East Gobi Basin Research Articles

Conditions for uranium biomineralization during the formation of the Zoovch Ovoo roll-front-type uranium deposit in East Gobi Basin, Mongolia

The Zoovch Ovoo uranium roll-front-type deposit is hosted in the Sainshand Formation, a Late Cretaceous siliciclastic reservoir, which constitutes the upper part of the post-rift infilling of the Mesozoic East Gobi Basin in SE Mongolia. The Sainshand Formation consists of unconsolidated medium-grained sand, silt and clay intervals deposited in fluvial-lacustrine settings. The uranium deposit is confined within a 60–80 m thick siliciclastic sequence inside aquifer-driven systems. The overall system experienced shallow burial and was never subjected to temperatures higher than 40 °C. This study proposes a comprehensive metallogenic model for this uranium deposit. Sedimentological and mineralogical observations from drill core samples to the microscopic scale (optical and Scanning Electron Microscopy) together with in situ geochemistry of late-formed phases (Laser Ablation–Inductively Coupled Plasma Mass Spectrometry, Electron Probe Microanalysis, Fourier Transform–Infrared Spectroscopy) were considered for the reconstruction of the main stages of U trapping.In the mineralized zone, the uranium ore is expressed as Ca–enriched uraninite (UO2) and less commonly as Ca–enriched phospho-coffinite (U, P)SiO4. Trapping mechanisms include i) complexation (i.e. uranyl-carboxyl complexes), ii) adsorption on organic or clay particles) and iii) reduction by pyrite and by bacterial activity to amorphous uraninite. In all cases, the organic matter plays either the role of trap for uranium or nutrient for bacteria that can trap uranium through their metabolism. The shallow burial diagenesis conditions do not allow direct reduction of U(VI) by organic carbon. The δ34S values of the iron disulfide are very diverse, fluctuating in extreme cases between −50 to + 50‰, with an average δ34S value for framboidal pyrite at 2‰, and −20‰ for euhedral pyrite. The positive and negative values reflect close versus open fractionation systems, while bacterial sulphate reduction (BSR) is active during the whole diagenetic history of the deposit as an essential source of reduced sulfur. Therefore, using detrital organic matter as a carbon source, microorganisms play a significant role in uranium trapping, either as a direct reducing agent for uranium or pyrite formation, which will trap uranium through redox driven epigenetic processes.

Dolomite cements in Cenomanian continental sand deposits: Time evolution and significance (Zoovch Ovoo U-deposit, East Gobi Basin, Mongolia)

Shallow buried unconsolidated sands generally provide very little information about diagenesis as most detrital minerals remain unchanged. However, in rare cases carbonate cemented nodules or sandstone layers may occur inside unconsolidated series. These cements could help to reconstruct the chronology of events from early to late phase stages of diagenesis. The Late Cretaceous sequence of the Zoovch Ovoo depocenter in East Gobi Basin is represented by 600 m of clastic deposits. The 60–80 m of Cenomanian unconsolidated sands and clays, deposited in alluvial-deltaic to lacustrine settings, compose the upper part of the post-riftmedium-grained siliciclastic reservoir, the Sainshand Formation which hosts uranium roll-front systems. Dolomite cemented sandstone layers with 10–20 cm thickness occur among the unconsolidated rock facies. Calcite is absent from this formation, but is present only in the overlying, also outcropping, Bayanshiree Formation. Samples from the dolomite cemented sandstone layers were investigated in detail to uncover their origin and diagenetic history. Four dolomite cement types were recognized that indicate recrystallization episodes and were classified based on the size and shape of the crystals, namely: (i)microcrystalline dolomite frequently associated with siderite, (ii) euhedral dolomite also associated with siderite, (iii) subhedral dolomite and (iv) finally anhedral dolomite. Their REE content varies significantly from dolomite I to IV, in particular by a strong depletion in LREE about 30 times. The contrasted precipitation of calcite in Bayanshiree and dolomite cements in Sainshand formations could be attributed to different Mg/Ca ratio of the circulation fluids in the two aquifers. Both carbonates display however rather homogenous oxygen and carbon isotopic compositions for δ18O (−10 ± 1‰ V-PDB) and δ13C (−7 ± 1‰ V-PDB). The δ18O values are interpreted as inherited from typical meteoric waters quite close to present day waters. The δ13C values indicate a mixed source of both organic and mineral carbon. All data taken into account, a full paragenetic succession was constructed. It includes the evolution of dolomite during burial diagenesis and the effects of the oxidizing roll-front uranium rich waters in the system. The latter induces partial dissolution followed by precipitation of a dolomite phase typical of roll-front zones. Carbonate cements can be thus considered as the best and rather unique geochemical indicators for the recognition of a burial history framework to paleofluid circulations and fluid-rock interactions in these intracontinental series of unconsolidated sands.

