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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. [ABSTRACT FROM AUTHOR]

The Barremian and Aptian correspond to periods of marked changes in the atmosphere-ocean system, and offer valuable information about global tectonic events. Here, we present integrated high-resolution carbonate carbon-isotope (δ13C carb) and geochemical data that precede the Early Aptian oceanic anoxic event (OAE 1 a) in the Qiangtang Basin (Eastern Tethys). We identified a characteristic and well-correlatable Barremian-Lower Aptian carbon-isotope pattern that can be categorized into 10 Segments. The most notable features of the δ13C carb record are multiple negative carbon-isotope excursions in the Barremian-Aptian, and each is possibly linked to environmental changes. Two carbon isotopic plateaus are observed in the Upper Barremian, which are generally related to increased primary productivity, and to the concomitant development of dys-to anaerobic conditions. Our data indicate the development of anoxic conditions during shale deposition, which occurred under the conditions of a semi-enclosed lagoon environment. Such conditions, were likely localized, and not the primary mechanism responsible for the deposition of organic-rich sediments within the Shenglihe succession. The deposition of organic-rich shales was very likely due to global/regional warming. Rising temperatures and hydrological cycle strengthening during shale deposition are believed to have increased primary productivity, and thus resulted in the expansion of the local oxygen minimum zone within the water column. • Barremian was a time of marked changes in the atmosphere-ocean system. • δ13C record is subdivided into 10 δ13C segments prior to the OAE 1a. • Positive carbon isotope excursion is prominent in the Upper Barremian. • Global warming would raise surface productivity. [ABSTRACT FROM AUTHOR]

Abstract The Mesozoic Qiangtang Basin is investigated to reconstruct the tectonic evolution of the Paleo- and Meso-Tethys 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. Graphical abstract Image 1 Highlights • The volcanism and deepening-upward transgressive successions imply a rift basin. • Onset of the Mesozoic rift Basin is ca. 221.7 Ma and lasted for at least 19.4 Ma. • The rift basin is dominated by the rapid expansion of the Meso-Tethys Ocean. [ABSTRACT FROM AUTHOR]

The Carnian Pluvial Episode (CPE; ∼233 Ma) is marked by global warming associated with an intensified hydrological cycle and increased siliciclastic input, coinciding with a widespread crisis in carbonate production. It was further linked to the eruption of the Wrangellia Large Igneous Province (W-LIP), seawater hypoxia, and significant biotic turnover. While some studies have investigated the CPE in Tibet, the environmental and oceanographic processes leading up to this event have received limited attention. In this study, total organic carbon (TOC), biomarker, inorganic geochemistry, and pyrite framboid petrography of samples collected from the Upper Triassic (Carnian) Bagong Formation in the QZ8 well of the Qiangtang Basin (Tibet) were performed to explore the environmental, paleoceanographic, and organic matter controlling processes preceding the CPE. Results exhibit moderate to high enrichments of trace element redox proxies, such as U, Mo, and V, suggesting that the Bagong Formation was deposited under deoxygenation conditions, from severe anoxia to intermittent euxinia prior to the CPE. The presence of pyrite framboids with small mean diameters and narrow distribution ranges further support this interpretation of deficient redox conditions in bottom and pore waters. Additionally, the moderate to strong positive correlations between enrichment factor values of Mo and V and TOC content indicate that the available organic matter was controlled by oxygen-depleted conditions. Enrichment of Cu, Cd, and Zn, along with their Al-normalized ratios that show strong positive correlations with TOC content, reveal high marine primary productivity before the CPE. Meanwhile, the Si/Al, Ti/Al, and Zr/Al ratios showed cyclic patterns around moderate values, suggesting a modest terrigenous sediment supply, consistent with high Sr/Ba and Sr/Al ratios indicating a predominantly saline environment interspersed with enhanced terrestrial/riverine runoff during deposition. Biomarker analysis reveals a substantial contribution of tricyclic and tetracyclic terpanes, with the C 19 /C 23 TT and C 20 /C 23 TT ratios indicating increased terrestrial organic matter input, supported by the presence of Type III kerogen in the Bagong Formation. Prior to the CPE, the climate was warm and humid, leading to accelerated hydrological cycling and freshwater influx of terrestrial organic matter into the Qiangtang Basin. This likely triggered an increase in nutrient supply under severe deoxygenation conditions and a high sedimentation rate, resulting in enhanced organic matter production and preservation regimes while minimizing the carbonate dilution effect. • Deoxygenation conditions, from severe anoxia to euxinia, occurred prior to the CPE. • Pyrite framboids with low mean diameters and narrow ranges support anoxic settings. • Enrichment of Cu EF , Cd EF , and Zn EF reveals high marine productivity before the CPE. • Detrital and biomarker proxies indicate a modest sediment supply and terrestrial OM sources. • Anoxia and paleoproductivity controlled OM production and burial versus a minimal dilution. [ABSTRACT FROM AUTHOR]

