The dolomite to the west of Namtso in Baoji area of Bange County was previously correlated to be Lower Cretaceous Langshan Formation, but the typical Early Triassic conodont species such as Ellisonia agordina Perri and Andraghetti 1987, Hadrodontina anceps Staesche 1964, and Pachycladina cf. obliqua Staesche 1964 are newly discovered in it, therefore, we revise the dolomite from Langshan Formation to Mujiuco Formation. The age range of the Mujiuco Formation is revised from Late Permian only to Late Permian to Early Triassic based on the above discovery. Oilimmersion phenomena is first discovered in the Mujiuco Formation, and the preliminary test of the oilimmersed dolomite shows that the percentage of the saturated hydrocarbon 34%, aromatic hydrocarbon 1.1%, nonhydrocarbon 51%, and asphalt 13% respectively. The discovery of the oilimmersed dolomite indicates that the Mujiuco Formation in Baoji area is oilbearing strata, furthermore, indicates that the Guge Sequence is not merely potential strata for petroleum exploration, but a realistic oilbearing sequence for petroleum exploration. Based on the present study, Guge Sequence is divided into Lower Assemblage and Upper Assemblage, and the Late Permian to Early Triassic dolomite in the Mujiuco Formation is the reservoir bed of the Lower Assemblage. The palaeoenvironment of both Coqen Basin and South Qiangtang Basin is restricted carbonate platform during Late Permian and Early Triassic, which is similar to the sedimentary setting of the North Field in Arab peninsula and the Puguang Field in South China. The restricted carbonate platform is favorable to form large Oil and Gas Field. The Late PermianEarly Triassic dolomite in north Tibet is mostly buried under the ground except its exposure on the ground occasionally, therefore, its condition for the oil preservation is advantageous to make a positive evaluation for the oil exploration. The petroleum exploration and evaluation for the vast area of Coqen Basin and South Qiangtang Basin inevitably requires to start at a breakpoint, and the Mujiuco dolomite to the west of Namtso is proposed to be the breakthrough.