Due to lower exploration degree,the stratal pattern and correlation is still controversial in the oil-bearing Qiangtang Basin,one of the largest sedimentary archives with thick organic carbon-rich strata in the Tibetan Plateau,which impacting the basin evaluation of petroleum conditions and oil-bearing systems.Based on investigations of biostratigraphy,chronostratigraphy,sequence stratigraphy and basin evolution of the Qiangtang Basin,we conduct a comprehensive data compilation to analyze the depositional history and ages of the basin rock record.The results reveal that:(1)New isotopic chronology data confirm that there is a Precambrian metamorphic crystalline basement in the Qiangtang Basin.The Paleozoic basement fold system is buried at a depth of 7-15 km,which is overlaid unconformably by the Mesozoic deposits.(2)The proposed age of the Nadikangri Formation is the Upper Triassic,contradicting previous suggestion of Lower-Middle Jurassic.(3)The dolomites of the paleo-reservoir in the southern Qiangtang Basin belong to the Late Triassic rather than the Middle Jurassic in age,and the dolomites are probably"allochthonous root-less"and tectonic contact with the Buqu Formation.(4)The Bilong Co oil shales in the southern Qiangtang Basin belong to the Lower Jurassic rather than the Middle Jurassic.(5)The Quemo Co Formation has an age not younger than the Middle to Lower Jurassic rather than the original suggestion of the Middle Jurassic.(6)Facies changes and paleontological analysis combined with isotopic data indicate that the Shenglihe-Changsheshan oil shale was deposited in a marine environment during the Early Cretaceous.(7)Isotopic chronology and field geological survey confirm that the Kangtuo and Suonahu Formations comprise simultaneous strata with different sedimentary facies.The data synthesis and progress in stratigraphic research of the Qiangtang Basin provide a new basis for the development of the stratigraphic division and correlation scheme of the basin,as well as the analysis of its petroleum system and potential.

The overall research level of the Qiangtang Basin is at its primal stages due to lack of data and uneven distribution,which makes the predictions of hydrocarbon source rocks and reservoirs imprecise.Based on the analysis of basin attributes,sedimentary systems and their combination-coupling relationships,this article solves the problems of multiple interpretations of sedimentary facies in regions with abundant data and lack of sedimentary model guidance in regions lacking data.Hence,three geological acknowledgments are acquired as follows.First,we specify two distinct consecutive stages of basin property of the north and the south subterrains in Qiangtang Basin based on the systematically summarized closing history of the Koh Xil-Jinsha River Ocean,Bangong Lake-Nujiang Ocean and Longmuco-Shuanghu Ocean.During the Late Triassic-Early Cretaceous epoch,the north Qiangtang terrain evolved from a compound foreland basin to a weak-compressional in-land residual sea basin and entered into a rapid uplift-denudation period with multiple folds and faults.The south Qiangtang terrain,meanwhile,transferred from a passive continental margin to an active continental margin,and also entered into a rapid uplift denudation period with multiple folds and faults.Basin attributes of eight key sedimentary periods are clarified.Second,we compile ten tectonic-lithofacies paleogeography maps in the Late Triassic-Early Cretaceous epoch of Qiangtang Basin and distribution maps of igneous rocks of Nadigangri Formation based on basin properties,sedimentary systems,and the coherent relationships among them.These maps reveal the controlling effect of tectonic background on sedimentary process in Qiangtang Basin and enhance the precision of sedimentary facies prediction in data-poor areas.Third,we figure out the tectonic-lithofacies paleogeography in developing periods of hydrocarbon source rocks and reservoirs explicitly.The distribution of hydrocarbon source rocks of Bolila Formation-Bagong Formation is controlled by foreland depression and deep-water shelf of the passive continental margin,and that of Quse Formation is controlled by a back-arc extensional basin and weak-compressional in-land residual sea basin.The dolomite reservoirs of the Buqu Formation are developed at the platform margin belt.With this knowledge,we compile four distribution maps of hydrocarbon source rocks and reservoirs.All these new acknowledgments will boost the oil-gas exploration process in the Qiangtang Basin.

