Pore Structure Characteristics and Fluid Mobility of Tight Reservoir Based on Nuclear Magnetic Resonance: A Case Study of Jurassic in Wudun Sag, Dunhuang Basin

The pore throat of tight reservoir is small, and the fluid mobility evaluation is difficult, which affects the characterization of tight reservoir physical properties and development potential analysis. In this study, low-field nuclear magnetic resonance technology was applied to the Jurassic tight reservoir core in the Wudun Sag of Dunhuang Basin, combining with XRD whole-rock mineral analysis, cast thin section and scanning electron microscope photos to systematically clarify the corresponding relationship between NMR T₂ spectrum and pore throat radius, thus realizing the quantitative characterization of tight reservoir pore diameter and illustrating the distribution characteristics of pore throat. The fluid mobility of different pore structures is also defined. The results show that the NMR T₂ spectra of Wudun sag are mainly bimodal, with the left peak dominant type accounting for 65% and the bimodal equilibrium type accounting for 35%. The radius of intergranular pore is 6×10⁻³～600×10⁻³ μm, the radius of dissolution pore is 0.6～4 μm, and the radius of microfracture is greater than 4μm. The distribution of pore throat radius varies greatly among all Wells: XC1 is dominated by intergranular pore dominance with large change in the proportion of dissolved pore; D1 is dominated by intergranular pore dominance with small change in the distribution curve of pore throat radius; D2 is characterized by intergranular pore dominance and double pore equilibrium with large change in the proportion of intergranular pore and dissolution pore; DX3 is characterized by double pore equilibrium with small overall signal intensity; Through the nuclear magnetic test of saturated water, bound water and residual oil state of the core, it shows that the contribution of the dissolution hole to the fluid mobility is great. The exploration direction of the late Wudun sag is to seek for the well area with the development degree of pore structure and the proportion of dissolution holes "double high". The above results and understanding have important guiding significance for the exploration and development of tight oil reservoirs in this area.