The Late Barremian (Early Cretaceous) is a global/regional warming interval. Deciphering the mechanism of organic matter (OM) accumulation in this interval could cast lights on OM accumulation mechanism under global/regional warming. This study presented high-resolution geochemical data of the Late Barremian marls and calcareous shales in the Qiangtang Basin to unravel the effect of paleoclimate and hydrothermal activity on OM enrichment, and to further discuss OM enrichment mechanism under warming climates. Our work revealed that the Late Barremian calcareous shales in the Qiangtang Basin showed high total organic carbon contents and were deposited under an anoxic-euxinic condition, high bioproductivity, and relatively stable high-salinity water mass. Hydrothermal activity was intense during this calcareous shale interval deposition. Hydrothermal fluids not only released reducing gas to increase the reducibility of water column, but also provided sufficient nutrients to enhance bioproductivity, thus facilitating the preservation and accumulation of OM. A climate transition from cold-arid to warm-humid occurred at the onset of calcareous shale deposition. A warming climate evidences in the late Barremian succession in the Qiangtang Basin suggested at least a regional warming event. The OM enrichment in the Late Barremian calcareous shales in the Qiangtang Basin was caused by the combined effect of regional (warming climate) and local (hydrothermal activity) factors. Accelerated hydrological cycle under warming climates promoted fluvial delivery of nutrients to sedimentary basins, and thus flourished bioproductivity. Local factors (e.g., hydrothermal activity and water-column stratification) facilitated OM accumulation and preservation under a warming setting.

Tectonic setting of the Late Carboniferous Cameng Formation in the north of South Qiangtang terranes (SQT) remains controversial. Here, we report new geochemical results for clastic rocks from the Cameng Formation. Our results show that the Index of Compositional Variability (ICV) is greater than 1, indicating these sediments experienced limited recycling and were probably derived from acid to intermediate magmatic rocks. The extent of weathering, with the Index of CIA vary from 67 to 75 (mean = 71), ICV > 1 (mean = 1.25) and the Th/U ratios > 4, is moderate in the source area. These new findings show that the Cameng Formation was most likely deposited in an unstable environment (continental island arc or active continental margin). Combined with other evidence, we further infer that the Early Paleozoic rocks ( 206 Pb/ 238 U ages varying from 438 to 502 Ma) presently exposed in the South Qiangtang terrane are the potential source of the Late Carboniferous depositions in the northern SQT, and were most likely deposited in a forearc basin under the in situ suture model, that is, the southward subduction of the Longmu Co-Shuanghu Tethys Ocean beneath the SQT.

