Shale gas production from the Longmaxi Formation in the Sichuan Basin, southern China, faces rapid decline and challenges in maintaining stable output. The upper part of the Long-1 Member is considered a potential replacement interval, yet detailed investigations into its reservoir heterogeneity characteristics and gas content controlling mechanisms remain lacking. This study conducted geochemical analysis, connate water content measurement, porosity testing, FE-SEM imaging, and low-pressure N₂/CO₂ adsorption experiments on core samples from the upper and lower Long-1 Member in the Jiaoshiba gas field. Multifractal characteristics were analyzed, and their influence on gas content was compared and discussed. The results show that the upper interval is jointly influenced by organic matter pores, clay mineral interlayer pores, and carbonate cementation, with water likely occurring as clay-bound water, exhibiting higher structural heterogeneity at the non-micropore scale. In contrast, the lower interval is affected by carbonate mineral dissolution, with dissolution pores and microfractures developed, and water likely occurring as water clusters within organic matter pores, showing stronger heterogeneity at the micropore scale. Furthermore, in the upper interval, higher pore uniformity and continuity correspond to higher gas content, whereas in the lower interval, higher micropore heterogeneity and more pronounced dominant pore size characteristics correspond to higher gas content. These results reveal significant differences in the heterogeneity characteristics of the upper and lower intervals of the Long-1 Member at different pore size scales and their distinct responses to shale gas enrichment, providing a reference for detailed comparison in three-dimensional shale interval development and for the evaluation of lower-quality shale intervals.