Shale petrofacies division of Wufeng-Longmaxi formations in Sichuan Basin and its periphery

The research of shale petrofacies is significant for the exploration of shale gas "sweet spots". Different petrographic division schemes have been proposed but no consensus has been reached. This paper has studied the criteria and boundary values of the petrographic classification reference parameters of the Upper Ordovician Wufeng-Lower Silurian Longmaxi formations in the Sichuan Basin and its periphery by considering shale quality and compressibility. (1) There are three petrofacies: marlstone, silicolites and claystones with >50% of calcium content, >75% of siliceous content and >50% of clay content, respectively. (2) Four organic petrofacies have been classified as low-carbonaceous shale, medium-carbonaceous shale, high-carbonaceous shale and enriched carbonaceous shale with 1%, 2% and 4% of organic matter as boundaries, respectively. There are five inorganic petrofacies, such as low-siliceous shale, medium-siliceous shale, high-siliceous shale, low-calcareous shale and medium-calcareous shale with 25% and 50% of silicon or calcium mineral components as boundaries, respectively. This set of petrographic comprehensive division schemes based on organic carbon-mineral components and combined with siliceous genetic analysis has strong applicability and popularization in scientific research and practical applications, which is significant for the study of the lithofacies distribution law in the Wufeng-Longmaxi formations.