Abstract: In light of the strong microscopic heterogeneity and varied development outcomes in the low-permeability glutenite
reservoir of the Baikouquan Formation in the Mahu Sag, Junggar Basin, this study carried out an integrated investigation of core description, thin-section observation, CT scanning, and constant-rate mercury injection techniques to quantitatively characterize and classify the microscopic pore structure of the reservoir. The results show that reservoir pores in the study area are dominated by residual intergranular pores and intragranular dissolved pores, exhibiting overall “small pores, fine throats” characteristic. The connectivity of pore-throat networks is generally poor, with coordination numbers concentrated in the range of 1-3. The reservoir can be classified into four pore structure types ( Ⅰ , Ⅱ , Ⅲ , and Ⅳ ), corresponding to sandy small conglomerate, gravel-bearing coarse sandstone, sandy fine conglomerate, and argillaceous fine conglomerate, respectively. Type Ⅰ reservoirs show large throat radii (average 5.94 μm), low pore-throat radius ratios (average 45.85), and optimal configuration, resulting in the strongest seepage capacity. In contrast, Type Ⅳ reservoirs are characterized by the smallest throat radii (average 2.62 μm), the highest pore-throat radius ratios (average 79.33), and poor configuration, leading to extremely weak seepage capacity. The development of the throat system is identified as the key factor controlling reservoir quality and seepage capability, while the influence of pore size is relatively limited. Pore structure, within the glutenite reservoir are the main cause of severe pore-throat configuration imbalance, forming a seepage “bottleneck” that contributes to low permeability, significant productivity variations, and low recovery efficiency in the reservoir.

Key words: micro pore structure, constant rate mercury injection, CT scan, reservoir classification, Mahu depression