QK-1 is the first scientific drilling designed to solve the key problems in petroleum geology in the Qiangtang Basin on the northern Tibetan Plateau. Its altitude is 5030 m. and the well depth is 4696.2 m. It is also the highest-altitude scientific drilling on the globe, and is located in the Lake Bandaohu area in the northern Qiangtang Basin. By performing the QK-1 and its affiliated shallow drillings, a series of new discoveries have been made in regard to high-quality regional seals, source rocks, petroleum displays and the Jurassic strata succession. (1) Two sets of caprocks, which could be regional high-quality seals, were discovered through drilling, i.e., an anhydrite layer in the Quemoco Formation (J(1-2)q) (354-365 m thick) and a gypsum-bearing mudstone in the Xiali Formation (J(2)x) (230-252 in thick). Comprehensive analyses of drilling and seismic data show that the distribution of these caprocks is regionally continuous and stable. Moreover, petroleum displays and well logging data reveal that these two caprocks are characterized by low porosity, good permeability (the mean permeability is 0.265x10(-3) mu m(2), and the mean breakthrough capillary pressure is 9.0 MPa), and good scaling. (2) The northern Qiangtang Basin contains high-quality hydrocarbon source rocks in Upper Triassic deltaic facies. The affiliated shallow drillings reveal a good Upper Triassic hydrocarbon source rock in a coastal swamp-deltaic plain facies on the northern Qiangtang Basin margin. The two mudstone source rock members have a thickness of 36.0 m and total organic carbon (TOC) values of 2.0%-3.6%, and a thickness of 61.0 m and TOC values of > 1.0%, respectively. The organic matter type is II1-, II2- and III-type kerogen with high to over-thermal maturation. Well logging, seismic data and facies analyses indicate that the pro-deltaic to shelf mudstones in the covered area in the central northern Qiangtang Basin are better source rocks than those in the deltaic plain on the basin margin. (3) Well logging petroleum displays provide important insights for petroleum reservoir formation and destruction. Based on petroleum displays and evidences of gas release in 250 outcrops. the QK-1 scientific drilling has also discovered 14 petroleum anomalies in the Eocene Suonahu Formation, Middle Jurassic Buqu Formation and the Upper Triassic Nadigangri Formation. One anomaly is oil-bearing, while thirteen anomalies arc gas-bearing. The total hydrocarbon value of gas logging is 10%, and is mainly methane in composition. The petroleum displays of well logging occurred after drilling through the Xiali or Quemoco formations which are caprocks, while outcrop petroleum displays are mainly distributed on the basin margin or central uplift which are strong tectonic deformation zones. The widespread petroleum displays indicate good hydrocarbon-generation conditions and petroleum reservoir formation/destruction processes, while the distribution characteristics of petroleum displays indicate that the high-quality regional-seals-covered area in central basin could be favorable plays of petroleum reservoirs. (4) The QK-1 drilling cut through a sedimentary succession from the Upper Jurassic to Upper Triassic. i.e., from the upper to the bottom, the Upper Jurassic Suowa Formation (J(3)s), Middle Jurassic Xiali (J(2)x) and Buqu Formations (J(2)b). Middle-Lower Jurassic Quemoco Formation (J(1-2)q) and Upper Triassic Nadigangri Formation (T(3)nd), in which the whole Jurassic strata is completely continuous. The Jurassic sedimentary succession shows a transition from alluvial facies to shore. shallow shelf and carbonate platform facies up section. This finding breaks the previous view that suggests a lack of Lower Jurassic strata. and provides new basics for Qiangtang Basin analysis. Overall, the discovery of two regional high-quality seals in the Jurassic Quemoco and Xiali formations in the Qiangtang Basin refutes the previous traditional perception, which suggests poor petroleum preservation conditions. The northern Qiangtang Basin contains high-quality source rocks with great hydrocarbon generation potential in the Upper Triassic deltaic facies. Petroleum display anomalies and distribution characteristics reveal that the Qiangtang Basin not only underwent large-scale hydrocarbon generation, reservoir formation/destruction processes, but also developed favorable display of petroleum preservation. The establishment of a continuous Jurassic strata succession provides a new evidence and valid framework for petroliferous basin analysis of the Qiangtang Basin.

Compared to organic matter (OM) accumulation in marine environments, OM enrichment mechanism in terrestrial-marine transitional environments remain unclear. The Late Permian was an important coal-forming period in China. Late Permian coal-bearing mudstones are important petroleum source rocks in the Qiangtang Basin. In this study, the coal-bearing rocks in the Late Permian Nayixiong Formation from the Qiangtang Basin are selected to address OM accumulation process in transitional environments. Our work demonstrates three-stage climatic changes during the deposition of coal-bearing sedimentary rocks: stages I, II and III, corresponding to a semi-warm and semi-humid climate, warm and humid climate, and cold and arid climate, respectively. Based on degree of pyritization (DOP), enrichment factor (EF) of elements, the Late Permian coal-bearing rocks were deposited under oxic-dysoxic environments. Low excessive barium (Baxs) and biogenic silica (Sibio) contents in the coal-bearing rocks indicates a low primary productivity. We suggest that warm and humid conditions, freshwater and terrestrial plants input are the primary factors controlling OM accumulation during the Late Permian coal-bearing deposition. We summarized a global/regional climate-driven model for OM accumulation in transitional environments. The warming climate created a habitable environment for both terrestrial and marine organisms. Subsequent freshwater input brought abundant nutrients and terrestrial OM to the aqueous system to build high OM accumulation.

