This study addresses key scientific challenges regarding reservoir space in deep coal reservoirs (>2400 m). The multi-scale pore structure in coal rocks, as well as the occurrence state of CBM governed by this structure, such as the proportional fractions of adsorbed gas and free gas, plays a crucial role in comprehending the permeability and adsorption capacity of deep coal rocks. The pore structure and gas content of Jurassic Xishanyao Formation (J2x) coal seams (2460–2585 m) in the central southern Junggar Basin were investigated using multiscale experimental techniques, including FE-SEM, micro-computed tomography (μ-CT), high-pressure mercury intrusion porosimetry (MIP), low-temperature N2/CO2 physisorption, nuclear magnetic resonance (NMR), and isothermal adsorption experiments. Results indicate that the full-scale pore structure is dominated by macropores (>50 nm, avg. 67.7 % of pore volume), followed by micropores (<2 nm, avg. 27.9 %). Critically, micropores contribute 96.43 % of the specific surface area, serving as the primary reservoir for adsorbed gas. Confining pressure (3.45–35 MPa) significantly degrades reservoir quality, causing 23.2 %–63.5 % porosity loss and 97.5 %–99.4 % permeability decline. Deep coals host both adsorbed and free gas. Adsorbed gas content (4.63–10.09 cm3/g) is controlled by micropore volume and specific surface area, while free gas content (6.16–8.04 cm3/g) correlates positively with macropore volume and constitutes a proportion of 44.8 % in average, significantly exceeding levels in shallow CBM reservoirs. Effective porosity (avg. 2.06 %) provided by macropores (including micro-fractures) forms the storage space for free gas. The results provide a crucial theoretical foundation for evaluating deep CBM reservoirs.
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