Integration of High-Pressure Mercury Injection and Nuclear Magnetic Resonance for Pore-Throat Structure Characterization in Tight Sandstone Reservoirs

Tight sandstone reservoirs are typically characterized by complex pore-throat structures and poor porosity-permeability properties. Conventional evaluation methods often fail to accurately characterize their pore structure, significantly hindering the scientific assessment and efficient development of such reservoirs. To address this challenge, this study focuses on the tight sandstone reservoir in the Yanqi 2 well block of the Yan’an Gas Field, Ordos Basin. By integrating high-pressure mercury injection (HPMI), nuclear magnetic resonance (NMR), and other analytical techniques, a comprehensive multi-parameter evaluation system was established, providing a critical theoretical foundation and robust data support for the precise assessment of tight sandstone reservoirs. The key findings include: ① A maximum eigenvalue method was developed to achieve detailed pore-throat structure characterization across different lithologies. ② Based on full-range pore-throat distribution analysis from core samples, cast thin sections, and scanning electron microscopy (SEM), a quantitative reservoir space identification method was established. Results revealed that the average volumetric proportions of intragranular pores and microfractures in quartz sandstone, lithic sandstone, and lithic quartz sandstone were 21.89%, 11.80%, and 9.53%, respectively. ③ Further integrated characterization demonstrated that: Quartz sandstone exhibited the lowest bound water saturation (28.7%) but the highest cutoff radius (0.15 μm), indicating favorable reservoir quality. Lithic quartz sandstone displayed intermediate properties, with bound water saturation (35.2%) and cutoff radius (0.12 μm), suggesting moderate reservoir development. Lithic sandstone had the lowest cutoff radius (0.08 μm) and highest bound water saturation (42.5%), reflecting poorer reservoir conditions.