Multiscale Heterogeneity and Pore–Throat Controls on Upper Paleozoic Tight Sandstone Reservoir Quality in the Northern Yanchang Gas Field

Abstract

Reservoir evaluation in the northern Yanchang Gas Field is complicated by variable permeability within low-porosity Upper Paleozoic sandstones. This study compares the He 8, Shan 1, Shan 2, and Benxi intervals using core petrophysical summaries, formation- and well-level heterogeneity indices, petrographic images, and high-pressure mercury intrusion (HPMI). Constant-rate mercury injection (CRMI) provides a separate comparison of eight samples from seven wells, comprising two He 8, four Shan 2, and two Benxi samples; Shan 1 is absent. Reported mean porosities range from 4.47% to 5.22%, whereas arithmetic mean permeabilities range from 0.207 to 0.591 × 10−3 μm2. In the HPMI dataset, mean and median entry radii range from 0.11 to 0.98 μm, but its petrophysical distribution differs from that of the broader core dataset. The CRMI samples display dominant throat radii of 0.32–2.05 μm and total mercury saturations of 21.01–67.48%. These observations describe variable capillary access and pore–throat configurations within the sampled rocks. Unequal sampling, unresolved data provenance and method metadata, and the absence of dynamic validation limit formation-wide ranking and productivity prediction. The results provide hypotheses for matched-sample testing rather than calibrated development criteria.