Multifractal Characteristics of Pore Structure in Deep Coal Reservoirs of the Suide Block, Ordos Basin

Exploration and development of deep coalbed methane represents a critical strategy for ensuring national energy security. This research concentrates on the No. 8 coal seam of the Benxi Formation in the Suide Block. A comprehensive analytical suiteincluding field emission scanning electron microscopy, low-temperature CO₂ adsorption, low-temperature N₂ adsorption, and high-pressure mercury intrusion porosimetrywas integrated with multifractal theory to quantitatively characterize the pore structure and heterogeneity of deep coal reservoirs and to decipher their geological controlling factors. Results indicate that the No. 8 coal is dominated by organic pores, with micropores accounting for an average of 86.26% of the total pore system. Micropores, mesopores, and macropores all exhibit distinct multifractal characteristics. Their generalized dimension spectra [D(q)-q] display an inverted "S" shape, while the multifractal singularity spectra [f(α)-α] follow a convex parabolic profile. Compared to micropores and mesopores, macropores possess the highest heterogeneity and the poorest connectivity. Analysis of controlling factors reveals that ash yield is negatively correlated with pore volume but positively correlated with fixed carbon content and thermal maturity. Furthermore, high ash yield significantly impairs the connectivity of mesopores and macropores, whereas its impact on micropores is primarily limited to the reduction of pore volume. This study provides a quantitative basis for evaluating the pore structure and heterogeneity of deep coal reservoirs.