Niubao Formation Research Articles

Magnetostratigraphy of the Niubao Formation in the Bangong‐Nujiang Suture Zone: Constraints on the Amount of Crustal Shortening of the Tibetan Plateau Since 60 Ma

AbstractThe Early Cenozoic sedimentary archives of Tibet are crucial for elucidating the geodynamic processes related to continental collision, particularly crustal deformation and shortening. However, the lack of a reliable chronostratigraphic framework for the Cenozoic sedimentary basins in central Tibet has hindered a comprehensive understanding of the influence of the continuous India‐Asia convergence process on the tectonic evolution of the Tibetan hinterland. Here, we report new magnetostratigraphic ages and paleomagnetic results for the Niubao Formation exposed in the northern bank of the Siling Co. Positive field tests and microscopic observations suggest that the remanence is primary. We determine that the depositional age of the measured section was between 59.2 and 48.6 Ma. Our results reveal that the paleogeographic location of the Bangong‐Nujiang Suture Zone during the Late Paleocene–Early Eocene was 25.4 ± 2.1°N, thus supporting the model of two‐stage collision between India and Asia.

First fruit record of Pterocarya (Juglandaceae) from the upper Eocene of the central Qinghai-Tibetan Plateau, China

The Juglandaceae family experienced significant diversification during the early Tertiary, as evidenced by fossil records showing a broad expansion of both extant and extinct taxa. The genus Pterocarya is characterized by its distinctive fruit with butterfly-shaped wings and a small nutlet. Macrofossil records suggest that this genus was distributed widely in the Northern Hemisphere. However, the fossil record of Pterocarya in China is limited. In this study, we describe a well-preserved Pterocarya fossil winged fruit from the middle-upper member of the Niubao Formation (the upper Eocene) of the central Qinghai-Tibetan Plateau, China. The winged fruit is identified as Pterocarya liae sp. nov. based on detailed morphological comparison, representing the earliest known record of Pterocarya winged fruit in Asia. The new finding extends the paleobiogeographic distribution of Pterocarya during the Eocene and provides new insights into the early stage of the diversification of this genus.

Provenance Analysis in the Nima Basin during Paleogene and Its Implications for the Decline of the Tibetan Central Valley.

It is unclear what caused the Bangong Nujiang suture zone in the central Tibetan plateau to rise from less than 2 km in early Cenozoic to more than 4 km at present. The zircon U-Pb ages and trace elements of samples from the Niubao Formation in the Paleogene of the Nima basin were analyzed and tested. Combined with the isostasy theory, the surface uplift height of the Nima Basin during the Cenozoic period was calculated. The zircon U-Pb age results of the Niubao formation are consistent with the ages of the Lhasa terrane on the south side of the basin, the Qiangtang terrane on the north side, and the uplift in central. The zircon Eu/Eu* results show that the crust in central part of Tibetan plateau thickened by ∼20 km in Paleogene, resulting in ∼3 km surface uplift. Sediments created a total of about 1 km of surface uplift throughout the Paleogene, and the deposition rate began to slow down significantly at ∼40 Ma. Therefore, it is inferred that in the early Cenozoic, the uplift of the valley was mainly caused by sedimentation. With the continuous downward subduction of the Indian plate, at about 40 Ma, factors such as crustal shortening dominated the uplift of the central valley, and the uplift caused by deposition only accounted for a very small part. In general, the uplift of the Central Valley in the Paleogene was mainly affected by crustal shortening, but a quarter of the surface uplift was caused by the accumulation of sediments.

Cold climatic snaps during the Eocene-Oligocene transition in the central Tibetan Plateau: Implications for ice-induced sedimentary structures and isotope geochemistry

