Abstract: The reservoir in N area of XX Sag is very diffcult to develop due to its low porosity and permeability. According to the analysis of a series of data including those of physical property and mercury injection from core samples, gas test and well logging etc., the reservoir can be divided into three categories by flow zone index(FZI) values. The study of capillary pressure curves, physical property data, pore structure parameters and percolation parameters in different flow unit shows that there are obvious differences in the capillary pressure curves (pore structure) and permeability values among the different types of reservoirs, but good consistency in each type of reservoir. The comparison and verifcation with well data reveal that the classifcation of reservoir type by FZI is reasonable. It is signifcant in adjusting injection-production well patterns and enhancing oil recovery.