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Structure and hydrocarbon prospects of the North Urals thrust belt

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The North Urals thrust belt holds significant hydrocarbon resources hosted mainly in thrust-related anticlines mapped at the surface. N e w seismic and drilling data have permitted the identification of n e w exploration targets. The most attractive
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  Structure and hydrocarbon prospects of the North Urals thrust belt Konstantin Sobornov 1  and Vladimir Rostovshchikov 2 1  All-Russian Research Geological Oil Institute (VNIGNI) 105819 Moscow, sh. Entuziastov, 36, Russia (Present address: Yukos Oil Corporation, 121170 Moscow, Kutuzovsky Prospect   34/21,  Russia). 2 Pechorageoflzika, 169400 Ukhta, ul. Stroiteley, 10, Russia. ABSTRACT:  The North Urals thrust belt holds significant hydrocarbon resources hosted mainly in thrust-related anticlines mapped at the surface. New seismic and drilling data have permitted the identification of new exploration targets. The most attractive structural plays are considered to be associated with the buried antiformal stacks in front of the belt. Stacks of folded thrust sheets involve the regionally productive Silurian-Lower Permian carbonates and sandstones encased by thick Artinskian shales, which provide both the source rocks and sealing. Untested structural closures in the buried wedge-shaped thrust sheets are the primary targets for exploration. Additional prospects are predicted in the autochthonous section of the thrust belt. This play includes a subthrust continuation of inverted anticlinal zones of the Pechora-Kolva aulacogen. KEYWORDS:  Urals, thrust, tectonic wedging, inversion INTRODUCTION The most important discovery in the North Urals, the Vuktyl gas-condensate field, was made in 1964. Since then petroleum exploration has led to only marginally economic discoveries. All fields were found in anticlinal structures in the allochtho-nous section. The long history of unsuccessful exploration efforts based on the traditional tectonic paradigm, suggesting limited shortening across the belt due to thick-skinned thrusting, have resulted in pessimism about the future of petroleum exploration in this area. Reinterpretation of the structural framework of the North Urals, constrained by recently acquired seismic reflection and drilling data, provide important insights into the subsurface structure of the belt and allow identification of new targets for petroleum exploration. These new exploration plays are discussed in this paper. REGIONAL STRUCTURE The Urals fold belt is a transcontinental orogen separating Europe from Asia. It extends for over 3000 km from the Barents Sea in the north to western Kazakhstan in the south (Fig. 1). For the most part, the Urals includes ranges of hills and low mountains rising to 1000 m on average. The highest point is Narodnaya mountain, 1894 m, in the north of the Urals. The formation of the Urals resulted from the closing of the Palaeozoic Uralian ocean and subsequent collision in late Palaeozoic times of a collage of Kazakh and Siberian microcontinents and volcanic arcs with the East European Platform (Hamilton 1970; Zonenshain  etal.  1990; Sengor  etal. 1993). This episode of deformation led to intense deformation of oceanic and suboceanic rocks. In late Triassic-early Jurassic times the Urals were again subjected to an intraplate Presented at the EAGE Conference in Glasgow, June 1995 (EO!8). compression phase and significant shortening occurred within platformal sediments to the west. Estimates of the shortening  within  the shelf   deposits  across the North Urals foreland thrust belt, based on the areal balancing of cross-sections, yielded magnitudes of the order of 50 km (Sobornov 1994).  