The analyses of the accumulation conditions of lithologic reservoir in Tanan sag showed that the tectonic framework of the convex concave and concave-uplift alternated formed in Tanan sag, which created favorability for the formation of the lithologic reservoir. The high-quality source rocks developed in the sedimentary center of Nantun Formation were material foundation of the development of the lithologic oil reservoirs. According to the lithologic oil reservoir types, spatial distribution characteristics and oil accumulation characters in Tanan sag, four kinds of lithologic oil reservoir forming modes are summarized.