Quantitative characterization of microscopic pore throat structure in tight sandstone oil reservoirs:A case study of Chang7 reservoir in Xin'anbian oil field, Ordos Basin

The Chang7tight sandstone oil reservoir in the Xin'anbian oil field in the Ordos Basin has good potential for hydrocarbon exploration. However, the studies of microscopic pore throat structure were limited, which restricted tight oil exploration and long-term development. Size, shape and spatial distribution of pore throats in tight oil reservoirs were studied in this paper by using scanning electron microscope (SEM), casting thin section, high-pressure mercury injection and Nano-CT technologies. Pores in the Chang7tight oil reservoir in the Xin'anbian oil field were divided into three categories, namely intergranular pores, dissolution pores and microfractures. A large number of nanoscale pore throats developed in this area, which contributed to reservoir capacity and permeability. Pore throat distributions on capillary pressure curves of different samples showed that when threshold pressure is less than 1MPa, there are many micron-sized pores with a good connectivity and pore throats are large tubular and striped with throat diameters being in the range of 100-380nm. When the threshold pressure ranges from 1to 3MPa, there are many nanoscale pores with a good connectivity locally, intragranular dissolution pores are developed and pore throats are tube bundle and spherical shape with throat diameter being in the range of 75-250nm. When the threshold pressure is greater than 3MPa, small isolated spherical nanopores occur, leading to a poor vertical connectivity, only local micro-fracture development provides storage space, and throat diameter is concentrated in the range of 15-75nm.