The Eocene-Oligocene transition was a time of major climate instability and a shift from an ice-free world to a predominant icehouse climate. Identifying the ancient glacial deposits and their associated sedimentary structures are of paramount significance for understanding the Earth's climate in the Cryogenian Period. Ice-induced sedimentary structures (IISS) are considered as types of primary structures preserved on surfaces and in upper parts of unconsolidated sediments by physical cryospheric processes of seasonal ice freezing and thawing, and therefore sensitive not only to climate, but also to paleogeography, hydrology, and type of environment. Despite the common occurrence of modern IISS in a wide variety of environments, from high-latitude regions to low-latitude areas of high altitude, available records from ancient geologic archives are scarce. Herein, we integrated sedimentological and geochemical proxies to study IISS and geochemical signals as reliable indicators of cold climate in an outstanding example of the Eocene-Oligocene interval from the Niubao Formation in the Lunpola Basin, central Tibet. Detailed field sedimentological, petrographic microscopic investigations, and bulk carbonate oxygen (δ18Ocarb) and carbon (δ13Ccarb) isotope geochemistry were conducted in this lacustrine succession. Seven sets of IISS are represented by ice crystal marks, frozen cracks, ice-frozen bubbles, ice-expanding laminae, ice-rafted debris (dropstones), ice water pea-like, and ice water pits. They are classified into three groups based on major deriving mechanisms: ice-related crystal marks, deformation-related ice structures, and gravitational fall IISS. Positive δ18Ocarb excursions at the Eocene-Oligocene transition provide evidence of a shift toward cold or near-freezing climatic snaps in response to global cooling episodes at this time. Integrating the paleogeomorphic scenario of central Tibet with the stratigraphically reported IISS and glendonite clusters, the results support that the Lunpola lake sustained cold or near-freezing conditions at Eocene-Oligocene transition. These findings provide one of the most comprehensive sedimentological and isotopic analyses of ancient IISS and a more direct spatial-temporal constraint for the regional climate system of central Tibet.

Eocene–Oligocene glaciation on a high central Tibetan Plateau

Abstract The nature of glaciation (bipolar vs. unipolar) during the Eocene–Oligocene transition (EOT) remains unresolved. Here, we report the occurrence of frost marks, ice-rafted debris (IRD), and glendonites from the Upper Eocene to Lower Oligocene Niubao Formation (Fm.) deposited in a proglacial lake above glaciolacustrine conglomerates and diamictite facies in the Lunpola Basin, central Tibetan Plateau (CTP). Magnetostratigraphy dates these cryospheric deposits to ca. 36.2–31.8 Ma, synchronous with a stratigraphic interval containing IRD offshore of SE Greenland and in the Barents, Chukchi, and Laptev Seas, suggesting a strong continental-oceanic coupling. Our results provide robust continental evidence for intermittent cryospheric processes in the midlatitude Northern Hemisphere during the late Eocene and EOT. The global cold snap EOT-1 influenced already glacierized high-altitude mountains, lowering equilibrium line altitudes (ELAs) of glaciers and leading to local development of ice fields, ice caps, and valley glaciers with proglacial lake systems, such as the one recorded in the Niubao Fm. The record of IRD, glendonites, and frost marks before the onset of EOT-1 points to an active cryosphere on a plateau already elevated by ca. 36.2 Ma.

An Oligocene-Miocene intermontane narrow lowland in the central Tibetan Plateau: Insights from provenance analysis and palynological record of a Cenozoic sedimentary succession

• The Niubao Formation in the Avengco Basin, the western part of the BNSZ, was deposited during Oligocene-Miocene with bidirectional sources. • The Oligocene-Miocene paleoelevation of the Avengco Basin was 3090-3290 m. • A 3 km high intermontane lowland on the BNSZ were formed during the Oligo-Miocene. • This intermontane lowland was probably formed by the reactivation of thrust belts and cloud extend westward to the Avengco Basin. A series of Cenozoic basins are distributed along the Bangong-Nujiang suture zone and amalgamate into an east-west trending topographic lowland. The reconstruction of the formation process for the lowland is critical to understanding the evolution of the Tibetan Plateau and paleoclimate. Here we present results from sedimentology, zircon U-Pb ages, and palynological records of the Niubao Formation in the Avengco Basin, the western part of central Tibet. The Niubao Formation consists of conglomerate, sandstone, siltstone, marl, and limestone, which were deposited in alluvial fan and shallow lake. The palynological assemblages suggest that the Niubao Formation in the Avengco Basin was deposited during the Oligocene-Miocene. Provenance analyses suggest that the Niubao Formation can have bidirectional sources, including the Southern Qiangtang terrane and/or Bangong-Nujiang suture zone in the north and the Northern Lhasa terrane in the south. By using the co-existence approach, palynological records from the Niubao Formation yielded a paleoelevation of 3090-3290 m after temperature and lapse rate correction, which was 1-2 km lower than those of the Lhasa and Qiangtang terranes during the Oligocene-Miocene. Based on the above observations and previous studies from the coeval sediments of the Lunpola and Nima Basins in central Tibet, we propose that the intermontane lowland along the Bangong-Nujiang suture zone was formed as a whole during the Oligocene at the latest and extended to the Avengco Basin in the west.