位于羌塘盆地东部的羌资-16井首次在羌塘盆地钻遇了完整的上三叠统—中下侏罗统地层,清晰的记录了盆地东部晚三叠世—早中侏罗世时期的沉积环境转变.本文在对羌资-16井岩芯的野外详细观察和室内分析的基础上,结合前期在该区实施的羌资-7井和羌资-8井资料,综合分析了羌塘盆地东部晚三叠世—早中侏罗世过渡时期的沉积环境变化特征.上三叠统地层主要包括波里拉组、巴贡组和鄂尔陇巴组,分别形成于局限台地、三角洲和火山喷发环境,标志着羌塘盆地前陆盆地演化结束和新一轮裂谷盆地的开启.中—下侏罗统雀莫错组底部紫红色底砾岩沉积代表了裂谷盆地开启后的初始沉积,形成于河流沉积环境,而后随着南部海水侵入,逐渐过渡为陆缘近海湖沉积环境.总体上,晚三叠世—早中侏罗世时期,羌塘盆地东部表现为前陆盆地海相沉积的消亡 →裂谷盆地的开启 →裂谷盆地早期充填的演化过程.晚三叠世诺利期开始(～220 Ma),羌塘盆地晚三叠世海相沉积结束,逐渐向早中侏罗世陆相沉积转变,开始了羌塘侏罗纪盆地的沉积演化历史.

As part of the eastern Cimmerian continent, the Qiangtang terrane played an important role in the evolution of the Pangea supercontinent. Recent studies suggest the Qiangtang terrane be divided into two parts along the Longmu Co-Shuanghu Paleotethyan suture, which connected with the Changning-Menglian suture as the boundary of the Cimmerian continents from the Laurasia. However, this proposal has been challenged by the absence of ophiolite me ' langes in 700-km length in eastern Tibet. To fill this gap, here we investigate an ophiolite me ' lange suite, named Bitu, in eastern Tibet. The me ' lange includes blocks of serpentinites, gabbros, basalts, and minor radiolarian-bearing cherts. Zircon U-Pb dating indicates that the gabbro crystallized at similar to 265 Ma and one basalt sample has zircon ages that cluster at similar to 426 Ma. Geochemical analysis shows that the mafic rocks have oceanic island basalt (OIB) or mid-ocean ridge basalt (MORB) characteristics. This indicates an expanded ocean from the Late Silurian to the Middle Triassic in eastern Tibet, comparable to that of other Paleotethyan sutures. Thus, based on regional comparison, we infer that the Longmu Co-Shuanghu, Bitu, and Changning-Menglian sutures represent the vanished Paleotethyan Ocean that spanned over-4000 km between the eastern Cimmerian and the Laurasia continents. Reconstruction of the Paleozoic plate tectonics provides robust evidence for terrane correlation from Pamir to Southeast Asia. Moreover, the pre-Cenozoic plate configuration proposed by this study questions the suggestions of 400-500 km offset in the Cenozoic along the Karakorum-Jiali-Ailaoshan fault system based on the correlation of the Rushan-Pshart with the Bangong-Nujiang suture and the correlation of the Jinshajiang with the Dien Bien Phu suture.

The Carnian Pluvial Event (CPE) is recorded widely across the Tethys region. Here we present high-resolution carbonate (delta C-13(carb)) and organic (delta C-13(org)) carbon-isotope records and mineral data from the Boli La to lower Bagong formations in the Quemo Co area of the Qiangtang Basin, Qinghai-Tibet Plateau. In this studied section, a calcareous fine-grained sandstone bed that yields a detrital zircon maximum depositional age of 232.5 +/- 3.3 Ma, together with ammonoid species and bivalves, allows identification of the Carnian Stage. An intensification of siliciclastic supply and a large drop in calcite and illite is recorded in the Carnian section, suggesting a palaeoclimatic shift from arid to warm and humid during the CPE. A negative carbon-isotope excursion (CIE) in both delta C-13(org) and delta C-13(carb) is also reported, which could be explained by a global perturbation of the carbon cycle and the release of CO2 enriched in C-12. The magnitude of the negative CIE in the Quemo Co area displays a significant shift towards lighter values compared to other reported carbonate carbon-isotope values across the onset of the CPE. A combination of both volcanic emissions from the synchronous Wrangellia large igneous province and release of methane clathrate hydrate is the most likely explanation for the significant negative excursion in delta C-13.

