Geological features of Sinian-Early Cambrian intracratonic rift of the Sichuan Basin

Abstract

With the major breakthrough in Sinian-Cambrian gas exploration in the Sichuan Basin, the geologic understanding on this formation has changed. One change concerns the discovery of a nearly SN negative structure in the west of Gaoshiti-Moxi area, but there is controversy over the nature, boundary features and distribution scope, sedimentary features, formation stage and evolution of the structure [11–14]. Since the negative structure belt is closely related to the formation of the Anyue Sinian-Cambrian giant gas field, it is necessary to get a better understanding of its geological features in order to guide the gas exploration in Sinian-Cambrian of the Sichuan Basin in the future. Through an analysis of the drilling data of Sinian-Cambrian in this area, an observation of outcrop profiles and the use of 27000 km 2D seismic data and latest drilling data (Well Gaoshi 17), and in combination with regional structure background, the boundary and distribution scope, sedimentary features and evolution of the Mianzhu-Changning rift in craton were investigated. The results showed that: ① the rift in overall SN direction had a steep and fairly stable eastern boundary, and gentler western boundary with variable positions in different stages; ② at the end of Sinian Deng2 member, the rift was a nearly symmetrical depression with an area of 3 × 104 km2; ③ at the end of Deng4 member, it was a fault depression which was steep in the east and gentle in the west controlled by its eastern boundary fault with an area of 8 × 104 km2; and ④ the evolution of the rift can be divided into four stages: embryonic stage in Deng1-Deng2, formation stage in Deng3-Deng4, filling and subsidence stage in Early Cambrian Maidingping-Qiongzhusi Formation, shrinkage and disappearing stage in Early Cambrian Canglangpu-Longwangmiao Formation. Through the study, it was concluded that both sides of the rift are in good Sinian-Cambrian gas-reservoir-forming conditions, which are major prospects for future exploration.