Fractal Characteristics and Controlling Factors of Pore Structure in Different Lithofacies of Longmaxi Formation Shale

Using 48 shale samples from the lower member of the Longmaxi Formation in Well YL, in the Middle Yangtze region, we investigate the fractal characteristics of pore structures across different shale lithofacies based on total organic carbon (TOC), X-ray diffraction (XRD), and low-pressure N2 adsorption analyses. The shale succession is dominated by three lithofacies—clayey shale, mixed shale, and felsic shale—with mesopores and micropores forming the principal pore systems. N2 adsorption–desorption isotherms exhibit pronounced hysteresis loops, and ln(V) versus ln(ln(P0/P)) plots show distinct two-segment behaviour, indicating dual fractal dimensions within the pore network. The fractal dimension of small pores (Df1 = 2.75–2.87) is consistently higher than that of large pores (Df2 = 2.01–2.39), suggesting stronger structural heterogeneity in micropore–mesopore systems. Felsic shale exhibits the highest fractal dimensions, followed by mixed shale, whereas clayey shale shows the lowest values. Fractal dimensions correlate positively with TOC, clay minerals, and pyrite content, but negatively with quartz, feldspar, and carbonate minerals. Lithofacies therefore exert a first-order control on pore fractal characteristics through their influence on mineralogical composition and organic matter abundance. These results demonstrate that fractal dimensions provide a robust quantitative metric for evaluating reservoir heterogeneity in Longmaxi Formation shales. Unlike previous studies that examined pore complexity at the bulk-rock scale, this study adopts a lithofacies-resolved dual-fractal framework to quantify multiscale pore heterogeneity and explicitly elucidate the roles of mineralogy and organic matter in controlling pore complexity.