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The Bilong Co oil shale is one of the most significant source rocks in the Mesozoic Qiangtang Basin (Northern Tibet); however, its absolute chronology remains controversial. In this study, in situ carbonate U–Pb isotope dating analysis was carried out for the first time. Detailed field geological investigations yielded some age-diagnostic ammonites, enabling a re-evaluation of the stratigraphic age of the Bilong Co oil shale. A total of 61 spots of U–Pb isotope dating from the middle part of the Bilong Co oil shale section suggests an average age of 181 ± 13 Ma. Elemental geochemistry and diagenetic analysis indicate that the proposed age represents the early deposition of the calcite, and the oil shale was deposited during the Early Jurassic time. This estimated age is further supported by the newly discovered ammonite assemblage of Hildoceratidae–Tiltoniceras sp. at the top part of the oil shale section, which confirms the deposition of the oil shale during the Toarcian age of the late Early Jurassic. Consequently, the Bilong Co oil shale can be assigned to the Quse Formation, which is attributed to the Lower Jurassic rather than the Middle Jurassic. The re-assessment of the stratigraphic age of the Bilong Co oil shale is of great significance for regional evaluation and exploration activities of hydrocarbon source rock layers in the Qiangtang Basin as well as for global stratigraphic correlation of the late Early Jurassic Toarcian oceanic anoxic event. [ABSTRACT FROM AUTHOR]

The Qiangtang Basin is one of the largest Mesozoic petroliferous basins in China that is still unexploited for the petroleum industry. The basic geological issues and synthesis of the Qiangtang Basin related to the Tethyan evolution are still poorly understood. This volume presents 20 research papers to address the tectonic settings, basin evolution, and sedimentary environments of the Qiangtang Basin, which are key factors for a better understanding of the overall petroleum system. The Qiangtang Basin holds a Precambrian base developed in response to the Paleo-Tethys Hol Xil-Jinshajiang Ocean evolution, with enhanced sedimentary basin infilling during the opening and expansion of the Neo-Tethys Bangong-Nujiang Ocean in the Mesozoic. The Paleozoic basin framework of the Qiangtang demised as the Paleo-Tethys Ocean closed at the latest Permian, while the Early-Middle Triassic foreland basin underwent a negative basin inversion during the Late Triassic. The latter initiated a new sedimentary filling cycle of the Qiangtang Basin, which was triggered by the active extensional rifting and the onset of the bimodal volcanic eruption. Recent scientific exploratory drilling (QK-1 well) found that the Qiangtang Basin is characterized by multiple source rocks, of which the upper Triassic Bagong Formation and the lower Quse Formation exhibit the most hydrocarbon generation potential. The development of high quality and quantity source rocks in the Qiangtang Basin is ensured by regional to global-scale bottom-water anoxia and high primary productivity in the eastern Tethys realm in the Late Triassic and the Early Jurassic. The main reservoirs in the Qiangtang Basin include the Middle Jurassic reef-beach facies, the Upper Triassic carbonates, and the Lower Jurassic terrestrial clastic rocks. Two very-thick caprocks of gypsum in the Lower Jurassic and gypsum mudstones in the Middle Jurassic strata are found during the scientific drilling, which is interpreted as favorable regional-scale seals for petroleum entrapment in the Qiangtang Basin. [ABSTRACT FROM AUTHOR]

[Display omitted] • The minimum paleoelevation of the Qiangtang terrane was ∼2830 m in the Eocene-Oligocene. • Pollen analyses indicate subtropical-temperate vegetation in the Eocene- Oligocene. As the largest and highest continental plateau on the Earth, the Tibetan Plateau has attracted worldwide attention to its formation and its remarkable paleoenvironment changes. Here we used the new stable isotope results of the lacustrine carbonate cement and pollen assemblages from the middle Eocene Suonahu Formation that was drilled from the QD17 well to infer the middle Eocene paleoclimate and paleoelevation of the northern Qiangtang basin in the central Tibetan Plateau. According to geological logging of the QD17 well, the sedimentary succession in the Suonahu Formation consists of purple-red calcareous mudstone, marlstone, siltstone, white gypsum, and gray-green mudstone. Analyses of the lithofacies indicate that the Suonahu Formation was deposited in lacustrine-evaporation environments. Both petrographic observations and isotopic data suggest that the lacustrine carbonate cement likely experienced an early-stage digenesis. The results of pollen records show that palynological taxa are mainly dominated by Ephedripites-Betulaceoipollenites-Pterisisporites and Ephedripites-Nitrariadites- Taxodiaceaepollenites assemblage, which suggest the vegetation types of the Suonahu Formation during the depositional period were mainly subtropical-temperate. The δ 18O psw values of paleo-surface water reconstructed from the lowest (i.e. the least evaporated) δ 18O c values (−10 ‰) of the weakly altered carbonate cement are used to make the minimum estimation of the average paleoelevation of the drainage basin. The paleoelevation of the northern Qiangtang terrane is reconstructed as about 2830 m by the middle Eocene-early Oligocene, approximately 2200 m lower than the present-day elevation. [ABSTRACT FROM AUTHOR]