Abstract: The natural gas reservoir in the deep sand and gravel strata of the Xujiaweizi fault depression in the Songliao basin is a typical unconventional resource type of “low porosity and low permeability”, with locally developed high porosity and high permeability zones, meeting the requirements for tight gas reservoir formation. Using techniques such as thin section identification, scanning electron microscopy, mercury intrusion detection, and two-dimensional nuclear magnetic resonance, an effective analysis of the sand and gravel reservoirs in the Shahezi Formation of the Xujiaweizi fault in the Songliao basin was conducted to reveal the structure of the sand and gravel reservoirs and their impact on productivity. The research results indicate that the lithology of natural gas reservoirs is mainly composed of sandstone and conglomerate, with low compositional and structural maturity. The reservoir space is mainly composed of secondary dissolution pores, and locally developed microcracks connecting pores, which improve the permeability of the reservoir; The distribution range of pore throat radius in gravel reservoirs is large (nanoscale to micrometer scale), belonging to the micropore throat type reservoir. Influenced by rock sorting, clay minerals, and the development degree of carbonates and other cementitious substances, the connectivity of pore throats in gravel reservoirs is better than that in sandstone, and that in coarse sandstone is better than that in fine sandstone. In the vertical direction, the pore throat structure of the fourth sequence is better than other sequences; The porosity of the reservoir is mainly distributed between 0.3% and 8.0%, and the permeability is mostly less than 0.987 0×10^-3 μm². Based on the empirical statistical method combining physical properties and gas production capacity, the minimum flow pore throat radius is estimated to determine the lower limit of porosity for tight sandstone as 3.0%, with a permeability of 0.019 7×10^-3 μm². The lower limit of porosity for tight conglomerate is 2.8%, and the permeability is 0.019 7×10^-3 μm².

Key words: tight reservoir, sandy conglomerate, pore structure, physical property lower limit, comprehensive evaluation； Shahezi Formation, Xujiaweizi fault depression, Songliao basin