Dispositional characteristics of Ordovician Wufeng Formation and Silurian Longmaxi Formation in Sichuan Basin and its adjacent areas

Abstract

According to data of gas wells and typical sections of Wufeng Formation and Longmaxi Formation in Sichuan Basin, shale of various graptolite zones were analyzed to determine depositional environment, lithology and thickness characteristics of the graptolite shale interval of WF2-WF3 in the lower part of Wufeng Formation, the graptolite shale interval of WF4 in Guanyinqiao Member of Wufeng Formation and the graptolite shale interval of LM1-LM4 in the bottom of Longmaxi Formation, and characteristics of shale horizontal distribution were also investigated. During the depositional period of the graptolite shale interval of WF2-WF3, the study area was less affected by the Guangxi movement, the depositional environment was the deep water of open sea, where black shale was mainly deposited; the sedimentation center was developed in northeast Guizhou-northeast Sichuan and south Sichuan, the maximum thickness was from 4 to 6 m in the sedimentation center. During the depositional period of the graptolite shale interval of WF4, the depositional environment in the study area changed greatly due to global sea level fall and enhanced Guangxi movement; the central Sichuan paleouplift, the central Guizhou paleouplift and the Jiangnan-Xuefeng palaeouplift were further expanded, and the area of the sedimentary basin decreased; the depositional environment was mainly carbonate bioclastic shoal of shallow sea, and partially deep sea which only was distributed in the Shizhu-Fuling-Wuxi area in east and northeast Sichuan and the Gongxian-Yongchuan area in south Sichuan; sediments of shallow water were dominated by limestone and argillaceous limestone with abundant Hirnantia, sediments of deep water were dominated by calcareous mudstone and shale with Hirnantia. During the depositional period of the graptolite shale interval of LM1-LM4, due to rise of global sea level and Guangxi movement, the sedimentary area was larger than that in the depositional period of Guanyinqiao Member, and the sedimentary environment mainly was stagnant deepwater; thickness of black shale in the graptolite shale interval of LM1-LM4 was large, and the maximum thickness was over 20 m. Furthermore, control of the central Sichuan paleouplift, the central Guizhou paleouplift and the Jiangnan-Xuefeng paleouplift on black shale was discussed, and control of the Zhiliujing underwater highland/uplift, Huayingshan highland and Dingshan highland as well as western Hubei-Hunan underwater highland/uplift on shale deposition and preservation was also investigated.