Abstract: Through the research on Permian System on the west slope of Mahu sag in Junggar Basin, it is verified that the dolomite reservoir of Fengcheng Formation is favorable for acquiring large-scale high-quality and high-efficiency reserves. These reservoirs are mainly dolomite siltstone and arenaceous dolomite, formed in stable and low-energy still water environment, where microfractures developed. The development degree of fractures, the layout relationship between dissolved pore and fracture, and the volume of dolomite in reservoirs are key factors influencing whether the quality of reservoirs is good or bad. The area where high-quality reservoir of Permian Fengcheng Formation is generally where the dolomite reservoir of the first member of Fengcheng Formation is horizontally and stably distributed and thick vertically and the gypsum reservoir of the second member of Upper Fengcheng Formation developed. At the same time, the area is located at the fault zone or fracture developing zone.

Key words: Junggar Basin, Permian dolomite, microfracture, controlling factor, favorable area