A  new contractional phase took place in Neogene time. It is responsible for the recent uplift of the Urals and the development of east-vergent thrusts in the transition zone between the Urals and West Siberia (Budanov 1957). This episode is thought to be related to orogeny in the Alpine-Himalaya region. Due to the heterogeneity of the structural grain of the eastern margin of the East European craton, the Urals foreland fold-and-thrust belt is divided into three structural segments. (1) The North Uralian segment borders the Pechora plate, which was accreted to the East European Platform in late Proterozoic time (Zonenshain  et al.  1990). The boundary between these plates is marked by the Timan Ridge, which represents a Proterozoic suture reactivated as a west-verging thrust zone in late Palaeozoic-early Mesozoic time. (2) The Middle Uralian segment stretches along the ancient East European platform. (3) The South Uralian segment, situated mostly in Kazakhstan, is located along the margin of the Pre-Caspian basin and is superimposed on rifted continental crust. In profile view, the Urals comprise an asymmetric west-verging fold-and-thrust belt. The major suture zone of the Urals is the Main Uralian fault which divides the orogen into two tectonic realms: external (to the west) and internal (to the east) (Zonenshain  et al.  1990). Seismic data show that the Main Uralian fault is an east-dipping thrust (Sokolov 1992; Juhlin  et al.  1995). The internal or palaeo-oceanic zone lies to the east of the Main Uralian fault. It is composed of intensely deformed subduction-accretion complexes (Fig. 2). The external or Petroleum Geoscience,  Vol. 2, 1996, pp.177-184 1354-0793/96/$07.00 © 1996 EAGE/Geological Society, London  178 K Sobornov and   V.  Rostovshchikov 50°E 60°E 70°E -60°N 0. t°.°"l  2 ri4 Bile 7 50°N 60°E Fig. 1.  Structural sketch map of the Urals. Legend: 1, thrust; 2,  Jurassic-Cenozoic continental strata; 3, Permian-Triassic molasse; 4,  Ordovician-Permian shelf deposits; 5, allochthonous terranes of the inner Urals; 6, Ordovician-Permian allochthonous slope-and-rise rocks;  7, cratonic basement and Proterozoic metasediments. palaeo-continental part of the Urals is a foreland fold-and-thrust belt involving both sedimentary cover and basement of the East European Platform and Pechora plate (Yudin 1983). Large allochthonous complexes made up of Ordovician to Permian slope-and-rise deposits are thrust over their shelf counterparts in several areas within the external part of the Urals (Voinovsky-Kriger 1945; Puchkov 1979). Estimates of shortening within the external part of the North Urals have yielded figures of the order of 100 km (Yudin 1983; Shishkin 1989). The structural geology of the foreland fold-and-thrust belt of the northern segment of the Urals is considered in this paper (Fig. 3). STRATIGRAPHY AND EVOLUTION The sedimentary section of the North Urals foreland comprises upper Proterozoic to Cenozoic deposits (Fig. 4). The total thickness of sedimentary cover including thrust repetition reaches 14 km (Dedeev  et al.  1994). The upper Proterozoic, Riphean and Vendian sediments are represented mostly by clastic rocks. Their deposition is related to rifting and subsidence of the Pechora plate. Hydrocarbon shows from these rocks have been reported both in outcrops and boreholes (Shablinskaya  et al.  1990). Late Proterozoic -Cambrian orogeny and uplift in the Urals and Timan produced a regional depositional break. Fig.  2. Schematic geological map of the Timan Pechora basin and the North Urals. Legend: 1, Ordovician-lower Permian shelf deposits; 2,  allochthonous terranes of the inner Urals; 3, Ordovician-Permian allochthonous slope-and-rise rocks; 4, cratonic basement and Proterozoic metasediments; 5, thrust in pre-molasse rocks; 6, Permian-Cenozoic platform deposits; 7, Artinskian-Triassic molasse; 8, zones of wedge-shaped thrusts in the base of molasse fill, involving backthrusts at the surface. Unlike the condensed Phanerozoic section found in the Middle Urals foreland and much of the Volga-Ural basin, involving Devonian-Permian deposits, the sedimentary fill of the North Urals foreland is notable for its completeness. After the late Proterozoic uplift, sedimentation in the North Uralian foreland was resumed in Ordovician time due to the opening of the Uralian Ocean (Perfil'ev 1979). From Ordovician through early Permian time the North Urals foreland evolved  The North Urals thrust belt 179 60° W&////M-** 0 Quaternary Triassic Fig.  3. Schematic geological map of the Timan Pechora basin and the North Urals showing the location of cross-sections (Figs 5-11) and oil and gas fields. For geological symbols, see Fig. 2. Oil and gas fields: 1,  Pagimey; 2, Kochmes; 3, Inta; 4. Kozim; 5, Vuktyl; 6, Yugid-Vuktyl; 7,  Rassokhinskoye; 8, Pachginskoye; 9, Kur'inskoye. as a vast shelf basin dominated by carbonate sedimentation. General subsidence of the Pechora plate was interrupted by rifting in late Silurian-lower Frasnian time. The geochemical composition of basalts from the Pechora-Kolva aulacogen, which bisects the Pechora plate, suggests