Abstract: In the exploration and development of marine shale gas in southern China, traditional lithofacies classification methods have not adequately considered the impact of shale thermal maturity on reservoir space, leading to suboptimal development outcomes in highly thermally mature marine shales. To address this issue, a new method for classifying reservoir facies based on thermal maturity, organic matter content, and mineral composition is proposed, aiming to improve the accuracy of reservoir evaluation and identify optimal reservoir facies types. Through thin section observation, scanning electron microscopy analysis, and various experimental tests, a systematic analysis of the reservoir facies characteristics of the Lower Silurian Longmaxi Formation shale in the southern Sichuan Basin was conducted, followed by a comprehensive reservoir evaluation. The research results show that the primary reservoir facies of the Longmaxi Formation shale in southern Sichuan are overmature organic-rich siliceous shale (OR-S) and overmature organic-rich mixed shale (OR-M). Vertically, the reservoir facies exhibit a transition from overmature organic-rich siliceous shale (OR-S) to overmature organic-rich mixed shale (OR-M) from bottom to top. Laterally, the reservoir facies display significant heterogeneity and poor continuity. From southwest to northeast, the content of siliceous minerals gradually decreases, showing a transition from organic-rich siliceous shale (OR-S) to overmature organic-rich mixed shale (OR-M) and overmature organic-rich argillaceous shale (OR-A). Using grey correlation analysis, a facies index grading standard was established, and key evaluation indicators, including total organic carbon (TOC) content, gas content, porosity, reservoir facies thickness ratio, siliceous mineral content, and clay mineral content, were selected. The study identified overmature, organic-rich siliceous shale (OR-S) as the optimal reservoir facies. This facies is characterized by moderate thermal maturity, high TOC content, high gas content, relatively large porosity, high siliceous mineral content, and a greater reservoir facies thickness, exhibiting superior reservoir performance.

Key words: thermal maturity, shale reservoir facies, reservoir characteristics, Longmaxi Formation, Luzhou Block