Yanbin Yao

Traditional models linking coal rank and pore evolution regard middle-rank coals as an interval of weak pore development, yet their applicability under deep burial remains uncertain. Here, we test this view using full-scale pore-size distribution (PSD) and coal composition evidence from 20 deep middle-rank coal samples in the Shenmu–Jiaxian Block, Ordos Basin, supported by microscopic observation, gas adsorption tests, and mercury intrusion analysis. Instead of the expected pore weakening pattern, total pore volume (PV), total specific surface area (SSA), micropore PV, and micropore SSA increase overall with vitrinite reflectance (RO), with fitted R2 values above 0.9. High-maturity middle-rank coals have the highest total PV and SSA, reaching 0.0623 cm3/g and 174.300 m2/g, respectively, about 3.7 and 4.5 times those of low-maturity middle-rank coals. The full-scale PSD curves show a unimodal pattern dominated by micropores, which provide most of total PV and more than 97% of total SSA. The anomalously high micropore proportion in the low-maturity group is better explained by modification of larger pores during deep burial under high pressure, mineral filling and interfacial pores development than by thermogenic generation of organic micropores. These results indicate that thermal maturity governs the overall trend through the adjustment between pore loss and pore generation, whereas maceral and mineral heterogeneity explains scatter among samples with similar thermal maturity. Deep burial therefore modifies the expression and applicability of the traditional framework, thereby providing an adjusted basis for evaluating methane reservoirs in deep middle-rank coals.