Formation and evolution of intraplatform basin from the late Sinian to early Cambrian in Sichuan Basin, China

Based on the drilling, logging, seismic and outcrop data, the formation and evolution of the late Sinian to early Cambrian intraplatform basin in Sichuan Basin was studied. Through the analysis of the total residual thickness of Member 3 + Member 4 of Dengying Formation, the total thickness of Maidiping Formation + Qiongzhusi Formation, and lithofacies characteristics of Maidiping Formation, the distribution of the intraplatform basin was defined and its controlling effect on natural gas distribution of Dengying Formation was revealed. The intraplatform basin was characterized by small total residual thickness of Member 3 and Member 4 of Dengying Formation, the large total thickness of Maidiping Formation and Qiongzhusi Formation, and the deep-water phosphorous-carbonaceous-siliceous mudstone in Maidiping Formation. Horizontally, the intraplatform basin was mainly distributed in Qingchuan-Ziyang-Yibin area, and experienced four evolution periods. During the period of Member 1 and Member 2 of Dengying Formation, the Sichuan Basin was a rimmed carbonate platform with the well developed mounds and shoals on the platform margin and its interior. The intraplatform basin in western Sichuan Basin was initially formed due to the first episode of Tongwan movement. During the period of Member 3 and Member 4 of Dengying Formation, the depositional center was formed in Moxi, Changning and Zhengxiong area due to extension and rifting of the Upper Yangtze Craton, thereafter, the Sichuan Basin evolved into a rimmed carbonate platform with an intraplatform basin. The second episode of Tongwan movement between Sinian and Cambrian led to the overall uplift and exposure of Sichuan Basin characterized by weathering denudation at high position and downward fluid erosion at low position. During the period of Maidiping Formation, three lithofacies zones were developed horizontally, and the third episode of the Tongwan movement between Maidiping Formation and Qiongzhusi Formation modified the sedimentary geomorphology locally. During the period of Qiongzhusi Formation and Canglangpu Formation, continuous deposition made the Sichuan Basin evolve into a gentle siliciclastic ramp. The evolution of the intraplatform basin not only promoted formation of high-quality platform-margin dolomite reservoirs of Member 4 of Dengying Formation, but also controlled distribution of the high-quality source rocks of Maidiping Formation and Qiongzhusi Formation. The dolomite of Dengying Formation in the surrounding area of the intraplatform basin was the most favorable hydrocarbon exploration area because of its good source-reservoir configuration.