A New Genus of Spittlebugs (Hemiptera, Cercopidae) from the Eocene of Central Tibetan Plateau.

Simple SummaryThe superfamily Cercopoidea is commonly named as “spittlebugs”, as its nymphs excrete a spittle mass to protect themselves and hide from predators and parasites. Cosmoscartini (Cercopoidea: Cercopidae) is a large and brightly colored Old World tropical tribe, including 11 genera. A new genus Nangamostethos gen. nov. (type species: Nangamostethos tibetense sp. nov.) of Cosmoscartini is described from Niubao Formation, the late Eocene (ca. 39 million years ago) of central Tibetan Plateau (TP), China. Its placement is ensured by comparison with all the extant genera of the tribe Cosmoscartini. The new fossil represents one of the few fossil Cercopidae species described from Asia. It is likely that Nangamostethos was extinct from the TP due to the regional aridification and an overturn of plant taxa in the last 33 million years.The superfamily Cercopoidea is commonly named as “spittlebugs”, as its nymphs produce a spittle mass to protect themselves. Cosmoscartini (Cercopoidea: Cercopidae) is a large and brightly colored Old World tropical tribe, including 11 genera. A new genus Nangamostethos gen. nov. (type species: Nangamostethos tibetense sp. nov.) of Cosmoscartini is described from Niubao Formation, the late Eocene of central Tibetan Plateau (TP), China. Its placement is ensured by comparison with all the extant genera of the tribe Cosmoscartini. The new fossil represents one of few fossil Cercopidae species described from Asia. It is likely that Nangamostethos was extinct from the TP due to the regional aridification and an overturn of plant taxa in the late Paleogene.

The first Chinese representative of the Cenozoic hawker dragonfly Oligaeschna (Odonata: Aeshnidae) from the Eocene of North Tibet

ABSTRACT Oligaeschna sinica sp. nov., the oldest recorded and first Chinese representative of the Cenozoic aeshnid genus Oligaeschna, is described from the Bartonian of Tibet. This fossil is also among the oldest known representatives of the family Aeshnidae sensu stricto. Its presence in the Niubao Formation supports a temperate, cool to warm climate for the area during this period. urn:lsid:zoobank.org:pub:9BAD44C0-135D-40AF-B3DD-1B7D695720A8 urn:lsid:zoobank.org:act:6D7E18CF-1A22-4B37-B816-BAD0890C39F9

Fossil Capsular Valves ofKoelreuteria(Sapindaceae) from the Eocene of Central Tibetan Plateau and Their Biogeographic Implications

Premise of research. Koelreuteria encompasses three extant species that are discontinuously distributed in East Asia and the Fiji Islands. Koelreuteria had a wide distribution in the Northern Hemisphere during the Paleogene, with the first known fossil record being from the early Eocene of western North America. This study reports early fossil occurrences of Koelreuteria in Asia with a detailed morphological investigation and considers the history of its biogeographical dispersal in light of these finds.Methodology. Four capsular valve fossil species of Koelreuteria are recognized from the early middle Eocene of the Niubao Formation at the Jianglang site (present altitude 4850 m), Bangor County, central Tibetan Plateau, China. They are compared with previous fossil occurrences using existing literature and herbarium records of extant material.Pivotal results. We describe a new species, Koelreuteria kvacekii P.-R. Chen, C. Del Rio & T. Su sp. nov., characterized by its oblate capsular valve, lobed apex, and cordate base of the valve, as well as one or two suborbicular seeds per locule. Other material is assigned to species previously described from other locations, extending the geographic and stratigraphic ranges for K. miointegrifoliola, K. allenii, and K. lunpolaensis.Conclusions. Fossil records of Koelreuteria and other co-occurring plants evidence a subtropical climate in central Tibet during the middle Eocene, contrasting with the present cold and dry climate in this region. These are the earliest fossil records of Koelreuteria from what is now the Tibetan Plateau. It seems that the Tibetan region served as an early diversification center for Koelreuteria during the Paleogene in Asia. All species of this flora disappeared locally because of the formation of the Tibetan Plateau by the Neogene.

