Sedimentary characteristics of the Sinian Dengying Formation in the Middle-Upper Yangtze Region and their influence on high-quality carbonate reservoirs

The Sinian Dengying Formation in the Middle-Upper Yangtze Region is a high-quality reservoir for oil and gas exploration. Current targeted exploration remains concentrated in Sichuan Basin, while investigations outside the basin are relatively underexplored. Deyang-Anyue trough and Western Hubei trough within the Middle-Upper Yangtze Region were selected as key areas to further clarify the sedimentary processes and reservoir characteristics of Dengying Formation. The sedimentary evolution and spatial distribution of reservoirs were analyzed by integrating the surface outcrop data with the drilling records. Key findings showed that Dengying Formation could be subdivided into four members in most of the Middle-Upper Yangtze region, specifically designated as Hamajing, Shibantan, Baimatuo, and Tianzhushan Members in Yichang area of the western Hubei Province. Three phases of Tongwan tectonic movement during Dengying deposition period significantly reshaped carbonate reservoirs. Phase Ⅰ occurred at the top of the second member, Phase Ⅱ occurred at the formation's upper boundary, and Phase Ⅲ occurred at the top of Maidiping (Yanjiahe) Formation. Dengying Formation of the Middle-Upper Yangtze was characterized by "three-platform-two-trough" paleogeographic framework. The extensional settings created graben-horst platform-basin structures, and organic-rich shales deposited in these troughs during the Early Cambrian, providing premium source rocks. Steep slopes formed along both flanks of Western Hubei and Deyang-Anyue troughs, and favorable reservoirs in Dengying Formation developed in the higher parts of the platform, which were platform margins and intra-platform shoals. Sandwich-style hydrocarbon assemblage formed in high-quality dolomitic reservoirs in Dengying Formation, underlying Doushantuo Formation shales and overlying Qiongzhusi (Shuijingtuo) Formation. The preservation conditions was critical for natural gas accumulation. These results could provide new theoretical foundations and exploratory directions for Dengying Formation oil and gas exploration.