Abstract: Tanan sag is located in the south of Tamtsag basin. Its lithologic reservoirs are one of major realms for petroleum exploration in this area as the most realistic reservoir type for increase of reserves. This paper studies the inherent regularity of lithologic reservoirs and confirms the type of Nantun lithologic reservoir, combined with integrated structural reservoir exploration research. After in?depth analysisof the reservoir?forming conditions, it is shows that the source rocks of Nantun formation in Tanan sag are characterized by high abundance of organic matter, development of fan delta and delta sedimentary system as well as reservoir facies belts. The development of delta front, nearshore subaqueous fan?in?fan and offshore subaqueous fan facies provides effective reservoir spaces for lithologic reservoir; the multiple conducting types include fault, pore, fracture and sand body; the good conservative conditions allowed to form two sets of regional caprocks of Nantun formation and Damoguaihe formation. In this study area three types of lithologic oil and gas reservoirs are distributed: updip pinchout sandstone reservoir, sand lens reservoir and fault lithologic reservoir