The organic geochemical characteristics from the Palaeogene lacustrine source rock in the Nyima Basin, Central Tibet, and their geological significance

The Lunpola Basin is a fault‐depression type residual Palaeogene continental basin, and it is the only basin located in Tibet in which industrial oil has been discovered. In recent years, as research has progressed, quantities of oil shale and asphalt were discovered in the adjacent Nyima Basin, which shows a good prospect for exploration. In this study, the organic geochemistry, n‐alkanes, biomarkers, and maturity parameters of the samples were analysed using gas chromatography (GC) and mass spectrometry (MS). The organic geochemistry revealed that the Niubao Formation has a high organic matter abundance (total organic carbon content of 0.15–8.43 wt% and hydrogen index of 31.25–1721.15) and is dominated by type I–II2 kerogen. The GC and MS analyses indicate that the oil shales of the Nyima Basin have a high organic matter abundance and a good hydrocarbon production potential, and the main organic matter sources were aquatic algae and terrestrial plants, as evidenced from the unimodal/bimodal distributions of the n‐alkanes. Besides, the high phytoalkane, gammacerane, homohopane, and gypsiferous mudstone contents in the Nyima Basin indicate a deep, hypoxic, well‐stratified, saline lacustrine sedimentary environment. Under warm climatic conditions, accelerated continental weathering increased nutrient input, leading to water‐mass eutrophication and algal blooms in the water column, while arid conditions and intensified evaporation induced saline stratification, leading to enhanced preservation of organic matter and the formation of high‐quality source rock layers in this lake basin. Therefore, the Eocene global greenhouse effect may have been the main reason for the formation of the large sets of oil shales in the Nyima Basin.

The influence of sedimentary systems and paleo-geomorphology on the physical properties of a reservoir in the Niubao formation, Lunpola basin, central Tibetan Plateau

The influence of sedimentary systems and paleo-geomorphology on the physical properties of a reservoir in the Niubao formation, Lunpola basin, central Tibetan Plateau

Origin and formation model of Eocene dolomite in the upper Niubao Formation of the Lunpola Basin, Tibetan Plateau

Using fresh core samples, we have determined the origin and formation process of Eocene lacustrine dolomites in the Tibetan Plateau through petrological, mineralogical, and geochemical analyses. Dolomitic rocks were collected from the upper member of the Eocene Niubao Formation in the Lunpola Basin, consisting of dolomitic mudstone, argillaceous dolomite, dolomite-bearing mudstone, and mud-bearing dolomite. These dolomites are dominated by aphanotopic and microcrystalline dolomites, with minor amounts of euhedral or subhedral powder- and fine-crystalline dolomites. Carbon and oxygen stable isotopes, combined with ubiquitous gypsum in study area, indicate a semisaline continental lake under strong evaporative conditions. The revealed relatively high temperature of the dolomitization (33.8°C–119.1°C), combined with hydrothermal minerals such as cerous phosphate and barite, reflect the participation of dolomite from hot fluids. Moreover, the inferred dolomitization temperatures decrease gradually toward the center of the lake basin, suggesting the resurgence of hydrothermal fluids along a fault zone on the lake margin. This proves that frequent thermal events occurred at the boundary fault of the Lunpola Basin margin during early Himalayan orogenesis. In addition, Jurassic carbonates interacting with hydrothermal fluids, as well as strong evaporation conditions, likely provided favorable conditions for the formation of primary lime sediments. A rich source of [Formula: see text] brought by volcanic ash, hydrothermal fluids, and the Jurassic carbonates then created conditions for dolomitization during the depositional period. Strong evaporation under a relatively hot climate enhanced penecontemporaneous dolomitization, thus forming dolomite. Tibetan Plateau was under arid to semiarid climate conditions, and there was a widespread distribution of dolostones in western, central, and northern China during the Eocene period. The hydrothermal dolomites of the upper Niubao Formation testify for active hot springs, whereas lacustrine dolomite imply arid or semiarid climates during the Eocene, in the early stages of Himalayan orogenesis.

