Fractal Characteristics of Pore–Throat Structure and Their Relationships with Fluid Mobility in Tight Sandstone: A Case Study of the Sangonghe Formation, Mobei 2 Area, Central Junggar Basin

To elucidate the relationship between pore–throat heterogeneity and fluid mobility in tight sandstone reservoirs, a multiscale characterization was conducted on sandstone samples from the J1s21 oil-bearing interval of the Sangonghe Formation in the central Junggar Basin using constant-rate mercury intrusion (CRMI), nuclear magnetic resonance (NMR), cast thin-section analysis, and scanning electron microscopy (SEM). The pore–throat structure was first characterized by CRMI, followed by systematic clustering analysis of seven representative pore–throat parameters. Centrifugal NMR experiments were then integrated to quantify fluid mobility and further elucidate the microscopic origins of differences in mobility among reservoir types. The results show that the tight sandstone reservoirs can be classified into Types I, II, and III. From Type I to Type III, the average pore and throat sizes generally decrease, whereas the pore-to-throat radius ratio and displacement pressure increase, indicating progressively enhanced pore–throat heterogeneity. The mean pore fractal dimensions Df1 of Types I, II, and III are 2.266, 2.589, and 2.732, respectively, while the corresponding mean throat fractal dimensions Df2 are 2.422, 2.668, and 2.826. Their average movable fluid saturations are 23.74%, 13.08%, and 6.70%, respectively. Both Df1 and Df2 exhibit negative correlations with movable-fluid saturation, with R2 values of 0.9099 and 0.9042, respectively. Increases in the pore-to-throat radius ratio and displacement pressure are likewise unfavorable for effective fluid migration. Type II reservoirs retain a considerable proportion of medium- to large-sized pores, but their throats are markedly finer, resulting in a characteristic large throat pore–fine throat pore mismatch. The differences in fluid mobility among the three reservoir types are therefore not controlled by a single pore type, but arise from the combined effects of diagenetically modified pore bodies, throats, and pore–throat mismatch. The fractal dimensions of pores and throats can serve as supplementary parameters to characterize the complexity and heterogeneity of pore–throat structures. Unlike existing studies that mostly use bulk pore–throat parameters to characterize reservoir heterogeneity, this paper employs constant-rate mercury injection to extract mercury intrusion responses of pores and throats separately, characterizes the fractal features of pore bodies and throats, respectively, and combines centrifugal nuclear magnetic resonance to quantitatively analyze the relationship between pore–throat scale mismatch and fluid mobility. These findings provide a basis for evaluating tight sandstone reservoir quality and assessing development potential.