Oil and gas fields in natural conditions are rarely of homogeneous nature. Given the chaotic nature of changes in rock mass permeability within a single layer, its significant parts (areas) can be considered uniform in permeability. If significant in sizes homogeneous zones (regions) are distinguished within the formation, the parameters of such a macro-inhomogeneous formation significantly affect the characteristics of the filtration flows. The area of zonal inhomogeneity consists of the parts of different permeability. Within the same zone, the permeability is on average the same, but differs from the permeability of adjacent zones.