that the rifting might be related to back-arc extension caused by subduction beneath Eastern Europe (M. Wilson, pers. coram.). The Palaeozoic section contains Ordovician, Lower Devonian and upper Frasnian-Tournaisian marine shales which contain types I and II kerogen (Dakhnova  et al.  1995) and reservoirs at several stratigraphic levels (Dedeev  et al.  1994; Zhemchu-gova & Schamel 1994). From Artinskian to late Permian time, the onset of continental collision caused the development of the Uralian foredeep which accumulated sediments shed from the rising fold belt. Up to 6 km of clastic deposits interbedded with evaporites and coal measures were deposited in the foredeep basin. Artinskian marine shales are enriched in type II kerogen and are considered to be important source rocks (Dakhnova  et al.  1995). In latest Permian-early Triassic time (250-240 Ma), the North Urals, along with West Siberia and the Barents Sea, experienced an episode of extension which was marked by Carboniferous 1 km TV * * r h—M limestone & marlstone [rM-j dolomite \ r \  unconformity |»%*  basement sandstone & siltstone shale & claystone •••',<•   conglomerate *•• salt & anhydrite --— detachment 0 reservoir ^fo   source Fig.  4. Stratigraphic column of the North Urals thrust belt. basalt flows in the Kosyu-Rogovsk basin (Timonin 1975). A new onset of compression occurred in Late Triassic-early Jurassic time which is responsible for much of the shortening in the foreland (Sobornov 1994). This episode of compression is marked by a regional unconformity between deformed Permian-Triassic strata and almost undisturbed Jurassic beds. After a long period of tectonic quiescence in late Mesozoic-early Cenozoic time, during which the fold belt was denuded, the Urals and adjacent areas of the East European foreland were uplifted in late Cenozoic time (Budanov 1957; Stepanov et al.  1992). PETROLEUM EXPLORATION Petroleum exploration undertaken over the last four decades in the North Urals thrust belt has led to the discovery of 9 hydrocarbon fields, including the giant Vuktyl gas-condensate field (Fig. 3). The Vuktyl field contains 17TCF of wet gas (13.4% of ethane and higher alkanes) and a large amount of condensate (Spiridonov 1989). An oil leg was found in the southern part of the field. The major pay zones are within Visean sandstones and Middle Carboniferous-Lower Permian carbonates sealed by Kungurian evaporites and Artinskian shales. The height of the gas column in this field (1.4 km) is equal to the structural closure. Other fields are medium to small in size. The known pools occur mainly in the autochthonous section within anticlinal structures expressed in the surface strata. The  180 K Sobornov and   V.  Rostovshchikov WEST Verkhnepechora foredeep Vuktyl EAST 0 Fig.  5. Geological cross-section across the Verkhnepechora foredeep basin (from Bogatsky 1985). depth of the pays ranges from 1.5 to 4.5 km. Most of the discovered hydrocarbons are hosted in Visean sandstone and fractured Carboniferous-Lower Permian carbonates, sealed by Artinskian marine shales and Kungurian evaporites. Gases discovered in the Kosyu-Rogovsk basin are typically enriched in hydrogen sulphide. This is interpreted to be mainly due to the presence of Ordovician evaporites. Hydrocarbon pools are sourced mostly by Artinskian and Upper Devonian-Lower Carboniferous shales which contain kerogens of types I and II (Dakhnova  et at   1995). Discovered hydrocarbon resources are dominated by gas.  This is believed to be the result of the interaction of several factors. Among them are (1) the widespread occurrence of gas-prone type II organic matter; (2) high maturity of allochthonous deposits (values of vitrinite reflectance of Carboniferous-Permian deposits range from 0.8% to 1.1%; Kalmykov & Letunovsky 1979); (3) the late Cenozoic uplift which has led to the cooling and decompression of formation fluids and the subsequent dissolution of a large volume of hydrocarbon gas which could replenish numerous traps in the fold-and-thrust belt (Sobornov & Tarasov, in press). Studies of the temperature conditions in the Timan-Pechora basin showed that the difference between present and palaeotem-peratures in Carboniferous-Permian deposits may be in excess of 100°C (Kalmykov & Letunovsky 1979). This drop in temperature could constitute a significant mechanism for the liberation of gas and could be the source of large accumulations of gas. The traditional petroleum exploration