Discrimination of Tight Sandstone Reservoir Effectiveness Based on Pore-Throat Functional Fractal Characterization and Three-Dimensional Pore-Network Connectivity Constraints

Tight sandstone reservoir effectiveness is governed not by pore volume alone, but by the storage and flow contributions of different pore-throat scales, their structural complexity, and their three-dimensional connectivity. This study investigates tight sandstones of the Benxi Formation deposited in a marine–continental transitional mixed siliciclastic–carbonate setting in the Gaoqiao area, southern Ordos Basin. Petrophysical measurements, red-epoxy-impregnated thin-section petrography, mercury intrusion capillary pressure (MICP), segment-specific fractal analysis of functionally defined pore-throat regimes, X-ray micro-computed tomography (micro-CT), and pore-network modeling (PNM) were integrated. The MICP responses define three pore-throat structure types and two data-derived functional boundaries at 0.708 and 0.141 μm, which separate large-pore-throat-dominated, transitional pore-throat, and fine-throat-limited intervals. Using these nominal boundaries, Type I is strongly dominated by the large-pore-throat interval, which accounts for 88.7% of total mercury intrusion, whereas Type II exhibits a mixed large-to-transitional response, and Type III is characterized by negligible large-pore-throat intrusion and pronounced fine-throat restriction. Perturbing both functional boundaries by ±5% and ±10% does not alter these principal functional distinctions, although samples close to the second boundary exhibit the expected local transitional sensitivity. Among the three segment-specific fractal parameters, the fine-throat fractal dimension, DB, shows the strongest association with median capillary pressure (r = 0.834, p < 0.001) and remains significantly related to displacement pressure, median pore-throat radius, and permeability, whereas DT shows no significant linear correlation with the tested petrophysical and MICP parameters. The fractions of the largest connected pore cluster in representative Type I–III samples are 90.26%, 72.56%, and 64.65%, while their PNM permeabilities decrease successively from 64.32 mD to 0.850 and 0.121 mD. Together, these results indicate that, for the investigated samples, reservoir effectiveness reflects the combined influence of pore-throat size configuration, segment-specific structural complexity, and three-dimensional network connectivity.