The pore architecture of the Nahr Umr Formation sandstone reservoirs is highly complex and heterogeneous, severely limiting efficient oilfield development. Conventional methods often fail to adequately characterize such intricate pore systems, necessitating the application of fractal theory. Focusing on sandstone samples from the Nahr Umr-B Member, this study integrates thin section identification, XRD, HPMI, and NMR to characterize the fractal features of the reservoir pore structure and evaluate fluid mobility. The results indicate that from Type I to Type III reservoirs, displacement pressure and median pressure gradually increase, whereas the average and median pore-throat radius gradually decrease, and the pore-throat sorting coefficient decreases. For instance, Type I reservoirs exhibit an average displacement pressure of 0.15 MPa, a median pressure of 0.81 MPa, an average pore-throat radius of 1.96 μm, and a median pore-throat radius of 2.85 μm; in contrast, Type III reservoirs show averages of 14.43 MPa, 45.32 MPa, 0.02 μm, and 0.03 μm, respectively. These trends reflect a gradual deterioration in pore connectivity, increased resistance to fluid flow, and a reduction in the development of larger pore throats. From Type I to Type III reservoirs, both the total fractal dimension (D_H) and the movable fluid pore fractal dimension (D_N2) show a gradual increasing trend. This indicates that the pore structure becomes increasingly complex and heterogeneous, the complexity of the movable fluid pore space increases, and fluid mobility progressively weakens. Furthermore, higher quartz content and lower cement and clay mineral contents correspond to smaller reservoir pore fractal dimensions and stronger fluid mobility. For example, Sample No. 3 (Type I) has a quartz content of 91.97%, a cement content of 1.64%, and a clay mineral content of 6.4%, with a D_H of 2.4385 and D_N2 of 2.9323. Conversely, Sample No. 4 (Type III) has a quartz content of 49.72%, a cement content of 11.21%, and a clay mineral content of 39.07%, with a D_H of 3.9099 and D_N2 of 2.9762. Compared to D_H, D_N2 reduces the prediction error for dynamic quality by over 70% on average, offering a more reliable prediction of fluid mobility and providing a more precise scale for evaluating reservoir development potential.

The Shaximiao Formation in the Southwestern Sichuan Basin is rich in natural gas resources, but the genetic types of channel sandbodies remain unclear. This study investigated the sedimentary characteristics and sandbody genetic types of the Jurassic Shaximiao Formation using well core, logging, and reservoir property data. The results revealed that: (1) The Shaximiao Formation is predominated by shallow-water delta front facies, with delta plain facies present only in the Sha-1 Member. Lithologically, it exhibits a “mudstone-overlying-sandstone” pattern with channel sedimentary sequences portraying normal grading and various microfacies. (2) The sandstones of the Shaximiao Formation developed seven types of lithofacies. Based on the characteristics of lithofacies associations, six genetic types of sandbodies, i.e., deeply incised abrupt-transition, deeply incised gradual-transition, growth gradual-transition, progradational superimposed, progradational mouth bar, and suspension accretion sandbodies are recognized. The deeply incised abrupt-transition and deeply incised gradual-transition sandbodies have the best reservoir physical properties, while the suspension accretion sandbody has the worst. (3) These genetic types of sandbodies are controlled by the relative position of the channels and their relation to mouth bars, with reservoirs in deeply incised abrupt-transition and deeply incised gradual-transition sandbodies developed in the delta plain. (4) High-quality reservoirs are found in deeply incised abrupt-transition and gradual-transition sandbodies, where sedimentary processes strongly influence reservoir physical properties. This study provides new insights into the relationship between tight sandstone reservoirs and channel sandbody genetic types, offering guidance for exploration and development in similar regions.