Multiscale microscopic pore structure characterization and storage–flow coupling mechanisms in ultra-low permeability tight sandstone reservoirs

This study investigates the Chang 4 + 5 ultra-low permeability reservoirs in the Jiyuan area of the Ordos Basin using integrated thin-section analysis, SEM, XRD, nitrogen adsorption, high-pressure mercury intrusion, NMR, and micro-CT techniques. By calibrating NMR with adsorption–intrusion data, a continuous multi-scale pore size distribution was established. The reservoirs are characterized by diverse pore types and pore-throat assemblages, with pore systems dominated by nanopores exhibiting a bimodal distribution. Sedimentary processes exert a primary control on grain size and sandbody architecture, resulting in pronounced reservoir heterogeneity, among which distributary-channel sandstones generally show more favorable reservoir properties. Diagenetic processes further modify the pore system: compaction and cementation reduce storage and flow capacity, whereas dissolution enhances effective pore development. Seepage behavior is strongly controlled by pore structure and can be summarized as “pores dominate storage, pore throats dominate flow.” Pore-throat size and sorting regulate bound-water occurrence and oil-phase flow capacity, while water-phase relative permeability curves exhibit two representative patterns. These results provide microscopic support for sweet-spot evaluation and efficient development of ultra-low permeability reservoirs.