The Role of Organic Matter on Uranium Precipitation in Zoovch Ovoo, Mongolia

The Zoovch Ovoo uranium deposit is located in East Gobi Basin in Mongolia. It is hosted in the Sainshand Formation, a Late Cretaceous siliciclastic reservoir, in the lower part of the post-rift infilling of the Mesozoic East Gobi Basin. The Sainshand Formation corresponds to poorly consolidated medium-grained sandy intervals and clay layers deposited in fluvial-lacustrine settings. The uranium deposit is confined within a 60- to 80-m-thick siliciclastic reservoir inside aquifer driven systems, assimilated to roll-fronts. As assessed by vitrinite reflectance (%Rr < 0.4) and molecular geochemistry, the formation has never experienced significant thermal maturation. Detrital organic matter (type III and IV kerogens) is abundant in the Zoovch Ovoo depocenter. In this framework, uranium occurs as: (i) U-rich macerals without any distinguishable U-phase under SEM observation, containing up to 40 wt % U; (ii) U expressed as UO2 at the rims of large (several millimeters) macerals and (iii) U oxides partially to entirely replacing macerals, while preserving the inherited plant texture. Thus, uranium is accumulated gradually in the macerals through an organic carbon–uranium epigenization process, in respect to the maceral’s chemistry and permeability. Most macerals are rich in S and, to a lesser extent, in Fe. Frequently, Fe and S contents do not fit the stoichiometry of pyrite, although pyrite also occurs as small inclusions within the macerals. The organic matter appears thus as a major redox trap for uranium in this kind of geological setting.

Plate interior polyphase fault systems and sedimentary basin evolution: A case study of the East Gobi Basin and East Gobi Fault Zone, southeastern Mongolia

Structural interpretation of 2-D seismic reflection data and subsurface-outcrop correlations reveal six distinct phases of deformation recorded in the Paleozoic basement rocks and Mesozoic-Cenozoic basin fill of the East Gobi Basin (EGB), southeastern Mongolia. These phases include arc accretion and arc-continent collision in late Paleozoic time, Late Triassic sinistral shear-zone development, Early Jurassic fold and thrust belt style shortening, Middle Jurassic-Lower Cretaceous extension and rift basin development, middle Cretaceous shortening with basin inversion and regional unconformity development, and Late Cretaceous-Oligocene thermal subsidence with renewed Paleogene left-lateral strike slip faulting across the fault zone. The five post-amalgamation deformation phases are localized along the East Gobi Fault Zone, suggesting that preexisting structures and boundary conditions exert fundamental controls on the long term evolution of intracontinental basins such as the EGB. Subsurface geophysical data and outcrop correlations demonstrate that the main subbasins of the EGB contain major differences in basement metamorphic and structural fabrics, basin fill patterns, and distinct Mesozoic-Cenozoic fault generations. Potential causes related to far-field boundary conditions and other driving factors are suggested for each of the major deformation phases.

A short-lived but significant Mongol–Okhotsk collisional orogeny in latest Jurassic–earliest Cretaceous