Fossil fruits of Illigera (Hernandiaceae) from the Eocene of central Tibetan Plateau

AbstractIlligera (Hernandiaceae) is a liana genus distributed mainly in the tropical Asia and Africa. Previous fossil records suggested that Illigera was restricted in western North America during the Eocene. Recent paleobotanical investigation has unveiled a Paleogene flora that is totally different from today's vegetation in central Tibet. This provides novel insights into the paleoenvironmental change during the evolution of the Tibetan Plateau (TP). Here, we investigated 10 fruit impressions of Illigera from the early middle Eocene Niubao Formation in the Bangor Basin, central TP. The fossil winged fruits are characterized by their eroded fruit wings, well preserved fusiform locular areas with a median ridge bisecting the fruit, and short veins fanning radially outward. These features allow us to assign these fossils to Illigera eocenica, a species originally discovered in the Eocene of western North America. This is the second fossil occurrence of Illigera worldwide, and the first in Asia. Our finding suggests a warm and humid climate in the central TP during the early middle Eocene, and a close floristic link between Asia and North America during the Paleogene. We also propose a Northern Hemisphere origin and a Paleogene dispersal event from Northern Hemisphere to Africa for Illigera.

Evaluation of sedimentary features and biomarkers in the Paleogene Niubao formation in the Lunpola basin, Tibetan plateau: implications for the oil source rocks and exploration

The Paleogene Niubao formation is mainly exposed in the central part of the Lunpola basin in the hinterland of the Tibetan Plateau and has high potential for oil and gas exploration and development. The biomarkers of source rocks are systematically studied by selecting representative samples. The middle and upper Niubao formation contain rhythmically inter-bedded mudstone and sandstone that show periodic variations characteristic of the lake water environment. Additionally, the large volumes of muddy dolomite in the upper Niubao formation (UNF) are associated with high salinity in the lake environment under a hot and arid climate. The organic geochemical analysis shows that the middle Niubao formation (MNF) and UNF have high organic abundance (total organic carbon (TOC) contents of 1.48–8.96% in the MNF and 1.52–12.83% in the UNF) and type I–II kerogen. The analysis shows that the UNF was deposited in a persistent, anoxic, stratified and highly saline paleo-lake environment, which was beneficial to the preservation of organic matter. The high phytoalkane and γ-paraffin contents in the UNF indicate a strongly reducing and highly saline lacustrine depositional environment. The high pregnane contents and ascending sterane in the source rocks in the MNF indicate high maturity, and the distribution of n-alkanes and regular sterane indicates the input of some terrestrial organic matter. The high maturity indicates that the source rock in the MNF may represent the main hydrocarbon-generating layer in the basin.

Fossil record of Ceratophyllum aff. muricatum Cham. (Ceratophyllaceae) from the middle Eocene of central Tibetan Plateau, China

Ceratophyllaceae Gray is a cosmopolitan submersed family in angiosperm. The fossil record is mainly composed of fruits ranging in age from the Cretaceous to the Neogene within the Northern Hemisphere. However, its fossil record in Asia is sparse. Here we report on a fossil fruit attributed to Ceratophyllum aff. muricatum Cham. from the early middle Eocene of the Niubao Formation in Bangor County, central Tibetan Plateau, China, which is characterized by marginal lateral spines, a style and two basal spines and a small smooth, possibly warty body. This is the earliest fossil record of Ceratophyllaceae in Asia, and it indicates a dispersion from North America to Asia as well as a much wider distribution of Ceratophyllum during the Eocene than previously thought. Moreover, this fossil record demonstrates that the depositional site corresponds to a shallow freshwater environment. The climate was tropical to sub-tropical, consistent with environmental conditions of other fossils findings in the same site. Besides, the climate condition was totally different from the present, largely due to the Neogene formation of central Tibetan Plateau.

