The Qiangtang Basin in Xizang as the target area for the oil and gas resources in China

Comparable with the Middle East petroleum provinces in the Tethyan domain, the Qiangtang Basin is selected as the target area for the oil and gas resources in China. The basin lies in northern Xizang, with an average elevation of 4900 m, and a total area of 18×10⁴km². Tectonically, it is assigned to the eastern part of the Tethyan tectonic domain, corresponding to the northern and middle subzones of the Middle East Tethyan tectonic domain. The basin behaved as a passive continental marginal basin during the Late Palaeozoic and a complex foreland basin during the Mesozoic, upon which the Tertiary strike-slip fault basin overlapped during the Mesozoic. The excellent reservoir-, seal-, and source-prone facies are well developed in the basin:the source rocks consist mostly of Triassic-Jurassic mudstones, limestones and oil shales, and the hydrocarbon-bearing formations are getting mature or supermature. The oil-gener tion peak period may be traced back to the Late Jurassic-Paleogene. The oil shales locally reach 70 m or more in thickness, with an estimated total reserve of ca. fifty milion tons. The highly porous and permeable clastic rocks, dolostones and organic reefs (banks) are also observed in the basin as the reservoir rocks. The secondary porosities were developed during the periods corresponding to oil-generation peak periods. There occur in the basin several suites of cap rocks consisting of mudstones, marls and gypsum, with a total thickness of more than 3000 m. The best gypsum reservoirs in the Middle East oil fields are also identified in the basin, with a total thickness of 177 m. The single bed of the Middle Jurassic Tongbuqu Formation may be 93 m in maximum thickness. Decades of well-preserved large-or medium-sized anticlines and domal trap structures formed during the latest Jurassic have provided the excellent conditions for the trap and migration of oil and gas. Other discoveries include 190 oil shows and several slab asphalt beds and one ancient oil pool with an area of about 10 km². The completely homogeneous tectonic uplift, less intense denunation, and widespread but small-scale magmatism on northern Xizang Plateau may facilitate the formation of oil and gas resources.