Abstract: Due to the complex pore-throat structures and strong heterogeneity of low-permeability reservoirs, a comprehensive classification and evaluation based on pore-throat characteristics and flow capacity is of great significance for guiding subsequent development. Taking the Chang 6 oil-bearing Formation in the southern Ordos basin as the research object, this study systematically evaluates the petrological characteristics, physical properties, microscopic pore-throat features, and flow capacity of the reservoir using an integrated suite of analytical methods, including thin-section petrography, scanning electron microscopy (SEM), X-ray diffraction (XRD), mercury intrusion capillary pressure (MICP), nuclear magnetic resonance (NMR), and oil–water relative probability. Based on K-means clustering method, the reservoir is classified and evaluated. On this basis, a comprehensive evaluation parameter is calculated, and furthermore, the factors controlling the quality differences among different reservoir types are discussed. The results indicate that the Chang 6 oil-bearing Formation in the study area is predominantly composed of fine-grained lithic feldspathic sandstones. The reservoir spaces are mainly residual intergranular pores and feldspar dissolution pores, and the pore–throat assemblages are characterized by medium-to-small pores coupled with fine throats. Accordingly, a comprehensive classification and evaluation system is established, dividing the reservoirs into four types: Types Ⅰ and Ⅱ are high-quality reservoirs, Type Ⅲ are moderate reservoirs, and Type Ⅳ are poor reservoirs. The distribution areas of different reservoir types correspond to distinct development strategies. The differences in reservoir quality are primarily controlled by sedimentary facies type, diagenetic intensity, and the content and type of clay minerals, among which a certain content of grain-coating chlorite could improve reservoir quality.

Key words: low-permeability reservoir, pore-throat structure, seepage capacity, reservoir quality, reservoir evaluation, Chang 6 oil-bearing Formation, Ordos basin