Nucleation and stabilization of Eocene dolomite in evaporative lacustrine deposits from central Tibetan plateau

AbstractIn past decades, the formation of dolomite at low temperature has been widely studied in both natural systems and cultured experiments, yet the mechanism(s) involved in the nucleation and precipitation of dolomite remains unresolved. Late Eocene dolomitic deposits from core in the upper Niubao Formation (Lunpola Basin, central Tibetan Plateau, China) are selected as a case study to understand the dolomitization process(es) in the geological record. Dolomite formation in Lunpola Basin can be ascribed to a different mechanism forming the large quantities of replacive dolostones in the geological record; and provides a potential fossil analogue for primary dolomite precipitation at low temperature. This analogue consists of an alternation of laminated dolomitic beds, organic‐rich and siliciclastic layers; formed in response to intense evaporation interpreted to take place in a continental shallow lake environment. Mineralogical, textural and stable isotopic evaluations suggest that the dolomite from those dense‐clotted laminated beds is a primary precipitate. At the nanoscale, these dolomitic beds are composed of Ca–Mg carbonate globular nanocrystals (diameter 80 to 100 nm) embedded in an organic matrix and attached to clay flakes. Micro‐infrared spectroscopy analyses have revealed the presence of aliphatic compounds in the organic matrix. Microscopic and elemental compositional studies suggest that clay surfaces may facilitate the nucleation of dolomite at low temperature in the same way as the organic matrix does. The dolomite laminae show values for δ18OVPDB from −3.2 to −1.76‰ and for δ13CVPDB from −2.62 to −3.78‰. Inferred δ18OSMOW values of the lake water reveal typical evaporitic hydrological conditions. These findings provide a potential link to primary dolomite formation in ancient and modern sedimentary environments; and shed new light on the palaeoenvironmental conditions in central Tibet during the Eocene.

Asclepiadospermum gen. nov., the earliest fossil record of Asclepiadoideae (Apocynaceae) from the early Eocene of central Qinghai-Tibetan Plateau, and its biogeographic implications.

Apocynaceae is common in the fossil record, especially as seed remains from the Neogene of Europe and North America, but rare in Asia. Intrafamilial assignment is difficult due to the lack of diagnostic characters, and new fossil and modern data are needed to understand the paleobiogeography of this group. We studied three Apocynaceae seed impressions from the Lower Eocene Niubao Formation, Jianglang village, Bangor County, central Qinghai-Tibetan Plateau. Morphological data from living and fossil species were phylogenetically mapped to enable systematic assignment. We describe a new genus, Asclepiadospermum gen. nov., and two new species, A. marginatum sp. nov. and A. ellipticum sp. nov. These species are characterized by an elliptical seed, a margin surrounding the central part of the seed, and polygonal, irregular, and small epidermal cells, and differ mainly in terms of the size of the margin and the shape of the apex. All these characters indicate that this new genus belongs to the subfamily Asclepiadoideae (Apocynaceae). These fossils represent the earliest fossil seed records of Asclepiadoideae. Asclepiadospermum indicates a humid tropical to subtropical flora during the early Eocene in central Tibet. Moreover, our discoveries indicate a close floristic connection between Eurasia and Africa during the early Eocene, which expands our knowledge of the floristic linkage between Tibet and other regions at that time.

Extinct genus Lagokarpos reveals a biogeographic connection between Tibet and other regions in the Northern Hemisphere during the Paleogene

AbstractThe biodiversity history on the Qinghai–Tibetan Plateau (QTP) has remained unclear for a long time. Recent paleobotanical investigations provide opportunities for revealing the phytogeographic history on the QTP in deep time and for testing phylogeographic hypotheses. Lagokarpos is an extinct genus with unknown modern affinities. It is easily distinguished by its fruits, which have two long wings and an elliptical fruit body. Previously, Lagokarpos fossils have only been found from North America and Germany, ranging from the latest Paleogene to early middle Eocene. Recently, we found fossil fruits of this genus from the Niubao Formation, near Bangoin County in the central QTP. A new species, namely Lagokarpos tibetensis H. Tang, T. Su & Z. K. Zhou sp. nov., is described. This is the first fossil record of the genus in Asia. The occurrence of L. tibetensis indicates a close floristic linkage between the QTP and other floras in the Northern Hemisphere during the Paleogene. According to the floristic assemblages, we suggest the central QTP experienced a tropical or subtropical humid climate during that period.