The organic-rich Changliang Mountain oil shale, located in the North Qiangtang depression, northern Tibet, is considered to be excellent mineral resource in China. The Changliang Mountain oil shale reported here was deposited in a tidal flat-lagoon environment and is characterized by black thin-layered oil shales intercalated with dark-gray marls. Here, we present geochemical data from the Changliang Mountain oil shale profile, in order to investigate the mechanism of organic matter (OM) accumulation and to establish the formation model for the marine oil shale deposition. Total organic carbon (TOC) values range from 2.96 to 23.47% in the oil shale samples, while the marl samples contains low TOC contents, ranging from 0.06 to 0.21%. In the organic-rich oil shale sediments, many redox indicators, including Mo/Al ratios, V/Cr ratios, Th/U ratios, Ni/Co ratios, and the relationship of Mo to TOC suggest a deposition under dysoxic to anoxic environments. Subsequently, the bottom water evolved into an oxic water body when the organic-poor marls were deposited. However, the negative and/or weak relationship between TOC and productivity indices P/Ti and Ba/Al indicates that OM accumulation was not controlled mainly by primary productivity, but dysoxic/anoxic bottom water environment. A stratified water column may be initiated by the supply of fresh water from the continent nearby, combining with warm and humid climate, which is beneficial to the reproduction of marine organisms. The death and bury of these organisms could lead to the formation of dysoxic/anoxic bottom waters and enhance the preservation of OM. In this study, a preservation model of the Changliang Mountain oil shale was established. The model implies that excellent preservation is the major controlling factor for OM enrichment in the oil shale layer. In addition, factors such as mixed deposition with clay minerals, and detrital matter input cannot be ignored for their influence on OM enrichment. [ABSTRACT FROM AUTHOR]

The late Albian-Cenomanian (mid-Cretaceous) greenhouse world was characterized by major eustatic sea-level variations and environmental change. However, little is known about the drivers of Cenomanian sea-level change. This work discusses stratigraphic trends in the palynofacies and palynomorph composition for upper Albian-Cenomanian strata from the North Qarun-1x well in the Gindi Basin (Egypt). These new data were combined with previously published data and embedded into a sequence stratigraphic framework. Results reveal four third-order (ca. 1.2 Myr) transgressive-regressive sequences for the upper Albian-middle Cenomanian upper Kharita and Bahariya formations and two sequences for the upper Cenomanian Abu Roash G Member. The increasing content of dinoflagellate cysts, mainly chorate and skolochorate forms, along with microforaminiferal test linings (FTLs) and an increase in the palynological marine index (PMI) compared to decreasing trends of spores and pollen grains are indicative for the deepening of the depositional setting upon sea-level rise and vice versa. Chorate and skolochorate dinoflagellate forms, such as Dinopterygium , Florentinia , Coronifera , Spiniferites groups, usually occur in transgressive systems tracts (TST), compared to marginal marine forms, such as Subtilisphaera and Aptea ?, which are recorded in regressive systems tract (RST) sediments. Trans-regional correlation between the upper Albian-Cenomanian stratigraphic sequences from the Gindi and Abu Gharadig basins in the North Western Desert and Gulf of Suez indicates synchronous major marine transgression during the Cenomanian. Trans-regional correlation with other coeval third-order sequences from Morocco, Algeria, and Germany provide evidence for a regional to global eustatic control of sea-level changes at the northern Gondwana paleo-margin at this time. The magnitudes and timing of the late Albian-Cenomanian sea-level cycles were modest (about 35 m), pointing to glacio-eustasy as a potential driver. • Palynological-based sequence stratigraphy of upper Albian-Cenomanian strata was conducted. • Upper Kharita and Bahariya formations and Abu Roash G Member are divided into five complete transgressive-regressive sequences. • Trans-Regional correlation indicates synchronous major marine transgression during the Cenomanian. • Correlation with coeval sequences in Tethys reveals a eustatic control of sea-level at the northern Gondwana paleo-margin. • Glacio-eustasy was the main trigger of Cenomanian eustatic sea-level rise. [ABSTRACT FROM AUTHOR]