The Central Asian Orogenic Belt (CAOB) is one of the largest orogenic collages on Earth, which records a long history of accretionary orogenesis from the late Proterozoic to the Mesozoic. Although paleomagnetic studies have unanimously suggested that the major Mongol–Okhotsk Ocean in the eastern CAOB closed mainly during the Late Jurassic–Early Cretaceous, few conclusive and self-consistent geological studies in Central Asia and East Asia have confirmed the occurrence of this significant collisional orogeny. Based mainly on basin evolution in the CAOB, Tarim Block, North China Block, and Qaidam–Qialian Block during the Late Jurassic–Early Cretaceous, the present study proposes that a short-lived but significant Mongol–Okhotsk collisional event occurred in the latest Jurassic–earliest Cretaceous. The rapid closure of the Mongol–Okhotsk Ocean was possibly related to the clockwise rotation of Siberia, which was driven by the continental collision between the Siberia Craton and the Kolyma–Omolon microcontinent in northeast Russia in the Late Jurassic. Owing to the clockwise rotation of Siberia and the subsequent closure of the Mongol–Okhotsk Ocean, a number of areas to both the east and the west of the Mongol–Okhotsk suture underwent uplift and erosion, including Lake Baikal, Transbaikalia, Sayan ranges, Altai Shan, Junggar Basin, Turfan Basin, Tarim Basin, Tian Shan, Mohe Basin, Great Xing'an ranges, Hailar–Tamsag Basin, Erlian Basin, Songliao Basin, East Gobi Basin, Yingen Basin, Bei Shan, Yin Shan–Yan Shan belt, Ordos Basin, Chaoshui–Yabulai Basin, and northern Qaidam Basin, forming a giant fold and thrust belt in northern China and Mongolia. The collision probably caused a prominent gap between Late Jurassic and Early Cretaceous magmatic episodes in the eastern CAOB and North China Block. After lasting for about 10millionyears, the main portion of the Mongol–Okhotsk Ocean closed, forming a northeast-trending suture. During the Early Cretaceous, a remnant ocean still remained in the easternmost part of the Mongol–Okhotsk Ocean, and the regional compressional setting in East Asia related to the closure of the Mongol–Okhotsk Ocean changed to escape tectonics, driven by the collision of the Lhasa Block at the southern margin of Asia. Strike–slip faulting triggered gravitational collapse of upper crust and delamination of lower crust and mantle lithosphere that was thickened during the Mongol–Okhotsk collision, resulting in the development of a continental-scale extensional setting in East Asia, associated with rift basins, metamorphic core complexes, and igneous rocks.

Hydrocarbon Prospect of Block VI, East Gobi Basin

2D seismic reflection data and magnetism data, gravity data and electrical data together define the overall subsurface structure of the East Gobi basin (EGB), and reflect Jurassic-Cretaceous intracontinental rift evolution through deposition of at least five distinct stratigraphic sequences. Three major NE-SW trending fault zones divide the basin[. In the paper, through strata sequence description and sedimentary facies analysis, as well as zone appraisal for the oil and gas in the study area, we conclude that western step-fault zone of the block VI of EGB was most favorable pay, then the reverse faulted-nose structure in the east, while in the western slope and eastern fault zone, their hydrocarbon prospect need further proved.

The circular Uneged Uul structure (East Gobi Basin, Mongolia) – Geomorphic and structural evidence for meteorite impact into an unconsolidated coarse-clastic target?

Abstract The Uneged Uul structure is a ∼10 km circular, complex, multi-ridged domal feature in the Unegt subbasin of the East Gobi Basin, southeastern Mongolia. As revealed by remote sensing and recent field reconnaissance, the central part of the Uneged Uul structure comprises a complex central peak of outward-radiating curved ridges, composed of stratigraphically uplifted greenschist-facies basement schists, surrounded by an annular moat. The most prominent feature of the structure is a central annular ridge ∼3 km in diameter composed of pebble-boulder conglomerates and gravels of the Upper Jurassic Sharilyn Formation, surrounded by three outer domal ridges composed of Lower Cretaceous conglomeratic sandstones and gypsum clays. Jurassic conglomerates forming the main part of the central annular ridge show effects of severe internal deformation. The original population of pebbles, cobbles and boulders appears moderately displaced and mostly broken but nowhere aligned along shear planes or foliated. Primary sedimentary features, such as cross-lamination or imbrication, have been obliterated. We explain this penetrative brecciation as a result of dissipative shearing caused by a strong and rapid singular event that in magnitude was beyond the range of the common crustal tectonics recorded elsewhere in this region. Disrupted and chaotically distributed conglomeratic sandstone beds in the central annular ridge dip in highly variable directions on a local scale but show an apparent SE-NW trend of bedding plane alignment. Further outside, the tilted and uplifted Upper Jurassic to Lower Cretaceous strata of the domal area are overlain by the flat-lying Upper Cretaceous, which stratigraphically constrains the timing of deformation at the Uneged Uul structure to most likely the Early Cretaceous. Endogenic formation models, such as magmatism and salt, gypsum, or mud diapirism, fail to explain the nature of the Uneged Uul structure. The Uneged Uul structure bears a set of geomorphic and structural features resembling those at some eroded complex impact structures on Earth. Morphologically similar central peaks are observed at the Spider and Matt Wilson impact structures in Australia; the central annular ridge reminds of that at Gosses Bluff in Australia; the outer domal ridges might correspond to ring-like features as known from Tin Bider in Algeria. We, therefore, cautiously propose that an impact may have produced the Uneged Uul feature causing structural uplift (∼1000 m) of basement rocks at its center. So far, no convincing evidence for shock metamorphism could be proven by field work and petrographic analyses. However, it is likely that at the time of the deformation event the unconsolidated conglomerates were highly porous and possibly immersed in groundwater buffering the propagation of sudden stress-reducing deformation. Further studies will be in order to unravel the nature of the Uneged Uul structure, which should be considered a promising possible impact structure.

