Transitional marine–continental shale reservoirs are typified by intricate pore architectures and pronounced heterogeneity; accurate characterization of their pore network and fluid mobility underpins reservoir appraisal and sweet-spot forecasting. Focusing on the Longtan Formation transitional shales in the Sichuan Basin, this study integrates NMR T₂ spectrometry, geochemical–mineralogical assays and multifractal analysis to elucidate multi-scale heterogeneity of the pore framework and its governing mechanisms. Results reveal that the investigated shales are characterized by low porosity (0.46–7.43%) and high bound fluid saturation (66.77–97.28%). Multifractal spectral width (Δα) and degree of multifractality (ΔD) serve as robust metrics of pore heterogeneity, correlating closely with rock composition (e.g., TOC and clay content). By combining multifractal indices, mineralogical assemblage and fluid movability, the samples are classified into three reservoir archetypes, with Type I (weakly heterogeneous—high quality) identified as the prospective developmental sweet spot. This work provides a theoretical and methodological backbone for quality assessment and play-ranking of transitional marine–continental shale reservoirs.

Currently, pore fractal characteristics of deep marine–terrestrial transitional coal-measure mudstones in the central Sichuan Basin remain poorly understood. To clarify the pore fractal characteristics and their controlling factors, seven representative deep mudstone samples were collected from the Longtan Formation of Well NT1H in the Suining area, central Sichuan Basin. These samples were subjected to total organic carbon (TOC) content determination, vitrinite reflectance (Ro) measurement, X-ray diffraction (XRD) analysis of whole-rock and clay minerals, and low-pressure nitrogen adsorption (LPN₂A) experiments. Pore fractal dimensions were calculated based on the Frenkel–Halsey–Hill (FHH) theoretical model. The influences of mineral composition, organic geochemical characteristics, and pore structural parameters on pore fractal dimensions were analyzed. The results indicate that shale pores in the study area are predominantly developed as mesopores, exhibiting dual fractal characteristics; fractal dimension D1 (structural fractal dimension at high-pressure segment) ranges from 2.6662 to 2.7366, and fractal dimension D2 (surface fractal dimension at low-pressure segment) ranges from 2.5895 to 2.6363. Mineral composition exerts differential control over pore fractal dimensions. The effects of organic matter content and thermal evolution degree on fractal dimensions exhibit stage-dependent characteristics. Correlations between pore structural parameters and fractal dimensions indicate that small-aperture pores (micropores and mesopores) constitute the primary factor controlling pore heterogeneity in shales. These findings provide a theoretical basis for “reservoir evaluation” and “sweet spot” optimization of deep marine–terrestrial transitional coal-measure shales in central Sichuan Basin.