strategy has targeted anticlinal structures mapped at the surface. According to the traditional thick-skinned structural interpretation, based on very limited subsurface data, these anticlines would have been associated with steeply dipping major faults cutting across most of the sedimentary section up to the erosional surface (Fig. 5). By the late 1980s almost all the large mapped anticlines had been drilled. As a result, the remaining hydrocarbon potential of the North Urals thrust belt was thought to be very limited. NEW STRUCTURAL INTERPRETATION Petroleum prospecting over the last decade has yielded a great deal of data on the subsurface structure of the North Urals thrust belt. Integrated interpretation of the new data has documented a more extensive occurrence of thrust faulting than was believed previously and provided important constraints on the structural geometry and evolution of the belt (Yudin 1983; Sobornov  et at   1991). In the eastern, inner, part of the foreland thrust belt, seismic data have supported earlier geological interpretations (Voinovsky-Kriger 1945; Puchkov 1979), suggesting overthrust relationship between allochthonous Ordovician to Permian slope-and-rise deposits and their shelf counterparts. Seismic reflection packages attributed to parautochthonous shelf deposits at depths of 1-5 km are visible 10-15 km to the east, beneath the allochthonous section (Sobornov & Tarasov, in press). Sedimentological studies suggest the possible presence of mid-Paleozoic reefs in the parautochthonous footwall, underneath the allochthonous deep water rocks (Shishkin 1989). This subthrust trend is a potential petroleum exploration play. In the outer part of the belt it was established that the western limit of the North Urals thrust belt is not confined to the surface exposure of thrust faulting, but continues well into the basin as a consequence of the wedge-shaped geometry of the leading edge of the belt (Sobornov  et at.  1991). This type of structure is analogous to the classical triangle zones recognized in the Southern Canadian Cordillera (Jones 1982) and more recently in many other thrust belts throughout the world. The interpretation of seismic data in the North Urals thrust belt has also revealed buried anticlines in the autochthonous sedimentary cover. The following discussion is focused on new, seismically identified, untested structures which are primary targets for petroleum exploration in the North Urals thrust belt. Tectonic wedges Figure 6 illustrates the characteristic seismic image of the wedge-shaped thrust sheets in the Verkhnepechora basin. Interpretation of this profile is constrained by surface mapping and drilling data. At the surface this zone of wedging is marked by drastically thickened Artinskian shales and by backthrusts (Yudin 1983), the srcin of which was not understood. The proposed interpretation suggests that the allochthonous tectonic wedge is emplaced into the sedimentary cover  The North Urals thrust belt 181 Fig.  6. Interpreted seismic reflection profile (migrated) across the North Urals-Verkhnepechora foredeep basin transition (from Sobornov & Rostovsh-chikov 1995). Projected positions of 1-Kyrtashor  and  1-Martyu  boreholes are shown. See Fig. 3 for location. Legend: P 2 ,  upper Permian; Pxk, Kungurian; P^, Artinskian; C-Pi, Carboniferous-Sakmarian; D, Devonian; S, Silurian. VERKHNEPECHORA FOREDEEP W  1  M NORTHERN URALS  / n 1K E -0 TWT.s Fig.  7. Interpreted seismic reflection profile (migrated) across the Vuktyl field and adjacent area of the North Urals (from Sobornov  et at,  1991). See Fig. 3 for location. Legend: P 2 , upper Permian; P^,  Kungurian; Piar, Artinskian; C-Pi, Carboniferous-Sakmarian; D, Devonian; S, Silurian. WEST VUKTYL EAST C-P 1  OKM TWT.s Fig.  8. Geological cross-section across the Vuktyl field and adjacent area of the North Urals. See Fig. 3 for location. Legend: 1, cratonic basement; 2,  Riphean-Vendian sediments; 3, Ordo-vician-lower Permian shelf sediments; 4,  Artinskian-Triassic molasse; 5, thrust. West Verkhnepechora foredeep Vuktyl anticline North Urals Thrust Beit 20km 3ji !] 2  g 3  i| 4  t^l NORTH-WEST Fig.  9- Interpreted seismic reflection profile (migrated) across the Kosyu-Rogov basin-North Urals transition. See Fig. 3 for location. Legend: P, Permian C-P, Carboniferous-Sakmarian; S-D, Silurian-Devonian; O, Ordovician; PR, Proterozoic. C-P S-D O. PR S-D O 10KM PR TWT.S
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