THE DISCOVERY OF COLD SEEP CARBONATE IN SHUANGHU REGION, QIANGTANG BASIN AND ITS IMPLICATIONS FOR GAS HYDRATE ACCUMULATION

Regional geological survey and outcrop sampling have been carried out in the Shuanghu region of Qiangtang Basin to evaluate the existence and the basic accumulation conditions of gas hydrate deposits. As evidence of gas hydrate leakage, cold seep carbonate was found in the Buqu Formation of Jurassic in Shuanghu region. The carbonate is different with those found in the northern South China Sea, the Gulf of Mexico and the North Sea. The carbon and oxygen isotopes of the samples vary between -30‰ and -25‰ (PDB) and 4‰ to 5‰ (PDB) respectively, indicating that the hydrocarbon gas is of thermal origin. Thus the Buqu Formation in Shuanghu region is considered as a potential exploration target after integrated geological evaluation. The factors favorable for the genesis of gas hydrate are as follows. (1) The oil shale in the Early Jurassic Quse Formation is rich in organic matter with favorable organic matter type and high thermal maturity, and could be a good source of hydrocarbon supply; (2) Fractures and cavities available in the grained dolomite of the Buqu Formation make the dolomite a good reservoir for gas hydrate accumulation; (3) The regionally distributed mudstone, gypsum, marls and permafrost zone provide good caprocks.