The heterogeneity of pore systems across micro- to macro-scales critically controls gas storage and transport in shale, yet it remains challenging to quantify it accurately, especially in complex marine–continental transitional facies. Here, we introduce a novel pore-volume-weighted comprehensive fractal dimension (Dₜ) model that integrates high-pressure mercury intrusion, low-temperature nitrogen adsorption, and carbon dioxide adsorption data. This approach overcomes the limitations of traditional single-fractal models by enabling a unified quantification of pore complexity from micro- to macro-pores. Applied to the Upper Permian Longtan Formation shale of the South Yellow Sea Basin, our model indicates that mesopores dominate the pore network. Samples with high total organic carbon (TOC) content exhibit distinctive dual-segment fractal features in nitrogen adsorption data, revealing the controlling role of organic matter on pore complexity. Furthermore, the Dₜ shows strong positive correlations with TOC, quartz content, and the Brunauer–Emmett–Teller specific surface area, suggesting that organic richness and brittle minerals synergistically govern pore system complexity. This study demonstrates that the Dₜ model provides a robust mathematical framework for full-scale pore characterization. Our findings offer a fractal-based theoretical foundation for predicting reservoir quality in transitional shale systems. Our understanding of the influence of geological conditions on pore systems has been deepened by applying fractal theory.