Tectonics and Petroleum Potential of the East Arctic Province

Russian Geology and Geophysics 50 (2009) 334–345 www.elsevier.com/locate/rgg

Tectonics and petroleum potential of the East Arctic province

V.E. Khain, I.D. Polyakova *, N.I. Filatova

Geological Institute, Russian Academy of Sciences,7 Pyzhevsky per., Moscow, 119017, Russia Received 17 July 2008

Abstract

Tectonics and petroleum potential of the underexplored East Arctic area have been investigated as part of an IPY (International Polar Year) project. The present-day scenery of the area began forming with opening of the Amerasia Ocean (Canada and Podvodnikov–Makarov Basins) in the Late Jurassic–Early Cretaceous and with Cretaceous–Cenozoic rifting related to spreading in the Eurasia Basin. The opening of oceans produced pull-apart and rift basins along continental slopes and shelves of the present-day Arctic fringing seas, which lie on a basement consisting of fragments of the Hyperborean craton and Early Paleozoic to Middle Cretaceous orogens. By analogy with basins of the Arctic and Atlantic passive margins, the Cretaceous–Cenozoic shelf and continental slope basins may be expected to have high petroleum potential, with oil and gas accumulations in their sediments and basement. © 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

Keywords: orogen; Cretaceous–Cenozoic basin; geodynamics; petroleum province; East Arctic; Hyperborea; Amerasia Ocean

