Low‐ to Middle‐Rank Deep Coal Reservoirs in the Taibei Sag, Tuha Basin: Pore Fractal Dimensions and Methane Adsorption Capacity

ABSTRACT Deep coalbed methane (CBM) reservoirs represent a frontier in unconventional gas exploration, yet the pore structure characteristics controlling their adsorption capacity remain insufficiently quantified. This study investigates the fractal pore structure of low‐ to middle‐ rank coals at depths exceeding 3300 m in the Taibei Sag, Tuha Basin, using core samples from the Kexin 1H well, by means of low‐temperature nitrogen adsorption and methane isothermal adsorption experiments. Two fractal dimensions, D 1 (pore surface roughness) and D 2 (pore structure complexity), were calculated based on the Frenkel‐Halsey‐Hill model. Results show that the coal adsorption–desorption isotherms in the study area can be categorized into three types, with Type I (dominated by open slit‐shaped and ink‐bottle pores) exhibiting the highest specific surface area and methane adsorption capacity. D 1 is significantly positively correlated with Langmuir volume (a key adsorption parameter), while D 2 has no correlation with Langmuir volume and limited predictive value. Fractal analysis also indicates that vitrinite reflectance and content significantly influence pore heterogeneity: D 1 shows no correlation with vitrinite, whereas D 2 decreases with increasing vitrinite content. This result quantifies the intrinsic pore structure related to adsorption in deep coal reservoirs, establishes D 1 as a practical evaluation index for the adsorption potential and quality of deep CBM reservoirs in the Taibei Sag, provides a precise quantitative basis for exploration target selection, development decision optimization and reservoir reconstruction in this structurally complex basin, and facilitates the efficient exploration and development of regional deep CBM.