Jurassic-Cretaceous lacustrine deposits in the East Gobi Basin, southeast Mongolia

Jurassic-Cretaceous lacustrine deposits in the East Gobi Basin, southeast Mongolia

Detrital modes of the East Gobi Basin (Ondor-Bogd area) sandstones (Late Jurassic-Early Cretaceous) in southeastern Mongolia and their geological implications

Sandstones of the Late Jurassic and Early Cretaceous of the East Gobi Basin, Ondor-Bogd area, South Mongolia are studied for the first time to infer the provenance and tectonic settings of the source areas. The sandstones of the Ondor-Bogd area are classified into three types: most sandstones belong to litharenite with some feldspathic litharentite and lithic arkose. Using various ternary diagrams, sandstone provenances are interpreted to be the magmatic ares and recycled orogens. Quartz-type diagram shows that sandstones were derived mostly from middle- and upper-rank metamorphic rocks with some from low-rank metamorphic rocks. Considering the sandstone modal composition, the studied sandstones are recycled sediments derived from collisional fold-thrust belts which formed during the accretion of Mongolian ares with the North China block. Upper Cretaceous sandstones of the Ondor-Bogd area are more enriched in volcanic rock fragments compared to the underlying sandstones, indicative of more exposure of paleo-volcanic arc massifs. The studied sandstones were deposited in a basin of an extensional tectonic setting, whereas provenance is interpreted to have been formed under a compressional setting, whose discrepancy points out the necessity of careful consideration when provenance of recycled sediments is concerned.

Sedimentology and Reservoir Architecture of a Synrift Lacustrine Delta, Southeastern Mongolia

Abstract In the East Gobi basin of southeastern Mongolia, non-marine strata fill Jurassic-Cretaceous intracontinental rift basins. Amphitheater-style cliff exposures at Har Hotol on the northern margin of the Unegt subbasin provide more than three kilometers of continuous outcrop, displaying early synrift strata that thicken and coarsen upward from prodelta lacustrine deposits to channelized fluvial sandstone. Outcrop investigations include photopanorama mapping and correlating measured sections, followed by interpretation of lithofacies units and architectural elements. Lithofacies distinctions are based on differences in grain size and bedding style, including massive to planar-bedded mudstone, and lenticular sandstone bodies with erosional bases and common soft-sediment deformation features. Architectural elements are categorized by first-order sedimentary structures (microscale,

Tectonics and hydrocarbon systems of the East Gobi basin, Mongolia

Mapping in the East Gobi basin, supplemented by seismic data, reveals a structural and burial history for basins adjacent to the Zuunbayan and Tsagaan Els oil fields. The tectonic framework was combined with available well and outcrop data to model the timing and magnitude of hydrocarbon generation.Five structural episodes are recognized: (1) pre-Jurassic northeast-directed shortening that formed the tectonic fabric; (2) Middle Jurassic to Early Cretaceous rifting along northeast trends that formed the subbasins of the East Gobi basin; (3) late Early Cretaceous north-south shortening and inversion on existing normal faults; shortening caused left-lateral and reverse displacements on northeast-trending faults; (4) middle Cretaceous uplift and erosion, followed by (5) east-west shortening and right-lateral movement on northeast faults. Folds formed by inversion over Middle JurassicEarly Cretaceous normal faults.Modeling suggests that the bituminous member of the Zuunbayan Formation should be mature over large parts of the Unegt and Zuunbayan subbasins. Oil migrated from mature source areas toward several traps, including the Zuunbayan and Tsagaan Els fields. Modeling suggests that early oil (104110 Ma) was generated in the Zuunbayan and Tsagaan Els area because of deep burial during the Cretaceous. Although generation began in the Early Cretaceous, peak generation in the Unegt subbasin occurred between 100 and 90 Ma. Generation continued at a decreasing rate up to the present day. Kerogen maturity (and oil field production) suggests that oil is the most likely product. Scoping calculations of hydrocarbon volumes generated indicate that the Unegt basin may have generated as much as 86 billion BOE.