Hydrocarbon Generation, Migration, and Accumulation in the Eocene Niubao Formation in the Lunpola Basin, Tibet, China: Insights from Basin Modeling and Fluid Inclusion Analysis

The Eocene Niubao Formation is the primary research target of oil exploration in the Lunpola Basin. Crude oil was extracted from Well Z1 on the northern margin of the basin in 1993. In this study, an integrated evaluation of the source rock, geothermal, and maturity histories and the fluid inclusion and fluid potential distributions was performed to aid in predicting areas of hydrocarbon accumulation. Due to the abundance of organic matter, the kerogen types, maturity, and oil-sources correlate with the geochemical data. The middle submember of the second member of the Niubao Formation (E2n2-2) is the most favorable source rock based on the amount of oil produced from the E2n2-3and E2n3-1reservoirs. One- and two-dimensional basin modeling, using BasinMod software, shows that the E2n2-2source rock started to generate hydrocarbon at 35-30 Ma, reached a maturity of Ro=0.7% at 25-20 Ma, and at present, it has reached the peak oil generation stage with a thermal maturity of Ro=0.8% to less than Ro=1.0%. By using fiuid inclusion petrography, fiuorescence spectroscopy, and microthermometry, two major periods of oil charging have been revealed at 26.1-17.5 and 32.4-24.6 Ma. The oil accumulation modeling results, conducted by using the Trinity software, show a good fit of the oil shows in the wells and predict that the structural highs and lithologic transitions within the Jiangriaco and Paco sags are potential oil traps. KEY WORDS:Niubao Formation, Lunpola Basin, source rocks, basin modeling, fluid inclusions, hydrocarbon migration and accumulation, petroleum geology.

Geochemical characteristics and Thermal Evolution of Paleogene Source Rocks in Lunpola Basin, Tibet Plateau

Lunpola Basin is the first exploration area with industrial oil flow in the hinterland of Tibetan Plateau and the target horizon is the Paleogene Niubao formation and Dingqinghu formation. Based on drilling data from multiple wells, the organic matter abundance, type and maturity of Paleogene source rocks in Lunpola Basin were measured and analyzed using organic geochemistry method. The 67 samples data from middle Niubao formation show that the average of TOC (wt.) is 3.50, S1+S2 is 30.55 mg/g; and the Tmax is 427 ∼ 442 °C, δ13CPDB is -25.6‰ ∼ -29.3‰ and Ro is 1.37%∼1.84%. The 53 samples data from upper Niubao formation show that the average of TOC (wt.) is 4.16%, S1+S2 is 30.16 mg/g. and the Tmax is 421 ∼ 440 °C, δ13CPDB is -26.9‰ ∼ -28.7‰ and Ro is 0.61%∼1.35%. The 70 samples data from lower Dingqinghu formation show that the average of TOC (wt.) is 4.16%, S1+S2 is 30.16 mg/g and Ro is 0.58%, and the Tmax is 421 ∼ 440 °C, δ13CPDB is -26.9‰ ∼ -28.7‰. These three sets of source rocks have high organic abundance and kerogen type of I- II1. Based on the previous studies of W-1 well in JiangjiaCo depression, XL-3 well in JiangriaCo depression and Z-1 well PaCo depression, a 2D model of north-south tectonic and thermal evolution in Lunpola basin is simulated by Schlumberger's PetroMod method. The results show that the XL-5 well of Niubao formation in Diezong area of Lunpola basin began to generate oil at 41.3Ma and reached the peak of hydrocarbon generation at 36.4Ma; and upper Niubao formation began to generate oil at 38.4Ma and hydrocarbon generation peaked at 31.9Ma. The lower Dingqinghu formation began to generate oil at 24.2Ma. At the end of upper Dingqinghu formation, the basin began the uplift and denudation stage at 23Ma, but the middle and upper Niubao formation and the Lower Dingqinghu formation are still active for hydrocarbon generation. The simulation also shows that the primary deposits in main source rocks are thin in the thrust uplift belt of southern basin and have not entered the stage of oil generation, and that the main oil source area is the Paleogene depression in central basin.