For lacustrine shale oil and gas reservoirs with coexisting hydrocarbon fluid properties, evaluating the adsorption capacity of shale is of significant importance for the exploration of lacustrine shale oil and gas. Taking the lacustrine shale from the Jurassic Lianggaoshan Formation in the northern Sichuan Basin as an example, this study conducted pyrolysis, scanning electron microscopy (SEM), and high-pressure methane isotherm adsorption tests to investigate the methane adsorption capacity of lacustrine shale and its controlling factors. The research findings are as follows: (1) The organic matter content in the study area’s lacustrine shale is moderate, with organic types ranging from II to III, and it is within the oil generation window stage. The mineral composition exhibits characteristics of high clay and low silica content; (2) Both the TOC (total organic carbon) and clay minerals promote the methane adsorption capacity of lacustrine shale; however, due to the overall moderate–low TOC levels, the storage space is primarily composed of inorganic pores; (3) A high clay mineral content provides more surface area, becoming a primary factor influencing shale adsorption capacity. This indicates that semi-deep lake deposits also possess exploration potential.