Multiscale Pore Architecture and Fractal Heterogeneity of Coal, Shale, and Tight Sandstone: An Integrated HMIP–N2/CO2 Adsorption–NMR Characterization Framework

Abstract

Comprehensive characterization of multiscale pore structures in unconventional reservoirs is crucial for evaluating gas storage, migration pathways, and production potential. This study presents an integrated, multimethod investigation of Lower Jurassic low-rank coal, low-maturity shale, and tight sandstone from the Fukang area, Junggar Basin, China. High-pressure mercury intrusion porosimetry (HMIP), low-temperature N2/CO2 adsorption, and low-field nuclear magnetic resonance (LF-NMR) were applied to quantify pore geometry, connectivity, and heterogeneity across nano–micro–macro scales. FHH and Brooks–Corey models were applied to evaluate pore heterogeneity and connectivity. Results reveal marked lithology-dependent contrasts. Shale is dominated by micropores (∼50%) with limited connectivity, functioning primarily as an adsorption reservoir. Coal exhibits a high proportion of medium-to-large pores (macropores ∼42.7%), comprising well-connected through/linked pores and ink-bottle geometries, facilitating both gas storage and migration. Tight sandstone presents multimodal pore-throat distributions, ranging from well-connected, low-displacement-pressure bodies to fine-throat, poorly connected bodies. LF-NMR and HMIP consistently indicate that coal and well-connected sandstones dominate fluid transport, whereas shale micropores contribute minimally to mobility. Fractal analysis quantifies multiscale heterogeneity: fractal parameters obtained from low-relative-pressure regions (D 1) are highest in coal (∼2.76), intermediate in tight sandstone (∼2.66), and lowest in shale (∼2.57), while fractal parameters obtained from high-relative-pressure regions (D 2) peak in tight sandstone (∼2.72), reflecting complex internal pore-throat networks. Brooks–Corey analysis reveals that wide-throat complexity primarily controls permeability. This multitechnique, fractal-based framework establishes a robust, cross-lithology methodology for evaluating pore connectivity, storage–flow partitioning, and production potential. The results offer transferable insights for continental unconventional reservoirs with heterogeneous pore systems.