Characteristics and main controlling factors of the marlstone reservoirs of the first member of Permian Maokou Formatin in Weiyuan area, southern Sichuan Basin

The exploration of the first member of Permian Maokou Formation (Maokou 1) in the Weiyuan area of southern Sichuan Basin has been limited. Previous analyses of cast thin sections and argon ion electron microscopy based on earlier drilling data suggested that talc pores were the primary reservoir spaces, with talc formation significantly contributing to the reservoir. However, the latest drilling data shows substantial differences in the types of reservoir spaces and the genesis of the Maokou 1 marlstone reservoirs compared to earlier understanding. It is necessary to further clarify the main controlling factors for the development of these reservoirs in the Maokou 1 member. Through core observation, thin section identification, physical property analysis, nitrogen adsorption, argon ion polishing scanning electron microscopy, and quantitative pore characterization, the primary reservoir spaces and types of the Maokou 1 marlstone were analyzed from both qualitative and quantitative perspectives. The main controlling factors for the development of these reservoirs were identified by combining total organic carbon (TOC) content determination, rare earth element analysis, and whole rock X-ray diffraction. High-quality reservoirs in the Maokou 1 member of the Weiyuan area are developed in marlstone, characterized by low porosity and permeability fractures and pore-type reservoirs, predominantly Type Ⅲ with some Type Ⅱ reservoirs. The main reservoir spaces include corroded pores, organic matter pores, and talc pores and fractures, with pore development closely related to TOC content. Talc formation, dolomitization, and silicification did not significantly contribute to secondary reservoir spaces, and their contribution to porosity is minimal. The development of marlstone reservoirs is jointly controlled by early dissolution, sedimentary facies, and organic matter abundance. Early dissolution is the key to the formation of corroded pores and fractures, while high primary productivity and high organic matter settling rate of the inner gentle slope shallow water environment provided the material basis for the formation of organic matter pores. This understanding provides theoretical support for the exploration of similar oil and gas reservoirs in the basin.