Quantitative evaluation and effectiveness criteria for the crystalline bedrock-type reservoir in the central uplift zone of the northern Songliao basin, NE China

The crystalline bedrock-type gas reservoirs in the central uplift zone of the northern Songliao Basin represent a significant target for deep natural gas exploration. However, few studies have evaluated and identified effective reservoirs within this specific zone. In this paper, we present a comprehensive study on several exploration drill cores to constrain the pore structure, fracture parameters, minimum porosity and permeability thresholds required for fluid migration in the crystalline bedrock-type reservoirs in the central uplift zone, in an attempt to establish the criteria for identifying effective reservoirs. The results reveal that nanometer- to micron-sized pores, microfissures, and fracture zones are well developed in the crystalline bedrock-type reservoirs, indicating that they can be classified as low-porosity, ultra-low-permeability reservoirs. The identified thresholds for an effective reservoir consist of porosity of 1.5 % and permeability of 0.015 × 10⁻³ μm², and a minimum pore-throat radius of 0.02 μm for fluid mobility. These criteria provide useful guides for identifying effective reservoirs within the crystalline bedrocks.