Movable Fluid Distribution Characteristics and Microscopic Mechanism of Tight Reservoir in Yanchang Formation, Ordos Basin

Tight sandstone reservoirs have smaller pore throats, complex structures, strong heterogeneity in the pore throat system, and significant differences in fluid distribution. To reveal the occurrence characteristics of movable fluids in tight sandstone reservoirs, the typical tight sandstone core samples from the Chang 4+5 reservoir of the Upper Triassic Yanchang Formation of the Ordos Basin were selected and the micro pore throat classification standard was established by using casting thin slices, field emission scanning electron microscopy (FE-SEM), high-pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR) technology. The test analysis results showed that the Chang 4+5 tight sandstone reservoir can be divided into Type I, II, and III reservoirs by the different pore throat structure. Type I reservoirs have well-developed pore throats, good connectivity, low threshold pressure (0.42 MPa on average), high movable fluid saturation (46.82% on average), and a minimum pore throat radius of movable fluid (0.056 mu m on average). The pore throat structure and fluid production degree of the other two reservoir types gradually deteriorated. Note that the tight sandstone reservoirs of the Chang 4+5 reservoirs have smaller pore throats but high movable fluid content in smaller pore throats (17% on average). With the deterioration of the pore throat structure in reservoir, the degree of fluid utilization in larger pore throats decreases (74.33%-57.33%), whereas the degree of fluid utilization in smaller pore throats does not change significantly (25.67%-13.82%). Many factors affect movable fluid parameters. The movable fluid parameters of tight sandstone reservoirs in the Chang 4+5 reservoir have a good positive correlation with permeability (R-2 = 0.85) and sorting coefficients (R-2 = 0.88). The movable fluid parameters of smaller and larger pore throats have no obvious correlation with a single factor. Multiple factors affect the fluid occurrence characteristics of different scale pore throats.