The Early Cretaceous Shengli River–Changshe Mountain marine oil shale belt, located in the northern Qiangtang Basin, Tibet, is estimated to be the largest marine oil shale resource in China. However, its palaeoweathering, provenance, and tectonic setting during the depositional history have received little interest. In this paper, 19 outcrop marine oil shale samples from the ChangsheMountain (CSM) area were studied to investigate the provenance, palaeoclimate, palaeoweathering, and tectonic setting during the marine oil shale deposition. The total organic carbon and organic sulphur contents of the CSM oil shales range from 2.2% to 13.44% and 0.19% to 1.08%, respectively. The CSM oil shale samples contain abundant carbonate (e.g., calcite and dolomite), quartz, and clay minerals. The clay minerals of the CSM oil shale samples is dominated by illite and then by smectite. Other minerals, including haematite, apatite, pyrite, baryte, and celestite, were also identified by scanning electron microscope–energy‐dispersive X‐ray spectrometer. The total rare earth element (∑REE) contents of the oil shales range from 9.01 to 105.96 ppm (average 57.56 ppm), close to the average REE contents of U.S. coals. The REE contents in oil shale samples are mainly controlled by terrigenous input. The CSM oil shales show LREE‐enriched, HREE‐depleted, with the LREE/HREE ratios ranging from 5.12 to 10.18, negative Eu anomalies (0.56–0.84) and slightly negative Ce anomalies (0.87–1.05). The chemical index of alteration values of the CSM oil shales range from 67 to 76 (average 73), indicating a moderate chemical weathering condition. Sr/Cu and Rb/Sr ratios suggest the CSM oil shales were mainly deposited under a warm and humid climate regime. Sr/Ba ratios, ranging from 0.54 to 7.84, suggest a palaeoenvironment with fluctuant salinity. Major element, trace element, and REE relationships indicate that the CSM oil shales are mainly from the felsic and intermediate igneous rocks. The REE distribution patterns of the CSM oil shales are similar to the Late Triassic Nadi Kangri felsic volcanic rocks in the Qiangtang Basin, indicating that the CSM oil shale may be derived from the Late Triassic volcanic rocks and the oil shales were mainly formed in rift tectonic setting. Favourable preservation condition (e.g., warmer palaeoclimate, higher palaeosalinity, and dysoxic to anoxic water column environment) could be the main controlling factor for organic matter accumulation.

The Mesozoic Qiangtang Basin is investigated to reconstruct the tectonic evolution of the Paleo- and MesoTethys Oceans. We present a detailed sedimentary facies analysis of the Nadi Kangri and Quemoco formations and the systematic zircon U-Pb ages of the volcanoclastic rocks in the Nadi Kangri Formation to understand the onset and nature of the Mesozoic Qiangtang Basin. The Nadi Kangri Formation, consisting of volcanic-volcanoclastic facies associated with alluvial-diluvial facies association, which represents continental volcanic succession. The Quemoco Formation, consisting of braided fluvial, restricted platform and tidal flat facies associations, represents continental siliciclastic and marine carbonate successions. The deepening-upward transgressive sequences reconstructed from the two formations are indicative of a rift basin. Zircon U-Pb ages of the volcanoclastic rocks in the bottom, middle, and upper sections of the Nadi Kangri Formation in the Fanghu area are 221.7 +/- 1.3 Ma, 217.0 +/- 1.5 Ma, and 207.1 +/- 1.3 Ma, respectively. Combined with the previous zircon ages of the Nadi Kangri Formation, we interpreted that 221.7-220.4 Ma is the initial age for the onset of the Mesozoic rift basin. An evolution pattern for the Tethys Ocean and Mesozoic Qiangtang Basin has been proposed based on the magmatism, metamorphism, sedimentary sequence, and deformation data obtained from the Kunlun (KLT), Hoh Xil-Bayan Har (HBT), North Qiangtang (NQS), Central Uplift Belt (CUB), South Qiangtang (SQS), and North Lhasa subterranes/terranes (NLS) during the Early Triassic-Middle Jurassic. Based on the new data, we interpret a double-sided subduction and subsequently a collision of the Paleo-Tethys Ocean beneath the KLT to north and NQS to south, probably occurring at ca. 252-237 Ma and 237-227 Ma, respectively. This interpretation is consistent with the Late Triassic residual sea (ocean) model that attributed to the origin of the Hoh Xil-Bayan Har Basin. The QT and NLS experienced a strong extension between ca. 225 and 201 Ma owing to the rapid expansion of the Bangong Lake-Nujiang Ocean (Meso-Tethys Ocean), which is consistent with the massive volcanic eruption and deposition of volcanic sediments in the QT and NLS. In addition, the QT and NLS experienced a stabilized subsidence during 201-166 Ma, due to the existence of the mature Bangong Lake-Nujiang Ocean.