Terrestrial pre-salt sediments are not only good lithologic reservoirs similar to marine pre-salt carbonates, but also are supposed to have high hydrocarbon generation potential because the overlying terrestrial thick-bedded salts are generally preserved in basin centers. However, their status as source rock has attracted much less attention than as reservoir. In this paper, we choose the Qaidam Basin as an example and study the minerology, organic geochemistry, and organic petrology of the pre-salt sediments in a hydrocarbon-rich unit, where abundant petroleum resources have been discovered under halite beds. LithoScanner logging of six wells and X-ray diffraction results of 378 samples show that terrestrial pre-salt sediments are characterized by mixed siliciclastic-carbonate-sulphate rocks and their relative proportions are laterally varied. Quartz, feldspar and clay minerals are of allochthonous origin and classified as argillaceous endmember while dolomite and ankerite are of autochthonous origin as carbonate endmember. Organic geochemistry analysis of 118 samples show great variability in TOC, S2, and Tmax values, and this variability is not related to mineral composition heterogeneity as hydrogen index is not well correlated with detritus, carbonate, and sulphate minerals contents, indicating the interacting controls of source rocks by both exogenetic and endogenetic processes in brackish-mesosaline lakes. Strong bacterial reworking of organic matter results in the dominant macerals of pre-salt sediments as bituminite and oil generation at low maturity levels. Sealed by impermeable halite beds and in close proximity to reservoirs, the generating hydrocarbon by pre-salt good source rocks is effectively accumulated, which entails hydrocarbon exploration under halite beds in other terrestrial salt-bearing basins. • Terrestrial pre-salt sediments are mixed with siliciclastic, carbonate, and salt minerals. • TOC contents and S2 show great heterogeneity in pre-salt sediments. • TOC contents are uncorrelated with any major mineral content. • The dominant macerals of pre-salt sediments are bituminite. • Terrestrial pre-salt sediments are self-sourced and self-reservoired. [ABSTRACT FROM AUTHOR]

The Late Triassic mudstones are considered to be the most significant hydrocarbon source rocks (TOC: 0.54%–3.29%) in the North Qiangtang Depression, eastern Tethys. Here, we present geochemical data from the Woruo Mountain Carnian mudstones, in order to investigate their paleoclimate, paleoenvironment, and provenance and to analyze the mechanism of organic material accumulation. The paleoclimate condition was warm and humid during the Carnian mudstones deposition, as indicated by moderate chemical index of alteration (CIA; 73–76), which may be connected with the Late Triassic Carnian stage global climate event in the Tethys. The low U/Th (0.17–0.25) and C org /P tot (7–33) ratio values and moderate manganese contents, reflect the oxidizing conditions during the Carnian mudstones deposition. The relatively high primary productivity in this study is supported by the relatively high P concentrations. The Al 2 O 3 –(CaO* + Na 2 O)–K 2 O ternary plot and Th/Sc–Zr/Sc crossplot reflect that the source areas have undergone a medium chemical weathering with weak sedimentary recycling. The TiO 2 –Zr, Co/Th–La/Sc, La/Th–Hf, and La/Yb–∑REE bivariate diagrams indicate that the provenance of Carnian mudstones was primarily from felsic igneous rocks. The collision setting has been identified based on the multi-major elements discriminate plots in the present study. The Riwanchaka and Mayigangri masses to the southwest and south of the study area consisting mainly of Middle-Triassic granodiorite and Late-Triassic granite are likely responsible for supplying provenance to the Woruo Mountain Carnian mudstones, which have similar REE patterns. The relatively high TOC contents of Carnian mudstones are related to high paleoproductivity and fast sedimentation rates, which will lead to preservation of some organic matter even when bottom waters are completely oxidizing. The detrital input during the Carnian mudstones deposition would result in dilution of organic matter. [ABSTRACT FROM AUTHOR]

