Relationship Between Pore-Throat and Pressure in Oil Charging in Tight Conglomerate Reservoirs of the Xiazijie Formation in Manan Area, Junggar Basin: Proved from NMR Experiment

The Xiazijie Formation in the Manan Area, Junggar Basin, is an extra-low- to ultra-low-porosity and -permeability deep conglomerate reservoir. The relationship between pore-throat and pressure in oil charging in this tight reservoir is undefined at the initial exploration stage. By combining high-pressure Hg injection with one-dimensional nuclear magnetic resonance (NMR) experiments, a conversion relationship between relaxation time in NMR and pore-throat radius during high-pressure Hg injection was established. Online NMR displacement experiments were conducted at pressures of 2, 5, 15, 25, 35, and 45 MPa. The variation patterns of reservoir oil saturation and the lower limit of the oil-charged pore-throat radius were investigated. The results show that the oil saturation of conglomerate reservoirs ranges from 8% to 32% at a charging pressure of 2 MPa, and from 19% to 52% at a pressure of 45 MPa. The average oil saturation follows a monotonically increasing power function with charging pressure. Oil saturation rapidly increases as charging pressure increases at low pressures, while it gradually increases at high pressures. The lower limit of the oil-charged pore-throat radius ranges from 12 to 82 nm at a charging pressure of 2 MPa, and from 8 to 20 nm at 45 MPa. This lower limit follows a monotonically decreasing power function with charging pressure. The lower limit of the oil-charged pore-throat radius rapidly decreases as charging pressure increases at low values, whereas it gradually decreases with a further increase in charging pressure. Understanding the relationship between oil saturation and the oil-charged pore-throat radius lower limit at different charging pressures could improve the understanding of the reservoir-forming rules and liquid-injection development in the tight conglomerate reservoir of the Xiazijie Formation in the Manan Area.