Establishment of classification and evaluation criteria for tight reservoirs based on characteristics of microscopic pore throat structure and percolation: a case study of Chang 7 reservoir in Longdong area, Ordos Basin

Taking the Chang 7 reservoir in the Longdong area of the Ordos Basin as an example, the microscopic pore throat characteristics were comprehensively analyzed through physical analysis, casting thin section, scanning electron microscope, CT scanning, high pressure mercury injection and constant rate mercury injection experiments. Fluid seepage was analyzed by NMR and visualized multiphase seepage experiments. Some reasonable evaluation parameters were selected to classify and evaluate tight reservoirs. The average porosity and permeability of the Chang 7 tight reservoir in the Longdong area are 9.33% and 0.18×10^-3 μm², respectively. The main pore types are feldspar dissolved pore and intergranular pore, with an average plane porosity of 1.89%. The pore throat radius is small, mainly distributed in the range of less than 1 μm. Its reservoir capacity is strong, but its connectivity is poor. The movable fluid saturation and oil displacement efficiency are mainly controlled by the fine pore throat, and the heterogeneity also has a certain influence on the oil displacement efficiency. Porosity, permeability, plane porosity, average pore throat radius, mean value coefficient, displacement pressure, movable fluid saturation and oil displacement efficiency are selected as evaluation parameters, and a multi-classification coefficient method is used to classify and establish an evaluation criteria for tight reservoirs, which are divided into Ⅰ, Ⅱ, Ⅲ and IV from good to bad.