Facies-Inherited Macromolecular–Microporous Coupling Controls Methane Adsorption Divergence in High-Rank Deep Coal Seams

Methane adsorption capacity in deep coal reservoirs exhibits significant heterogeneity among different coal facies; however, the governing mechanisms from coal submaceral to macromolecular structure and pore structure remain insufficiently clarified. In this study, the no. 8 coal seam was selected to systematically investigate the adsorption divergence among three coal facies through petrographic analysis, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, low-pressure N2/CO2 adsorption, and methane isothermal adsorption experiments. The results reveal that variations in submaceral composition fundamentally control coal macromolecular structure. Higher contents of matrix and homogeneous vitrinite promote the development of long and branched aliphatic chains and increase structural disorder, whereas increasing the level of semifusinite enhances aromaticity and aromatic ring condensation while reducing structural defects. More importantly, methane adsorption capacity is dominantly governed by micropore volume, specific surface area, and pore connectivity rather than aromatic ring condensation. Although the moist forest swamp facies exhibit higher aromaticity, their limited micropore development results in weaker adsorption capacity. In contrast, enhanced aliphatic branching and structural disorder in the water-covered herbaceous swamp facies promote micropore formation and surface free energy, leading to the highest adsorption capacity. These findings demonstrate that micropore structure inherited from coal facies exerts a primary control on methane adsorption, overriding the effect of aromatic ring condensation derived from inertinite. The identified coal facies-macromolecular structure-micropore-adsorption coupling mechanism provides a thermodynamically robust framework for predicting favorable facies belts in deep coal reservoirs.