Pore Structure and Movable Fluid Cutoff of High-Water-Bearing Tight Sandstone Gas Reservoirs: A Case Study of the Shaximiao Formation, Sichuan Basin

High-water-bearing tight sandstone gas reservoirs often exhibit highly variable gas well productivity, and conventional porosity–permeability cutoffs alone fail to capture movable fluid potential. This study aims to characterize the multiscale pore structure and establish a movable fluid cutoff for the Member 1 reservoir of the Shaximiao Formation in the Zitong area, Sichuan Basin. These reservoirs are shallow-water delta-front tight sandstone gas accumulations characterized by low-to-ultra-low porosity, ultra-low permeability, strong heterogeneity and high water saturation. High water saturation narrows the movable-fluid window and makes conventional porosity–permeability cutoffs unreliable, leading to uneven reserve utilization. To achieve this, core petrophysical measurements, casting thin-section observations, scanning electron microscopy, and saturated-centrifugal nuclear magnetic resonance experiments were integrated to investigate multiscale pore structure and movable fluid characteristics of the target reservoir. The reservoir is dominated by feldspar-litharenite and lithic-feldspathic sandstone, with laumontite as the principal cementing mineral. Reservoir storage spaces are mainly residual intergranular pores and intergranular dissolution pores, and flow channels are predominantly constricted and sheet-like throats. Porosity–permeability correlations are extremely poor, and the rock matrix exhibits weak percolation capacity, while internal micro-fractures make a considerable contribution to fluid flow. Cores are classified into four reservoir types (Type I–IV) according to porosity. Reservoir quality deterioration originates from the substantial reduction in large-mesopore fractions, whereas micropores are ubiquitously developed. Type I and Type II reservoirs feature bimodal connected pore-size distributions; Type III displays a unimodal micropore-dominated pattern; Type IV presents a multimodal micropore-dominated pattern with progressively degraded pore-throat connectivity. Movable fluid occurrence is jointly controlled by pore-size composition and pore-throat connectivity. Mesopores and small-pores serve as primary storage sites for movable fluids. The mobilization ratio of micropores is merely 21–24%, and most fluids within micropores exist in the form of irreducible water. A multi-parameter collaborative movable fluid cutoff criterion was established: the beneficial-production cutoff corresponds to porosity = 8%, permeability = 0.5 mD, movable fluid saturation = 60%, movable fluid porosity = 5%; the effective-production cutoff corresponds to porosity = 6%, permeability = 0.2 mD, movable fluid saturation = 50%, movable fluid porosity = 3%. Differential development strategies are proposed based on reservoir classification. This work provides experimental evidence and theoretical support for reservoir evaluation and development scheme formulation of high-water-bearing tight sandstone gas reservoirs.