The Permian-Triassic boundary (PTB) events such as carbon-isotope excursion, volcanism, and environmental disturbances are not well understood in an ocean island setting. Here, we develop a new case study for the Permian-Triassic transition in the Qiangtang Basin, Tibet, by combining existing biostratigraphy, new carbon isotope data, sedimentological data, U-Pb zircon age, mineralogical and geochemical data. These new data defined the Permian-Triassic transition age as about 252.3 +/- 0.9 Ma, corresponding to the lowermost sea level. A long-term negative carbonate carbon isotope trend is present at the same stratigraphic level in different depositional environments, strongly suggesting its global nature. However, our new ocean island setting is characterized by an abrupt shift in carbon isotope values across the level, which is different from many chemostrtigraphic studies that show a gradual shift in carbon isotope values across the event horizon. The most likely explanation for the abrupt shift in C-13 values is a sedimentary hiatus at this level in the study section. Three-stage climatic models through the PTB in the ocean island setting are identified: Stage 1 is characterized by a warm and humid climate with moderate chemical weathering; while a hot and humid climate with intense chemical weathering dominates stage 2; the climate in stage 3 is a relatively hot and arid climatic condition with weak chemical weathering.

The Permian Chert Event is of great significance to understanding the geological evolution of the entire Permian; however, the origin of widespread chert formation is debated. We report new geochemical data from deep-marine siliceous rocks of the upper Permian Da-long Formation, Lower Yangtze region, southeastern China. Their geochemical results show that these thin-bedded siliceous rocks have a clear biologic origin, with rare to no evidence of hydrothermal influence. The values of Al/(Al + Fe + Mn) and Eu/Eu* are 0.60–0.84 (mean = 0.72) and 0.45–1.08 (mean = 0.77), respectively, and Mn/Ti ratios are relatively low (mean = 0.72). The correlations of La N /Ce N , La N /Yb N , and Fe 2 O 3 /TiO 2 with Al 2 O 3 /(Al 2 O 3 + Fe 2 O 3 ), along with the Ce anomaly, indicate that the Da-long siliceous rocks were deposited at a transitional zone between a continental margin and the open ocean; i.e., relatively close to terrestrial sediment input and far from hydrothermal activity. The accumulation of chert is related to its unique paleogeographic location in an equatorial setting with many submarine paleo-highlands. Intense upwelling and frequent local volcanism are the main factors that promoted the development of siliceous rocks in the studied area. Ocean acidification triggered by large-scale volcanism (Large Igneous Province) during the late Permian led to extensive silica precipitation and preservation.

We report here U-Pb age and in situ Hf isotopic results for detrital and magmatic zircons from one conglomerate and four tuffite samples from the Late Triassic Nadigangri Formation across the North Qiangtang depression, Tibet. Coupled with previously published data in the region, this paper proposes new insights into the geochronological framework for the Nadigangri Formation. The deposition ages of tuffite from top to bottom in the Woruo Mountain, Quem Co and Dongqu River, are 203 Ma, 226 Ma, 221.5 Ma and 221.1 Ma, respectively. The detrital zircons yield a younger cluster of ages of 201.5–225 Ma from the conglomerate of the Quem Co Formation. The Late Triassic Nadigangri Formation defines a temporal range approximately between 201 and 225 Ma（Norian-Rhaetian）, including three predominant groups of 220–225 Ma, 210–217 Ma and 201–205 Ma, which correspond with the three main rifting episodes of initial rifting, further rifting and final rifting. Positive εHf（t） value and low model ages in younger detrital zircons suggests a juvenile character. However, the Hf isotopes of magmatic zircons display the presence of reworked ancient crust with 1.1–1.8 Ga. These results provide strong constraints not only on the temporal range of the Late Triassic Nadigangri Formation, but also on the onset of the Qiangtang Mesozoic rift basin.

