Evolution Of Mesozoic Sedimentary Basin In North Qiangtang And Its Significance In Petroleum Geology

The Qinghai -Tibet Plateau is located in the east part of the well-known Tethys realm that is the largest petroliferous region onshore in China which is under-explored. Because of the special structure, complicated tectonic evolution and badly natural environment, the research level of petroleum exploration in the Qinghai-Tibet Plateau is the lowest in China. Recently, the oil and gas survey and exploration for this area are being given more and more attention.The Qiangtang basin is located in the north and mid position of the Qinghai -Tibet Plateau, where widespread Mesozoic marine deposits. This basin is the No.1 to the potential of the oil and gas resources in major sedimentary basin on the Qinghai -Tibet Plateau. In this paper, based on the early and late evolution of basin, and sedimentary sequences, we present the open age, early evolution and close of Mesozoic sedimentary basin in North Qaingtang.Regionally, a paleo-weathering crust, a new find by us in 2006, occurred widely at the top of the Xiaochaka strata during the early evolution of Mesozoic sedimentary basin in North Qaingtang, revealing that a depositional hiatus was existed between the Nadi Kangri Formation and its underlying strata (Xiaochaka Formation). Then, judged from the basin evolution, the Nadi Kangri deposits in the North qiangtang depression should be assigned in Jurassic structural/tectonic interval but different from Carnian structural/tectonic interval (Xiaochaka Formation). Mesozoic sedimentary basin in North Qaingtang developed on the basis of the Nadi Kangri volcanic rocks. As a result, eruption ages of the volcanic rocks represent the open ages for the basin, and tectonic setting of the volcanic rocks is close related to basin nature. The dacitic tuff from the basal part of the Juhua Mountain section gives a SHRIMP zircon U-Pb age of 225.1±1.4 Ma, which represents an early eruption age of the Nadi Kangri volcanic rocks. While the age obtained in the Nadigangri section is more close to the upper part of the sequences, which may represent a late age of about 205±4 Ma. Therefore, we conclude that open age of Mesozoic sedimentary basin in North Qaingtang is about 220 Ma.The open of Mesozoic sedimentary basin in North Qaingtang is characterized by the eruption and sedimentary of the Nadi Kangri volcanic rocks and early sedimentary is characterized by alluvial-flood plain facies conglomerate, and then basin settled uniformly. This shows a rift basin evolution. On the tectonic discrimination diagrams, such as Ti/100-Zr-3Y, Zr/4-2Nb-Y, Th/Hf-Ta/Hf and Zr-Zr/Y, the Nadi Kangri basaltic rocks plot in the "within-plate" setting field which, in conjunction with the tectono-sedimentary evolution of the basin, indicates that the Nadi Kangri basic lavas did indeed erupt in an intracontinental rift setting (rift basin).The close age of Mesozoic sedimentary basin in North Qaingtang remains unclear due to few fossils and lack of the correlatable dating data at the regional scale. However, during late evolution of Mesozoic sedimentary basin in North Qaingtang, oil shale, a new find by us in 2006, occurred widely in Shengli River region. An isochron is obtained giving an age of 101±24 Ma for the oil shale. The Re-Os age agrees with geological age derived from biostratigraphic data, indicating an end of marine sedimentation within the Middle-late early Cretaceous. Based on oil shale characteristics, this paper also studies the sedimentary characteristics and lithofacies and paleogeography during late evolution of Mesozoic sedimentary basin in North Qaingtang.In addition, this paper also studies the oil and gas resources in the Qiangtang basin and discusses the relation between the evolution of basin and source rocks, reservoir, and cap rocks.