Control Mechanism of Pore Structure of Different Macroscopic Lithotypes on Gas Desorption Differences: A Case Study of High‐Rank Coal From the Western Ordos Basin

The “high gas content but low productivity” paradox in high‐rank coalbed methane (CBM) reservoirs stems from inefficient gas desorption, which is controlled by pore structure heterogeneity linked to macroscopic lithotypes. This study evaluates the production potential of three lithotypes (bright, semibright, and dull coals) and elucidates the causal chain of “macroscopic lithotype—pore‑fracture structure—desorption dynamics.” High‑pressure mercury intrusion porosimetry, polarized light microscopy, and field desorption experiments were integrated to characterize the pore‑fracture systems and desorption behaviors of 11 high‑rank coal samples from the western Ordos Basin. Gray relational analysis (GRA) quantified the contributions of pore parameters and coal rank to desorption rates at different stages. Results show that bright coal contains well‑interconnected fracture networks, providing superior conductivity and stable long‑term desorption pathways. Semibright coal exhibits the most developed micropores and the largest specific surface area, yielding the highest adsorption capacity and serving as the primary gas storage component. Dull coal, dominated by macropores with the largest average pore diameter, facilitates rapid gas release during the initial desorption stage. GRA reveals that the early (rapid) stage is dominated by coal rank (vitrinite reflectance), whereas the mid‑to‑late stages are governed by total pore volume and specific surface area. Conclusions clarify the distinct roles of each lithotype: semibright coal as the “storage core,” bright coal as the “seepage key,” and dull coal as the “initial high‑yield contributor.” This study establishes a theoretical framework for sweet spot identification and customized development strategies for deep CBM resources.