The Mangxiang Formation black shales are the most important hydrocarbon source rocks in the Wuyu Basin. Trace and rare earth element (REEs) of the black shales from the Wuyu Basin were studied in order to understand their depositional environments and palaeoclimate. Thirty one black shale samples from the Wuyu Basin were analysed by inductively coupled plasma-mass spectrometry and X-ray fluorescence. The black shales are characterized by moderate SiO2 (52.55-58.73%) contents and medium K2O/Na2O (2.21-4.83) ratio values but relatively high MgO+Fe2O3 (5.8-6.91%) and Al2O3 (15.1-17.5%) contents. The Chemical Weathering Index of Alternation (CIA) ranges from 57 to 70, together with medium Th/U (2.66-4.97) ratio values, reflecting a weak to moderate degree of chemical weathering of the source area. The palaeoredox condition of black shale was slightly oxic (or dysoxic) during black shale deposition as evidenced by slightly Ce anomalies (0.92-0.99) and Mn enrichment (EF = 1.6). The moderate palaeosalinity values (9.32 parts per thousand-17.75 parts per thousand), together with medium B/Ga (3.90-5.57) ratio values, indicate a brackish water environment. The palaeoclimate index Sigma(Fe + Mn + Cr + Ni + V+ Co)/Sigma(Ca + Mg + Sr + Ba + K+ Na) ranges from 0.28 to 0.50 and low Sr/Ba (0.15-1.22) ratio values, indicating a semiarid to semimoist climatic condition during the sedimentation of the black shale. The high omega(La)(N)/omega(Yb)(N) ratio values (1.02-1.09) indicate a fast sedimentary rate during black shale deposition. In this study, a preservation model of the Mangxiang Formation black shale was established. The model indicates that excellent preservation may be the major controlling factor for the accumulation of organic matter. Copyright (C) 2015 John Wiley & Sons, Ltd.

The Late Jurassic to Early Cretaceous depositional environments of the Qiangtang Basin in Tibet have the potential to provide significant insight into mechanisms of black shale deposition, organic matter accumulation, and the timing of closure of the Mesotethys Ocean. However, the depositional setting has not been well constrained. Here we apply multiple geochemical proxies and petrologic analyses to representative samples of black shale, marl, and micrite collected from a section in the region. These indicate a transitional marine–continental environment, with brackish to saline water. Redox conditions were weakly oxic to suboxic. Environments represented by the studied section varied over time. The lowermost marls were deposited in a low-salinity, weakly oxic shore to shallow lake environment, under a warm and humid climate regime. The micrites in the middle part of the section were deposited in a lagoonal environment, with intense evaporation, high salinity, and water column stratification. The uppermost shales were deposited in a reducing, semi-enclosed lagoon environment during a marine transgression. These results suggest that the Late Jurassic–Early Cretaceous succession was deposited in a tidal-flat or lagoonal environment.

The sediments of organic-rich oil shales in the Bilong Co. area can be correlated with those of the early Toarcian anoxic black-shale events in Europe. The Bilong Co. sediments are rich in trace elements Se, Mo, Cd, As and Ni, and, to a lesser extent, Li, F, V, Co, Cu, Cs, Hg and Bi, in comparison to the upper continental crust. Thirty-two oil shale samples were collected from the Bilong Co. oil shale to evaluate the controlling factors of trace-element enrichment in the lower Toarcian anoxic sediments. Minerals identified in the Bilong Co. oil shale include calcite, quartz, illite, feldspar and dolomite, and trace amounts of siderite, magnesite, halite, haematite, zeolite, amphibole, gypsum, anhydrite, apatite, pyrite, sphalerite, barite and mixed-layer illite/smectite. Mineralogical and geochemical data show that seawater and hydrothermal activities are the dominant influences on the mineralogical composition and elevated trace-element concentrations in the oil shale. The clay minerals, quartz and feldspar in the Bilong Co. oil shale were derived from the Nadi Kangri volcanic rocks. Input of sediment from this source may have led to enrichment of trace elements Li, Cr and Cs in the oil shale. Carbonate minerals and nodular- and framboidal-pyrite are authigenic phases formed from seawater. The enrichment of V, Co, Ni, Cu, Mo, As, Se, Bi and U in the oil shale was owing to marine influence. Barite, sphalerite and fracture-filling pyrites were derived from hydrothermal solutions. High concentrations of F, Zn and Cd were probably derived from hydrothermal fluids.

