one trillion cubic meters, was discovered in Central Sichuan Basin, China. It includes three main production layers from the bottom to 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 overpressured. In this study, We used the PVTsim software to calculate the minimum capture 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 are 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 clarifies that the oil and gas generated from kerogen and the gas generated through crude oil cracking are the main factors for development of abnormally high pressure in this area. Also, We established pressure compartment 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 hydrocarbons 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 reservoir in the Dengying Formation is a semi-closed pressure compartment. Natural gas preferentially accumulated in high-quality reservoirs in the platform margin, 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 into the platform. 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 reservoirs. This study is of significance for understanding the formation mechanisms of the pore pressures and the processes of gas accumulation of ancient carbonate rocks.

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