The preliminary study of petroleum geology of the Qiangtang Basin in northern Xizang

Ten successions of source rocks are developed in the Qiangtang Basin, of which four ones appear as important source rocks, including the Upper Jurassic Suowa Formation (J₃ s), Middle Jurassic Xiali (J₂ x) and Buqu (J₂ b) Formations, and Upper Triassic Xiaochaka Formation (T₃ x). The source rocks occur almostly throughout the basin besides the central uplift, with a total thickness of more than 3000 m, and consist chiefly of carbonate rocks and argillaceous rocks. The former are common in the Upper Jurassic Suowa Formation and Middle Jurassic Buqu Formation where the lower organic carbon contents ranging between 0.15% and 0.25% suggest the poor to moderate source rocks. The latter are observed in the Middle Jurassic Xiali Formation and Upper Triassic Xiaochaka Formation where the higher organic carbon contents ranging between 0.60% and 1.20% indicate the moderate to good source rocks. An additional sequence of oil shales has been found to be hosted in the Middle Jurassic Xiali Formation and Upper Jurassic Suowa Formation. The oil shales behave as the valuable source rocks due to their extraordinary high organic carbon and soluble organic matter contents. The above-mentioned four sequences of source rocks, without exceptions, have a marine origin; the organic matter is made up of low-grade marine organisms. The mixed (Ⅱ-type) organic parent material is interpreted to have good reservoir quality. The thermal evolutionary history shows that the organic matter is now in the maturation stages in the central part of the basin; the high maturation stages in the peripheral part and central uplift, and supramaturation stages outside the basin. It is estimated that the crude oil area appear in the central part of the basin, followed by condensates-wet gas area, and dry gas area towards the margins of the basin. The extents of the crude oil areas reach the maximum in the Suowa Formation. There is a gradual decrease in crude oil area from the Xiali Formation through the Buqu Formation to the Xiaochaka Formation.
The reservoir rocks in the basin are assembled by clastic rocks and carbonate rocks, with subordinate volcanic rocks. The clastic rocks occur mainly in the third member of the Upper Triassic Xiaochaka Formation, Middle Jurassic Quemocuo Formation, Middle Jurassic Xiali Formation, Upper Jurassic Xueshan Formation, and Paleogene Shuanghu Formation. The rock types consist of fine-grained sandstone and siltstone, with subordinate medium- and coarse-grained sandstone. The carbonate rocks are defined to the first and second members of the Upper Triassic Xiaochaka Formation, Middle Jurassic Buqu Formation and the Upper Jurassic Suowa Formation. The rock types consist of marls, grainstone, oolitic limestone, organic reef limestone and dolostone. The volcanic rocks are observed in the Permian and Upper Triassic strata. The dolostones as reservoir rocks are widespread in the basin, and their planar distribution is believed to be related to the paleouplift.
Several sequences of cap rocks appear in the basin, including, evaporates, carbonate rocks, condensed sandstone, volcanic rocks, siliceous rocks and coal-bearing series. The former three types, especially mudstone and evaporates, widespread in the basin have good sealing capacity, and thus may serve as the principal cap rocks in the basin.
In sum, the Qiangtang Basin has favourable geological conditions for the generation of the large-or medium-sized oil/gas fields, and thus is interpreted to be highly prospective.