The gypsolith and its bearings on the oil and gas exploration in the Qiangtang Basin in northern Xizang and southern Qinghai

The gypolith deposits crop out over a wide area of the Middle Jurassic Quemocuo Formation, Buqu Formation and Xiali Formation, and the Upper Jurassic Suowa Formation in the Qiangtang Basin in northern Xizang and southern Qinghai. These deposits may be arranged into three types of sedimentary sequences:(1) oosparite-dolostone (dolomitic limestone)-gypsolith, (2) siltstone (or mudstone)-gypsolith-siltstone (or mudstone)-gypsolith, and (3) gypeous dolostone (or mudstone)-gypsolith sequences, which are assigned to the lagoon-gulf, tidal-flat and sabkha sedimentary environments, respectively. The lagoon-gulf gypolith deposits are commonly larger in thickness and laterally extensive. The tidal-flat gypolith deposits become apparently thinner and laterally inextensive. The sabkha gypolith deposits occur as thin and very thin beds or as nodules and augens in the mudstone and dolostone of the basin. The gypolith deposits cited above have also been found to occur as salt-dome structures 100 to 1 000 m in diameter in Yanshiping, Wenquan and Chibuzhang Lake, where oil shows have been identified. It is possible, judged from the development of the salt domes and distribution of sedimentary facies, to search for piercement oil pools in favourable structures of the basin.