Lithofacies and pore features of marine-continental transitional shale and gas enrichment conditions of favorable lithofacies: A case study of Permian Longtan Formation in the Lintanchang area, southeast of Sichuan Basin, SW China

Abstract

In this work, the Permian Longtan marine-continental transitional shale in the southeast of Sichuan Basin was taken as study object. Through petrology and geochemical analysis, lithofacies types of the marine-continental transitional shale were classified, key controlling factors of physical properties and gas content of the different shale lithofacies were analyzed. The research results show that the Longtan Formation marine-continental transitional shale in the study area has four types of lithofacies, namely, organic-lean calcareous shale, organic-lean mixed shale, organic-lean argillaceous shale, and organic-rich argillaceous shale, among which the organic-rich argillaceous shale is the most favorable lithofacies of the study area. The pore types of different lithofacies vary significantly and the clay mineral-related pore is the dominant type of the pore system in the study area. The main controlling factor of the physical properties is clay mineral content, and the most important factor affecting gas content is TOC content. Compared with marine shale, the marine-continental transitional shale has low average values, wide distribution range, and strong heterogeneity in TOC content, porosity, and pore structure parameters, but still contains some favorable layers with high physical properties and gas contents. The organic-rich clay shale deposited in tidal flat-lagoon system is most likely to form shale gas sweet spots, so it should be paid more attention in shale gas exploration.