Major controlling factors for the high-quality shale of Wufeng–Longmaxi Formation, Sichuan Basin

Research on major controlling factors of the high-quality shale in Upper Ordovician Wufeng–Lower Silurian Longmaxi Formation is a major subject. Based on the information of outcrops and drilling data acquired from Southern Sichuan combined with geochemical elemental analysis and biostratigraphy, this paper analyzed the sedimentary characteristics of Wufeng–Longmaxi organic-rich shale in Sichuan Basin, including plate movements, fluctuation of sea level, paleoproductivity, deposition rates, and paleogeographic environment, and revealed the distribution and sedimentary controlling factors of high-quality shale. Four preliminary conclusions were drawn: (1) The collision and joint of Yangtze Plate and its periphery plates, as well as the intraplate deformation, were gentle in early stage, strong in late stage, gentle in northwest, while strong in southeast, thus forming the northwestward migration of sedimentation center in Sichuan Basin, and the evolution of southern Sichuan sea closeness from weak to strong. (2) Sea level changed following the cycle of deep→shallow→deep→shallow at the turn of Ordovician–Silurian. High sea level and stable ocean basin in early stage created extensive anoxic tectonic sedimentary space which was in favor of organic matter preservation. (3) Influenced by tectonic movements and sea closeness, paleoproductivity of Southern Sichuan marine presented the trend of high in early stage and low in late stage, while the deposition rate also changed from slow in the early stage to fast in the late stage. (4) Extensive deposition and distribution of organic-rich silicic shale was mainly controlled by stable sea basin of low deposition rate, relatively high sea level, semienclosed water, and low deposition rate. The research showed that the high-quality shale in Sichuan Basin is characterized by multiphase superimposition, horizontal extension, and northwest renewal of sedimentary age; Wufeng–Rhuddanian is the main depositional period of high-quality shale, Aeronian comes next; the main exploration layers are Wufeng–Rhuddanian in southern–eastern Sichuan depression, Rhuddanian–Aeronian in Weiyuan, and Wufeng–Telychian in central-northern Sichuan.