Lithofacies of Upper Ordovician Wufeng Formation in Sichuan Basin and its periphery

Abstract

The Upper Ordovician Wufeng Formation was divided into the shale and Guanyinqiao sections. Based on the comparative analysis of shale gas drilling cores and the whole rock mineral composition of the Wufeng Formation in the Sichuan Basin and its periphery, three end members including siliceous minerals (quartz), clay minerals and carbonate minerals were chosen for lithofacies classification. Four lithofacies types were identified in the shale section including siliceous shale, clayey siliceous shale, gray siliceous shale and gray clayey shale, while in the Guanyinqiao section, shell marl developed. The shale gas drilling showed that the four lithofacies types vary in TOC, brittle mineral and gas contents. The quality of siliceous shale is the best, and the quality of gray clayey shale is the worst. The lithofacies types of the shale section and the distribution characteristics of the Guanyinqiao section were controlled by the sedimentary environment. The shale thickness in Yongchuan area and other relatively shallow water areas in the southern Sichuan Basin is relatively large, but the thickness of siliceous shale is small, and the overall quality of the shale section is poor, so it is not recommended as the target window of horizontal wells. The thickness of siliceous shale in Weiyuan area in the southern Sichuan is small due to the influence of ancient land and topography, and it also is not recommended as the target window of horizontal wells. The thickness of siliceous shale in Jiaoshiba, Wulong, Dingshan and Nanchuan areas in the southeastern Sichuan Basin is more than 4 m, which can be regarded as a target window of horizontal wells.

Keywords

• shale gas
• target window of horizontal well
• siliceous shale
• ganyinqiao section
• wufeng formation
• sichuan basin