Abstract：The South Buir sag is a second-order Lower Cretaceous structural unit in Beierhu depression of Tamsag basin, in which the source rocks, the reservoir rocks and the well sealed regional cap rocks are mainly distributed in Nan-1 formation, Nan-2 formation and Da-1 formation respectively, shaping a normal sequence's source-reservoir-caprock assemblage. The analysis of hydrocarbon accumulation-controlled factors shows that the hydrocarbon distribution in this sag is controlled by the effective hydrocarbon-generating center in plane and by major source rocks in vertical direction, and the fan delta and sublacustrine fans around the fault belt seem to be good reservoirs. Bayantala structural belt and Wudong area are target areas for further exploration.