Organic-rich mudstones in the Lower Permian Zhanjin Formation may be primary source rocks in the Qiangtang Basin. However, the factors controlling organic matter accumulation remain unclear. Based on total organic carbon (TOC), geochemical, and mineral data, the paleoclimate, redox conditions, paleosalinity, hydrothermal activity, and primary productivity of the Zhanjin Formation mudstones (well QZ-5) were studied to discuss the organic matter accumulation mechanism. The Zhanjin Formation mudstones at 688-675 m and 672–622.8 m have higher TOC contents (avg. 1.52%) than those (avg. 0.94%) at 674-673 m and 662–659.5 m. Medium chemical index of alteration (CIA) values of samples at 688-675 m and 672-668 m indicate a warm and humid climate, whereas those at 674-673 m and 666–659.5 m indicate rapid cooling due to low CIA values. High Sr/Ba and S/TOC ratios indicate a high-salinity water column during the Zhanjin Formation mudstones deposition. Fluctuating U EF and Mo EF values and low to medium Th/U, V/Cr, and C org /P ratios suggest that these studied mudstones formed in an oxic to dysoxic environment. Relatively low Al/(Al + Fe + Mn) ratios, positive Eu anomalies, and Fe–Mn-(Cu + Co + Ni) × 10 and Co–Zn–Ni ternary diagrams indicate that these mudstones were influenced by atypical (weak) hydrothermal activity associated with relatively high femic terrigenous detrital influx. Relatively high P/Al ratios reveal that the level of primary productivity in the surficial water column was relatively high, but high Ni/Al and Cu/Al ratios may be influenced by paleoclimate and hydrothermal activity. Good correlations among the TOC concentrations and paleoclimate, detrital influx, and hydrothermal activity indicators combined with poor correlations between TOC and primary productivity, paleosalinity and redox proxies demonstrate that organic matter accumulation in the Zhanjin Formation mudstones was mainly controlled by detrital influx (adsorption of clay minerals) and hydrothermal activity, but not by primary productivity levels and preservation conditions such as paleosalinity and redox conditions. [Display omitted] • ►Zhanjin Formation mudstones formed in an oxic-dysoxic environment with high salinity. • ►Atypical hydrothermal activity was involved during the deposition of these mudstones. • ►OM accumulation was mainly controlled by hydrothermal activity and detrital influx. • ►Primary productivity, paleosalinity and redox conditions on OM accumulation are limited. [ABSTRACT FROM AUTHOR]

The Upper Triassic successions in the Qiangtang Basin are important targets for petroleum exploration, although they are poorly understood at present. The black mudstones collected from well QZ-16 are the deepest Upper Triassic source rocks to date and have over-mature organic matter (OM) (R o > 2). In this study, the total organic carbon (TOC) and systematic inorganic geochemistry were determined to reconstruct depositional conditions (palaeoclimate, palaeosalinity, primary productivity, redox conditions, sedimentation rate, and detrital influx) and to assess the OM accumulation mechanism. The TOC contents of the Bagong Fm. mudstones first decrease and then increase from bottom to top (Unit A to Unit D), and Unit C has the lowest TOC contents. These mudstones have relatively obvious correlations between TOC content and the palaeosalinity proxy (Sr/Ba), redox indexes (U/Al, V/Al, Mo/Al, U/Th, and V/Cr) and sedimentation rate indicator ((La/Yb) N), which indicate that OM accumulation was dominated by both high salinity and a slow sedimentation rate in a dysoxic environment. The high Al 2 O 3 /TiO 2 ratios indicate that the provenance of these mudstones was primarily from felsic igneous rocks. High detrital influx of felsic igneous rock provenance would have diluted the OM and inhibited OM preservation, as evidenced by the negative correlations between TOC content and detrital influx parameters, including SiO 2 , Al 2 O 3 , TiO 2 , and K 2 O. The primary productivity (P/Al and Ba/Al) is poorly correlated with TOC for these mudstones, suggesting that its effect on OM accumulation was negligible or limited. The relatively high TOC contents of mudstones from Units A and B are mainly related to the interaction of dysoxic conditions, a high-salinity water column, slow sedimentation rates, and a low detrital influx. An oxidizing water column with low salinity and a relatively high detrital influx and sedimentation rate upward in Unit C led to greater OM degradation and dilution and the lowest TOC contents. The significant TOC content fluctuations in Unit D may be related to vertical variations in the depositional conditions. [Display omitted] • OM accumulation mechanism of the over-mature source rocks are presented in detail. • The effect of primary productivity on OM accumulation was limited in these mudstones. • Preservation conditions were the main factors controlling the OM accumulation. [ABSTRACT FROM AUTHOR]