South Buir Sag is located in Tamsag Basin of Mongolia, which is a Mesozoic faulted basin. South Buir Sag shows the distinct characterization of faulted structure: many uplifts and many depressions, depressions alternated with many uplifts. This special structure framework results in the characteristic of multi-provenances, multi-depocencers. South Buir Sag experienced the whole process of Cretaceous basin development. South Buir Sag formed fan deltas-lacustrine system in the period of Tsagaantsav Formation, Lower Zuunbayan Formation which occurred in rifting stage. It developed deltas-lacustrine system during the period of Upper Zuunbayan Formation，Sainshand Formation，Bayanshire Formation which is in rifting-downwarping stage. From the period of Bayanshire Formation to present, South Buir Sag underwent downwarped and developed fluvial system. The sandstone of fan deltas front can provide favorable reservoir for hydrocarbon accumulation. Some regions of structure and deposition matching well can form lithology-structure petroleum reservoir.