The Bangong Lake-Nujiang suture zone is an extreme important tectonic boundary in the Tibetan Plateau which is not only closely related to evolution of the Tethyan tectonic domain,but also directly controls the formation and inversion of the Mesozoic southern Qiangtang oil and gas-bearing basin.Based on stratigraphic correlation,sandstone petrography analysis and detrital zircon U-Pb dating,this paper reconstructs provenance changes in the Dongqiao-Beila area and reveals multi-stage subduction processes and microblock amalgamations in the central Bangong Lake-Nujiang Ocean(BNO).The BNO branch(Amdo Ocean)at Dongqiao-Beila area began to subduct in the Early Jurassic,resulting in amalgamation of the Amdo block to the southern Qiangtang terrane and rifting the Dongkaco microblock away from the northern Lhasa terrane and formed two new BNO branches of the Dongqiao Ocean to the north and Beila Ocean to the south.The lower Xihu Group close to the Beila suture in the south of the Dongkaco micoblock shows recycle deposits from the underlying Upper Triassic Quehala Group,while the upper Xihu Group close to the Dongqiao suture in the north of the Dongkaco microblock began to receive detritus from the southern Qiangtang terrane,indicating the Dongqiao Ocean demised and the Dongkaco microblock has amalgamated on the southern margin of the Qiangtang terrane with a peripheral foreland basin developed during the Middle Jurassic.Subsequent subduction of the Beila Ocean droved the Dongkaco microblock continuing to converge with south Qiangtang terrane in the Middle Jurassic and promote abundant materials from the south Qiangtang terrane transporting into the Middle-Upper Jurassic Jienu Group in the foreland basin on the Dongkacuo microblock.Wide collision between the Qiangtang and Lhasa terrane occurred during the latest Jurassic and Early Cretaceous(147-141 Ma)and continued during Aptian(ca.120 Ma)until the Beila Ocean finally dimed at Dongqiao-Beila Area. In response to multi-stage northward subductions of the BNO,the south Qiangtang Basin changed from passive continental margin to active continental margin during earliest Jurassic and developed arc-related basin system of east-west extending back-arc basin,volcanic arc and fore arc basin from north to south.The Lower Jurassic Quse black shale and overlying Buqu bioclastic limestone deposited in the back-arc basin.Continuous convergence and microblock collision contributed to rapid regression and uplift of the south Qiangtang basin,resulting in the deposition of tidal Xiali Formation with gypsum and mudstone during the Middle Jurassic.As the result of collision between the Lhasa and Qiangtang terrane in the Early Cretaceous,the south Qiangtang Basin evolved in fold thrust belt of the peripheral foreland basin and resulted in differential burial and denudation of the Jurassic successions.During the Late Jurassic-Early Cretaceous,the Quse black shale and Buqu bioclastic limestone were correspondingly buried rapidly due to the tectonic compression and entered the stage of oil generation and dolomitization,making it become the most important accumulation period in the south Qiangtang Basin.

The Qiangtang Basin,located in the eastern part of the Tethys domain,is the only large-scale superimposed basin in China where no industrial oil or gas fields have been discovered.Based on the new round of comprehensive research on oil and gas geology in recent years,a set of widely distributed black mudstone with large thickness(600-800 m)of Bagong Formation is found being developed in the front delta-shallow water shelf during the foreland basin stage of Late Triassic in Qiangtang Basin,which is of great significance for oil and gas exploration.In order to study the organic geochemical characteristics of the black mudstone of Bagong Formation in the eastern margin of the North Qiangtang Depression,61 core samples from shallow well and 64 hand-drilled plunger samples are collected to carry out geochemical and mineralogical analysis,and the source rock potential and sedimentary environment are evaluated.The results show that:(1)The black mudstone of the Bagong Formation,with TOC of 0.06，-5.81，,is poor-medium source rock on the whole,and some reach the standard of high-quality.Formed in weak oxidation-weak reduction environment,the organic matter type is Ⅱ2―Ⅲ,mainly belonging mixed organic matter.It is in the late mature to high mature stage with Ro of 0.95，-1.90，.(2)The Bagong source rock has great hydrocarbon generation potential,due to high organic matter abundance,good organic matter type,and moderate maturity.(3)There developed a source-reservoir assemblage of"lower generation and upper reservoir"between the Bagong source rock and the glutenite reservoir at the bottom of the Quemocuo Formation,which lays a foundation for the formation of large and medium-size oil and gas fields.Besides,the Bagong mudstone is a favorable layer for shale oil exploration.In general,the Bagong source rock is of great significance for conventional and unconventional oil and gas exploration in the eastern margin of the North Qiangtang Depression.