Quantitative Characterization of Micropore Structure and Seepage Characteristics of Diagenetic Facies and Their Implications for Development in the Highly Inhomogeneous Chang 9 Reservoir, Ordos Basin

Despite the great exploration potential of the Ordos Basin’s Chang 9 reservoir, existing studies lack systematic quantitative links between diagenetic facies, 3D pore structure, and seepage behavior, which hinders efficient development. This study quantified the micropore structure and seepage traits of distinct diagenetic facies in the reservoir by integrating multiscale techniques: X-ray diffraction (XRD), core thin-section observation, SEM, physical property testing, high-precision CT, and NMR. First, five diagenetic facies were classified based on diagenetic intensity (compaction, cementation, dissolution). Class I (moderately compacted, chlorite-film, weakly dissolved-fracture facies) was identified as a key high-productivity indicator, exclusive to high-yield wells. Second, Class I and II facies feature developed microfractures and medium-large pores. They possess over 80% 3D pore connectivity, and their waterflood efficiencies (53.76% and 64.28%, respectively) are much higher than those of other facies. However, Class I’s “fracture–matrix” system risks early water channeling, leading to lower efficiency than Class II. Third, we quantified that residual oil is mainly enriched in small pores (0.1 ms < T2 < 10 ms, pore radius <20 μm). In contrast, medium-large pores (10 ms < T2 < 1000 ms) and microfractures (T2 > 1000 ms) achieve optimal displacement efficiency. Finally, this study established a quantitative correlation among diagenetic facies, pore structure, and seepage efficiency. It fills the gap in such quantitative analysis for the Chang 9 reservoir and provides a basis for facies-controlled development.