Pore-Throat Combination Characteristics and Their Effect on Gas–Water Seepage Behavior in Tight Sandstone Gas Reservoirs of the Dingbian Area, Ordos Basin, North China

The pore-throat structure and gas–water seepage behavior are important factors affecting reservoir quality and development efficiency. Sixteen tight sandstone samples from the He 8 and Shan 1 Formations in the Dingbian area of the Ordos Basin were analyzed using casting thin section, scanning electron microscopy, high-pressure mercury injection, and displacement experiments, integrated with the nuclear magnetic resonance technique, to clarify the pore-throat structure characteristics and their influence on the mobility of reservoir fluids. The results revealed four characteristic pore-throat structure combinations: large interparticle pore dominated pore-throat (LIPT), small interparticle and dissolution pore dominated pore-throat (SIPT), intercrystalline pore dominated pore-throat (IAPT), and nanopore dominated pore-throat (NPT). Among these, IAPT and SIPT are identified as the main spaces of movable fluids, and the number of IAPT and SIPT determines the seepage capacity of the tight sandstone. The best fluidity was observed in coarse-moderate sandstones in the riverine sedimentary phase with high IAPT and SIPT, while the worst fluidity was observed in siltstones in the NPT-dominated interfluvial bay phase. Linear correlation and gray relational analysis were used to examine the quantitative effects of nine factors on the flow of movable fluids with different pore-throat combinations. Among these factors, the average pore-throat radius exhibited the highest gray relational and weighting coefficient with movable fluid saturation, with values of 0.81 and 0.12, respectively, followed by the median radius and maximum mercury injection saturation. In general, the average pore-throat radius size and connectivity are key factors influencing the seepage behavior. Based on these findings, seepage patterns of movable fluids with four different pore-throat combinations were established in the context of the depositional environment. This can help in better evaluating the heterogeneity of tight sandstones and guide the exploration and development of tight oil and gas in shallow braided river delta front sedimentary environments.