Characteristics of Ultra-Low Permeability Conglomerate Reservoirs and Study on the Lower Limit of Gas Injection Mobilization

To address the challenges in understanding the reservoir potential of ultra-low permeability conglomerate reservoirs in the Junggar Basin, this study systematically investigated the lower limit of gas injection mobilization by integrating core analysis, nuclear magnetic resonance (NMR), and physical simulation experiments. The results indicate that the reservoir lithology in the study area is predominantly muddy fine conglomerates, exhibiting low porosity and ultra-low permeability characteristics, with an average porosity of 10.8% and an average permeability of 0.33 mD. The pore-throat structure is complex, with secondary dissolution pores accounting for 26.5%. Capillary pressure curves display a fine-skewed pattern, with an average displacement pressure of 0.93 MPa and a median pore-throat radius of 0.13 μm, reflecting strong reservoir heterogeneity. By combining the oil-bearing statistical method and the oil layer atlas method, the physical property lower limits of the reservoir were accurately determined as porosity ≥ 7.0% and permeability ≥ 0.05 mD. Gas injection experiments revealed that under reservoir conditions (injection pressure of 32 MPa), the effective permeability lower limit for gas mobilization is 0.03 mD, corresponding to a pore-throat radius of 0.06–0.075 μm. This is significantly lower than the traditional water flooding lower limit (0.4 mD), overcoming a key technical bottleneck in the development of ultra-low permeability reservoirs. Based on these new insights into reservoir limits, the development potential of Class IV reservoirs (permeability 0.03–0.05 mD) was re-evaluated. The findings of this study provide important theoretical and technical support for the efficient development of similar reservoirs.