Multifractal and Grey Relational Analysis of Pore Fluid Distribution in Tight Sandstone Using an Innovative NMR Dual T2 Cutoff Model

Accurate characterization of pore-fluid mobility is essential for evaluating tight sandstone reservoirs. This study investigates ten tight sandstone samples from the Sangonghe Formation in the Junggar Basin using petrophysical measurements, X-ray diffraction, scanning electron microscopy, low-field nuclear magnetic resonance (NMR), and multifractal analysis. Saturated–centrifugation NMR results show that the conventional single-T2-cutoff model cannot fully separate bound and movable fluids. A dual-cutoff framework was therefore used to classify pore fluids into totally bound, partially movable, and totally movable states. The experimentally determined T2C1 and T2C2 values range from 0.127 to 0.582 ms and 155.340 to 265.210 ms, respectively. An adaptive second-order difference method was further applied to the fully saturated T2 spectrum to estimate the dual cutoffs. Within the investigated dataset, the model-derived values show strong agreement with the centrifugation-derived results, with MAPE values of 3.040% for T2C1 and 3.820% for T2C2. Correlation, multifractal, and grey relational analyses indicate that T2C1 is more strongly associated with clay-mineral-related fluid retention and pore heterogeneity, whereas T2C2 is more closely associated with porosity and permeability. These results demonstrate the potential of the proposed approach for NMR-based evaluation of fluid mobility in tight sandstone, although further validation using larger and more diverse datasets is required.