The Qiangtang Basin is the biggest petroleum-bearing basin in the Qinghai-Tibet Plateau. This basin experienced a foreland basin evolution during the Early- Middle Triassic and a rift basin evolution during the Late Triassic-Early Cretaceous. Triassic and Jurassic hydrocarbon source rocks were widely distributed throughout the basin. The Triassic Tumen Gela Formation coal-bearing mudstones represent the best source rocks because of high total organic carbon (TOC) content (1.25-3.45%) and HI values (2.8-123 mg/g Toc), and the Xiali Formation mudstones are moderately-good source rocks with an average TOC content ranging from 0.55 to 7.30% and HI values ranging from 7.0 to 165 mg/g Toc. The Jurassic Buqu Formation and Suowa Formation carbonates, however, exhibit poor-to fair-quality as hydrocarbon source rocks. Excellent dolomite and paleokarst reservoirs and mudstones and bearing-evaporite marl cap rocks, together with well-developed structural traps are recognized in the basin. Additionally, a large paleo-oil-reservoir zone has also been discovered.Based on an integrated petroleum systems analysis, nine favorable hydrocarbon exploration areas are proposed, of which the Tuonamu area and Badaohu area are selected as the potential targets for the exploration for oil and gas resources in the basin. Good reservoir quality dolomites in the Buqu Formation are considered to have a significant exploration potential. (C) 2016 Elsevier Ltd. All rights reserved.

The Bilong Co oil shale zone is located in the South Qiangtang depression. This zone, together with the Shengli River-Changshe Mountain oil shale zone in the North Qiangtang depression, northern Tibet plateau, represents the potentially largest marine oil shale resource in China. Seventeen samples including oil shale and micritic limestone were collected from the Bilong Co oil shale area to determine the concentrations, distribution patterns, occurrences and origins of platinum group elements (PGEs) in marine oil shale. The oil shale samples from the Bilong Co area exhibit very low total PGE contents ranging from 1.04 to 2.96 ng/g with a weighted mean value of 1.686 ng/g, while the micritic limestone samples from the Bilong Co area exhibit a little lower PGE value ranging from 0.413 to 1.11 ng/g. PGEs in oil shale samples are characterized by high contents in Pd (average 0.79 ng/g), Os (average 0.123 ng/g) and Pt (average 0.644 ng/g) compared with Ru (average 0.068 ng/g), Rh (average 0.033 ng/g) and It (average 0.026 ng/g). The highest values for individual PGEs are not uniformly distributed in the section. Clearly, the PGEs are generally enriched in the oil shale samples near the boundary between micritic limestone and oil shale.The individual PGEs in oil shale samples from the Bilong Co area exhibit various modes of occurrence. Ruthenium and Pt occur mainly in pyrite, while Pd is associated mainly with organic matter and Mg-minerals. Rhodium and Os are controlled mainly by pyrite and organic matter. Iridium is present mainly in other Fe-bearing minerals, rather than pyrite. The PGEs in the Bilong Co oil shale are mainly of seawater origin and possibly influenced by terrigenous supply. (C) 2014 Elsevier GmbH. All rights reserved.

