Calibrated Multi-Method Fractal Characterization of Full-Scale Pore Structure and Geological Controls in Deep Anthracite: Case Study from Daning–Jixian Block, Ordos Basin

Deep coal reservoirs commonly exhibit strong multiscale heterogeneity, which directly affects coalbed methane (CBM) storage, diffusion, and flow. In this study, deep No. 8 coal samples from the Daning–Jixian block, Ordos Basin, were comprehensively and quantitatively characterized using low-pressure CO2 adsorption, low-temperature N2 adsorption, mercury intrusion porosimetry (MIP), and nuclear magnetic resonance (NMR). A method-constrained calibration framework was developed to assign reliable fractal dimensions to different pore-size intervals and to calculate a volume-weighted comprehensive fractal index. The scale-dependent pore structure was evaluated, and its relationships with coal quality, maceral composition, and maximum vitrinite reflectance (Ro,max) were analyzed. The results show that deep anthracite has a trimodal pore-size distribution, with micropores dominating both specific surface area and pore volume. Fractal behavior is strongly scale-dependent, and calibrated full-pore-size fractal dimensions provide a more reliable measure of reservoir heterogeneity than single-method interpretations. Pore development and heterogeneity are closely associated with coalification degree, coal quality, and maceral composition. Ash tends to inhibit pore development, whereas fixed carbon and vitrinite promote micropore development; inertinite mainly contributes to macropores and fractures. These findings provide a quantitative basis for evaluating pore-structure heterogeneity and optimizing deep CBM reservoir development.