Polyphase evolution of the East Gobi basin: sedimentary and structural records of Mesozoic–Cenozoic intraplate deformation in Mongolia

AbstractMapping and correlation of 2D seismic reflection data define the overall subsurface structure of the East Gobi basin (EGB), and reflect Jurassic–Cretaceous intracontinental rift evolution through deposition of at least five distinct stratigraphic sequences. Three major northeast–southwest‐trending fault zones divide the basin, including the North Zuunbayan (NZB) fault zone, a major strike‐slip fault separating the Unegt and Zuunbayan subbasins. The left‐lateral NZB fault cuts and deforms post‐rift strata, implying some post‐middle‐Cretaceous movement. This fault likely also had an earlier history, based on its apparent role as a basin‐bounding normal or transtensional fault controlling deposition of the Jurassic–Cretaceous synrift sequence, in addition to radiometric data suggesting a Late Triassic (206–209 Ma) age of deformation at the Tavan Har locality. Deposits of the Unegt subbasin record an early history of basin subsidence beginning ∼155 Ma, with deposition of the Upper Jurassic Sharilyn and Lower Cretaceous Tsagantsav Formations (synrift sequences 1–3). Continued Lower Cretaceous synrift deposition is best recorded by thick deposits of the Zuunbayan Formation in the Zuunbayan subbasin, including newly defined synrift sequences 4–5. Geohistory modelling supports an extensional origin for the EGB, and preliminary thermal maturation studies suggest that a history of variable, moderately high heat flow characterized the Jurassic–Cretaceous rift period. These models predict early to peak oil window conditions for Type 1 or Type 2 kerogen source units in the Upper Tsagantsav/Lower Zuunbayan Formations (Synrift Sequences 3–4). Higher levels of maturity could be generated from distal depocentres with greater overburden accumulation, and this could also account for the observed difference in maturity between oil samples from the Tsagan Els and Zuunbayan fields.

Tectonics and hydrocarbon systems of the East Gobi basin, Mongolia

Tectonics and hydrocarbon systems of the East Gobi basin, Mongolia

Tectonics and hydrocarbon systems of the East Gobi basin, Mongolia

Tectonics and hydrocarbon systems of the East Gobi basin, Mongolia

Geochemical characteristics and correlation of oil and nonmarine source rocks from Mongolia

New bulk and molecular organic geochemical analyses of source rock and oil samples from Mongolia indicate the presence of lacustrine-sourced petroleum systems in this frontier region. Samples of potential source rocks include carbonate, coal, and lacustrine-mudstone lithologies that range from Paleozoic to Mesozoic in age, and represent a variety of tectonic settings and depositional environments. Rock-Eval and total organic carbon data from these samples reflect generally high-quality source rocks, including both oil- and gas-prone kerogen types, mainly in the early stages of generation. Bulk geochemical and biomarker data indicate that Lower Cretaceous lacustrine mudstone found in core from the Zuunbayan field is the most likely source facies for the East Gobi basin of southeastern Mongolia. Oil and selected source rock samples from the Zuunbayan and Tsagan Els fields (both in the East Gobi basin) demonstrate geochemical characteristics consistent with nonmarine source environments and indicate strong evidence for algal input into fresh- to brackish-water source facies, including elevated concentrations of unusual hexacyclic and heptacyclic polyprenoids. Despite similarities between Zuunbayan and Tsagan Els oil samples, biomarker parameters suggest higher algal input in facies sourcing Zuunbayan oil compared to Tsagan Els oil. Tsagan Els oil samples are also generated by distinctly more mature source rocks than oil from the Zuunbayan field, based on sterane and hopane isomerization ratios.

Sedimentary record and tectonic implications of Mesozoic rifting in southeast Mongolia

