Abstract:

A supergiant oilgas field with geological reserves of more than one trillion cubic meters, located in the middle area of Sichuan Basin, has been found in the CambrianSinian strata. Previous studies revealed the importance of persistent MoxiLongnüsi paleouplift. The existence of DeyangAnyue intracratonic sag (aulacogen sag) and its important in controlling rich hydrocarbon generation depression was recognized. Nevertheless, the development, evolution and formation of the dynamic mechanism need to be further researched and investigated. Based on a large number of field work and analysis, combined with the existing underground data such as drilling and seismic data, focused on midwest area of Sichuan Basin, the structuresedimentary records during the Late Sinian to the Early Cambrian were analysed and discussed. The corresponding Ⅰ, Ⅱ, Ⅲ episodes of Tongwan tectonization correspond to the boundary between the Member 3 and 2 of Dengying Formation, Maidiping Formation and Dengying Formation, Qiongzhusi Formation and Maidiping Formation, and they includ the strara of the Member 3 of Dengying Formation, Maidiping Formation and Qiongzhusi Formation. Among those, typical gravity flow slump deposits and volcanic activity records such as tuff were found in the first episode of Tongwan tectonization. Superposed characteristics of structure interface between the second and third episodes of Tongwan tectonization occurred in most areas of Sichuan Basin and formed a regional unconformity. Typical debris flow deposits and supergene karst response can be found in southern region of Sichuan Basin. The evolution process of three episodic tectonization in general has characteristics of upliftingsubsiding, then subsidence filling. The uplift area and subsidnce area have a certain superimposition, and relative deep shelf area(previous intracratonic sag or rift trough) was formed during the sedimentary period of the Member 3 of Dengying Formation and the Maidiping Formation in the western margin of Sichuan Basin. It is the central deposition region during the sedimentary period of Doushantuo Formation and the Member 1 and 2 of Dengying Formation. The study suggested that the relative deep water area was continuingly inherited from the basement, and it has characteristics of episodic activity enhancement during the sedimentary period from the Member 3 of Dengying Formation to Qiongzhusi Formation. Finally, the dynamic genesis was discussed.

Key words: Tongwan tectonization, Late SinianEarly Cambrian, Sichuan Basin