Lithofacies, reservoir characteristicsand controlling factorsof the Middle Permian Gufeng Formation shale, an example from the Huaguoping area, western Hubei

The study of shale lithofacies and reservoir quality is important to geological evaluation and dessert selection of shale gas. In order to clarify the main controlling factors of shale lithofacies pore structure development in the Middle Permian Gufeng Formation in the Huaguoping area of western Hubei, an integrated analysis of thin section, total organic carbon (TOC) content, X-ray diffraction (XRD) analysis, major and trace elements, scanning electron microscopy, low-pressure N₂ adsorption were used. The results indicate that the Gufeng Formation mainly develops three lithofacies types, namely as carbon-rich and high silicon, medium carbon and medium silicon, and medium carbon and high silicon. The types of reservoir space are mainly organic matter pores, intergranular pores, intragranular dissolved pores and micro-fractures. The pore size distribution is mainly micropores and mesopores. Carbon-rich and high-silicon pores are the most developed, followed by medium-carbon and medium-silicon, and medium-carbon and high-silicon are the worst. The formation of Gufeng Formation shale is mainly controlled by paleoproductivity, paleoredox conditions and upwelling current. The high paleoproductivity, anoxic-anoxic environment and the prevalence of upwelling current in the early-middle sedimentary period are the main reasons for the formation of carbon-rich and high-silicon shale. The pore development of Gufeng Formation shale is mainly controlled by TOC content and quartz and carbonate mineral content. The clay content has little effect on pore development. The increase of TOC and quartz content is beneficial to the development of micropores and mesopores in shale, while the increase of carbonate mineral content will inhibit the development of micropores and mesopores and promote the development of macropores. The carbon-rich and high-silicon type is mainly distributed in the middle and lower part of the Gufeng Formation, with high gas content, high pore volume and specific surface area, and high brittle mineral content, which is a favorable layer for shale gas exploration.