The East Gobi basin of Mongolia is a poorly described Late Jurassic–Early Cretaceous extensional province that holds great importance for reconstructions of Mesozoic tectonics and paleogeography of eastern Asia. Extension is especially well recorded in the structure and stratigraphy of the Unegt and Zuunbayan subbasins southwest of Saynshand, Mongolia, where outcrop and subsurface relationships permit recognition of prerift, synrift, and postrift Mesozoic stratigraphic megasequences. Within the synrift megasequence, three sequences developed in response to climatic and rift-related structural controls on sedimentation. Where best exposed along the northern margin of the Unegt subbasin, each of the synrift sequences is bounded by unconformities and generally fines upward from basal alluvial and fluvial conglomerate to fluvial and lacustrine sandstone and mudstone. Resedimented ashes and basalt flows punctuate the synrift megasequence. Rifting began in the Unegt subbasin prior to 155 Ma with coarse alluvial filling of local fault depressions. Subsidence generally outstripped sediment supply, and fresh to saline lacustrine environments, expanding southward with time, dominated the Unegt- Zuunbayan landscape for much of latest Jurassic–Early Cretaceous time. Episodic faulting and volcanism characterized the basin system for the balance of the Early Cretaceous. A brief period of compressional and/or transpressional basin inversion occurred at the end of the Early Cretaceous, prior to deposition of a widespread Upper Cretaceous overlap sequence. The driver(s) of Late Jurassic–Early Cretaceous extension remain uncertain because southeast Mongolia occupied an intraplate position by the beginning of the Cretaceous. Extension in the East Gobi basin was coeval with collapse and extension of early Mesozoic contractional orogenic belts along the northern and southern borders of Mongolia and probably was a linked phenomenon. Strike-slip faulting associated with collisions on the southern Asian and Mongol- Okhotsk margins likely also played a role in late Mesozoic deformation of the East Gobi region, perhaps partitioning the Gobi from apparently coeval large-magnitude contractional deformation in the Yinshan- Yanshan orogenic belt south of the study area in Inner Mongolia.

Future Petroleum-Productive Regions of USSR and Mongolia: ABSTRACT

The potential for major discoveries of oil and gas is high in the USSR, but remote in Mongolia. Development of the USSR's potential is plagued by six factors: (1) remoteness of prospective basins from commercial markets, (2) lack of adequate infrastructure within prospective basins, (3) inadequate drilling technology for economic development below depths of 3,200-3,800 m (10,500-12,500 ft), (4) poor-quality indigenous equipment, (5) absence of offshore capabilities, not only in the warm Caspian and Black Seas, but also in the ice-plagued Arctic and Pacific Oceans, and (6) a chronic manpower shortage. Imports of foreign technology are alleviating the problems gradually, but they are far from solved. European USSR: The most important objectives of the future are: (1) Jurassic through Devonian in the Greater Barents basin and Svalbard platform, offshore Barents Sea, (2) deep Carboniferous-Ordovician of Timian-Pechora basin, (3) subthrust plays of western Urals, (4) pre-Kungurian salt section of the Pricaspian basin, (5) Devonian of the Dnepr-Donets graben, (6) Jurassic through Paleogene of the Black Sea shelf, (7) pre-Tertiary formations of the North Caucasus trough, and (8) deep Tertiary objectives near Baku in the central and southern Caspian Sea. Asiatic USSR: The most important targets are: (1) deep Mesozoic and Paleozoic (Carboniferous, Devonian, Silurian) carbonates of the Nyurol'ka, Frolovo, and other depressions in and below the West Siberian basin, including the Kara Sea, (2) late Paleozoic-Mesozoic of the Aral' Sea, the Ustyurt depression, the Chu-Sarysu basin, and the Turgay and Syrdar'ya syneclises, most of them unexplored, (3) the pre-Upper Jurassic of the central Asian basins, (4) the deep Tertiary of the Cheleken district, (5) the subthrust and pre-Tithonian salt section of the South Tadzhik basin, (6) the Proterozoic-Silurian marine sequences of the Tunguska, Lena, and Sukhana basins, and of the Nepa-Botuoba arch, (7) the Carboniferous-Lower Jurassic of the Vilyuy, Lena-Anabar, Khatanga, and Yenisey basins, with the associated sub hrust plays of the Taymyr and Verkhoyansk ranges, and (8) the numerous late Paleozoic-Mesozoic offshore basins of the Arctic shelf plus the Tertiary basins of the Pacific, especially the Severny basin. Mongolia: Principal objectives are Jurassic and Cretaceous fluviatile and lacustrine sandstones in southeastern Mongolia, especially in the East Gobi basin and in the Hailar basin, which is shared with China. Similar basins in China have giant fields, such as Karamay in the Junggar basin and Daqing in the Songliao basin. Resources: Through 1983, the USSR had produced about 75 billion bbl of oil and condensate and 217 tcf of gas. Estimated proved plus probable liquids was 35-36 billion bbl and, of gas, about 800 tcf. Resource potential, above and beyond proved plus probable, is estimated at about 90 billion bbl and 1,000 tcf (these numbers will increase as offshore icepack technology is improved). In contrast, Mongolia's produced and proved oil is less than 2 million bbl, but the resource potential could be large. End_of_Article - Last_Page 1205------------