Sinian–Lower Cambrian Anyue Gasfield in Western Yangtze Craton

Abstract

AbstractAnyue Gasfield is located in the western Yangtze Craton (YC), i.e., the current Sichuan Basin. It is a marine carbonate gasfield with the oldest stratigraphic age, the highest thermo-evolutional level and the largest individual gas reserve in China. By the end of 2016, its P1 gas geological reserve is up to 8488 × 108 m3 and 3P reserve 1.2 × 1012 m3 respectively in Sinian Dengying and Lower Cambrian Longwangmiao Formations. The emplacement of Sinian to Lower Cambrian gas reservoirs have been constrained by the giant successional Chuanzhong palaeo-uplift, especially at Moxi-Gaoshiti Anticlines, which have been always situated at a high position on the uplift and providing favorable geological conditions for the early entrapment of fossil-oil reservoirs and the late enrichment of cracking-gas reservoirs as well as for the formation of Anyue Gasfield. The Deyang-Ziyang Faulted-Sag is just between the Weiyuan Anticline and the Moxi-Gaoshiti Anticlines, where the fine-grained sediments of Lower Cambrian Qiongzhusi (or called Jiulaodong) Formation was filled, and controlling the occurrence of Qiongzhusi black shale/mudstone as the main hydrocarbon source kitchen, while the development of Dengying algal dolostone and Longwangmiao grain-dolostone at both sides of the faulted-sag as the high-quality reservoir beds. There are two sets of main hydrocarbon source beds, i.e., Lower Cambrian Qiongzhusi black-greyish black shale/mudstone and Doushantou black shale in the western YC. However, the Qiongzhusi shale/mudstone would be the effective source bed for both Dengying and Longwangmiao cracking-gas reservoirs, while the Doushantou black shale would be a potential one in Chuanzhong Uplift. In addition, three sets of reservoir beds are developed in Anyue Gasfield, i.e., two algal dolostone reservoir beds of Deng-2 and Deng-4 Members respectively with reservoir bed thickness 5.1–69.1 m and 60–110 m, and one grain-dolostone reservoir bed of Longwangmiao Formation with reservoir bed thickness 10.8–61.1 m. Three types of gas reservoirs have been found in Anyue Gasfield, including the structural-lithologic type of Longwangmiao gas reservoir, the structural-stratigraphic type of Deng-4 gas reservoir as well as the structural type of Deng-2 gas reservoir, all of which can be attributed to medium–low sulfur-contented and medium CO2-contented dry gas reservoir. Moreover, the Longwangmiao gas reservoir is also a deeply buried reservoir with high temperature and high pressure, while the Deng-2 and Deng-4 gas reservoirs are ultra-deeply buried reservoirs with high temperature and normal pressure.KeywordsMoxi-Gaoshiti AnticlinesDengying FormationQiongzhusi FormationLongwangmiao FormationChuanzhong UpliftDeyang-Ziyang Faulted-Sag