The early Jurassic black shales are the most important source rocks in the Qiangtang Basin, Tibet. These black shales may provide a useful example for understanding the sedimentary conditions during the early Jurassic in the eastern Tethys. Thirty‐two black‐shale samples were collected from the Qiangtang Basin to reconstruct the depositional environments during the early Jurassic in the eastern Tethys. The palaeoclimate index Σ(Fe + Mn + Cr + Ni + V + Co)/Σ(Ca + Mg + Sr + Ba + K + Na) varies between 0.58 and 1.14, together with low Sr/Ba (0.20–0.60) and Sr/Cu (1.68–7.88) ratios, indicating a warm humid to semi‐humid climatic condition during black‐shale deposition. The early Jurassic black shales exhibit slight Ce anomalies (0.89–0.94) and Mn enrichment (EF = 1.2), suggesting a slightly oxic depositional environment. The conditions of the sedimentary water system during black‐shale deposition were apparently brackish as evidenced by middle B/Ga (3.96–5.73) and Th/U (3.96–6.51) ratios. Copyright © 2015 John Wiley & Sons, Ltd.

The Mid-Cretaceous oil shale from the Qiangtang Basin represents the potentially largest marine oil shale resource in China. 23 samples, including oil shale and marl, were collected from the Basin's Shengli River area to determine Sr and Nd isotope compositions, discuss sedimentary sources and paleoenvironmental changes. The Nd isotopic compositions of the Shengli River oil shale are similar to those of the Nadi Kangri Formation volcanic rocks and the underlying Suowa Formation limestone, indicating the Nadi Kangri Formation volcanic rocks and the Suowa Formation limestone origins of oil shale. The T-Dm ages of oil shale may be considered as the mean age of the upper continental crust in the Qiangtang Basin. Therefore, just like other fine-grained continental sediments, oil shale comprises sampled source rocks that were particularly well mixed through multiple stages of sedimentary recycling. The oil shale samples from the Shengli River area have higher Sr-87/Sr-86 ratios than the contemporary seawater. The high Sr-87/Sr-86 ratio reflects the Sr isotope composition of their original precipitation fluids. Marl samples from this area exhibit slightly higher Sr-87/Sr-86 ratios (mean = 0.7084) than oil shale samples (mean = 0.7076), and show a dramatic Sr isotopic shift near the boundary between the oil shale seams and marl beds. This shift is closely correlated with a rapid change in Nd isotopic compositions, indicating paleoenvironmental changes across the oil shale-marl boundary.

The Shengli River–Changshe Mountain oil shale zone in northern Tibet represents a large marine oil shale resource in China. Thirty-four samples of oil shale were collected from the Changshe Mountain West oil shale, a new oil shale deposit discovered in this oil shale zone, to evaluate the minerals and geochemical anomalies in the oil shale. Minerals identified in the oil shale include calcite, dolomite, quartz, illite, and trace amounts of siderite, hematite, feldspar, apatite, zinc oxide, pyrite, sphalerite, barite, celestite, and Fe(Si, Al)-oxysulfate minerals. The oil shale samples are rich in trace elements compared to post-Archean average shales including Co (up to 15.8μg/g), Ni (up to 83.6μg/g), Cu (up to 101μg/g), Zn (up to 775μg/g), As (up to 255μg/g), Se (up to 6.91μg/g), Sr (up to 738μg/g), Mo (up to 87.9μg/g), Pb (up to 17.4μg/g), Cd (up to 13.0μg/g), Bi (up to 0.38μg/g), and U (up to 7.10μg/g), and to a lesser extent, Li, F, Sc, V, Cr, Ga, Sn, Hg, and rare earth elements. Trace elements of probable terrigenous origin in the oil shale were derived from two possible sources: a felsic volcanic rock source and a limestone source. High concentrations of lithophile elements (e.g., Li, Sc, Ga, and REEs) were derived from these provenances. However, the enrichment factor for these elements also is low. In addition, seawater and hydrothermal activity may have influenced the geological and geochemical compositions of the oil shale. Minerals such as calcite, dolomite, and a large proportion of pyrite are of marine origin, and the enrichment of V, Cr, Co, Ni, Cu, Se, Sr, Mo, Pb, and U in the oil shales was probably due to marine influence. Barite, celestite, sphalerite, Fe(Si, Al)-oxysulfate minerals, and fracture-filling pyrite are of hydrothermal origin, and F, Zn, and Cd were derived from hydrothermal fluids.