Introduction on tectonic types and ages of sedimentary basins within the Arctic shelf, which may be promising petroleum exploration Although recent geological and geophysical studies have targets (e.g., Claypool and Magoon, 1988; Houseknecht and largely improved the understanding of the Arctic tectonic Bird, 2005; Magoon, 1994; Thurston and Theiss, 1987). history, many issues remain underexplored, especially the Sediment thicknesses in the basins were estimated tentatively tectonics of the fringing seas and the continental shelf. Besides (see isopach patterns in Fig. 1) from maps of local gravity the purely scientific interest, gaining more knowledge of these and magnetic anomalies (e.g., Mazarovich and Sokolov, 2003) issues is important for metallogenic and petroleum exploration and few seismic profiling data (e.g., Drachev et al., 2001; implications. Franke et al., 2004; Lebedeva-Ivanova et al., 2006). Data of onshore and offshore geophysical surveys, remote sensing, drilling and analysis of marine sedimentary cores (Kaban’kov et al., 2004; Khain, 2001; Klemperer et al., 2002; Tectonic units and their history Kos’ko et al., 1993, 2002; Parfenov et al., 2003; Sokolov et al., 2002; Zamanskii et al., 1999; Zonenshain et al., 1990) indicate The East Arctic area includes fragments of the Precambrian that the East Arctic area consists of Precambrian to Cenozoic Hyperborean craton and Early Paleozoic to Middle Cretaceous tectonic units, which are still poorly understood in terms of (Late Kimmeridgian) fold belts. The polar part of the area is their settings and relationships. occupied by the Late Jurassic–Cretaceous Amerasia Basin, The synthesis we undertake aims at systematizing the major currently part of the Arctic Ocean. The continental crust of tectonic units, tracing their history, and predicting the petro- the region evolved through several orogenic events, namely leum potential of the area. Geographically the area mostly lies the Grenville, Baikalian, Ellesmerian, and Late Kimmerian in the eastern Eurasian Arctic shelf of the East Siberian and orogenies, associated with closure of the respective oceans, Chukchi Seas (Fig. 1) and covers the adjacent onshore area formation of fold-and-thrust belts, and attendant greenschist- on the south and a part of the Amerasia Basin of the Arctic amphibolite and granulite metamorphism (e.g., Khain and Ocean in the north. The main focus of the reported study is Filatova, 2007; Lawver et al., 2002; Parfenov and Kuz’min, 2001). The area encompasses fragments of two continental plates: an Early Paleozoic plate including Grenvillian Hyper- * Corresponding author. borea and a plate consisting of Precambrian Siberia and the E-mail address: [email protected] (I.D. Polyakova) Jurassic–Middle Cretaceous Verkhoyansk–Chukchi zone. The 1068-7971/$ - see front matter D 2009, IGM, Siberian Branch of Publishedthe RAS. bylsevier E B.V. All rights reserved. doi:10.1016/j.rgg.200 0 3. 09. 06 V.E. Khain et al. / Russian Geology and Geophysics 50 (2009) 334–345 335 modern shelves of the East Siberian and Chukchi Seas Canada and Podvodnikov Basins (including the Henrietta and bordering the Amerasia Basin involve Cretaceous–Cenozoic Jeannette Islands (Parfenov and Kuz’min, 2001). Early Paleo- structures (including systems of faults and basins) produced zoic marine, shelf, and island arc rocks in the Alaska and East by the Middle Cretaceous orogeny and spreading in the Arctic shelves have been interpreted jointly as the Franklinian Amerasia and Eurasia Basins (Fig. 1). sequence (Houseknecht and Bird, 2005; Sherwood et al., The Hyperborean craton (according to Shatski) or 2002) (Fig. 2) which shows up as an acoustic basement in Arctida (according to Zonenshain) has a Precambrian (Gren- seismic profiling sections. It includes particularly Ordovician villian) basement with pieces of older crust. In the present and Silurian turbidite, hemipelagic shale, jasper, chert, and framework, the craton has survived as a fragmentary body volcanics stripped by drilling south of the Barrow arch which includes the Kara and Svalbard microplates in the (Klemperer et al., 2002). Early Paleozoic rocks in the De Long western Arctic and is exposed in the New Siberian and De Islands presumably include turbidite and calc-alkaline vol- Long Islands, in the Peary Land Peninsula, in the Canadian canics. Arctic archipelago, and in the Lomonosov, Mendeleev, and Another arm of the Iapetus is reconstructed east of Siberia, Chukchi submarine ridges in the eastern Arctic (Fig. 1). By in the present frame of reference (Filatova and Khain, 2007), the end of the Neoproterozoic, Hyperborea was part of the where faulted fragments of mafic-ultramafic assemblages with Rodinia supercontinent assembled about 1 Ga ago. In the a Silurian (430–419 Ma) 40Ar/39Ar age of amphibolite-green- middle Neoproterozoic, Rodinia dispersed into several conti- schist metamorphism were found in the Kolyma Loop suture nents, including Hyperborea (Arctida) which drifted from the in the western Verkhoyansk–Kolyma orogenic system (Par- southern and equatorial latitudes to the Arctic region in the fenov and Kuz’min, 2001). Other allochthones are composed Paleozoic (Lawver et al., 2002). Proterozoic–Early Paleozoic of Cambrian siltstone with clasts of serpentinite, basalt, and terrigenous and carbonate shelf sediments on the greatest part jasper, as well as pelagic siliceous shale and basalt with of the Hyperborean craton are weakly deformed, with only Ordovician graptolites. the Cretaceous De Long magmatic swell in the De Long The continents of Laurentia, Hyperborea, Siberia, and Islands (Filatova and Khain, 2007). Baltica assembled during the Scandian and Ellesmerian phases The Baikalian orogeny at the Neoproterozoic-Cambrian of the Early Paleozoic orogeny into a continent of Laurussia boundary led to craton growth by accretion of Late Precam- and moved together to their modern latitudes in the Carbon- brian (Baikalian) orogens that currently exist as the Timanid- iferous. This resulted into change from redbed shelf deposition Protouralid fold belt in the west (e.g., Kuznetsov et al., 2007) (with evaportites) to deposition of mostly terrigenous gray and are exposed in the Taimyr Peninsula, Wrangel Island, East rocks. The convergence of the continents led to assembly of Chukchi, Seward Peninsula, and North Alaska in the east. The Pangea II subject to several stages of rifting. The first rifting passive margin setting is recorded in Neoproterozoic ophiolite, event at the Devonian–Carboniferous boundary gave rise to island-arc basalt, andesite, and rhyolite of tholeiite series several rifts, including the Hanna trough, with a volcanic- coexisting with 850–740 Ma plagiogranite found by Verni- terrigenous fill (Endicott Group and its equivalents). Conti- kovsky (Bogdanov and Khain, 1998). The collision and ocean nental rifting locally transformed into seafloor spreading closure were accompanied by granulite-amphibolite metamor- responsible for the opening of the Middle Paleozoic–Late phism and plutonism, with ages of granites 612–570 Ma the Jurassic Alazeya–South Anyui–Angayucham Ocean (Parfenov youngest. Farther to the east, the fragments of the Neoprotero- and Kuz’min, 2001; Sokolov et al., 2002) which evolved as zoic orogen are confined between continental blocks of an arm of the Pacific with its end at the onshore Oimyakon Hyperborea (Fig. 1). The Baikalian activity produced, specifi- Basin (Parfenov et al., 2003). cally, amphibolite and gabbro and gabbro-dolerite, about The Verkhoyansk–Chukchi zone (Fig. 1) formed through 700 Ma, in Wrangel Island (Kos’ko et al., 1993); 592–547 Ma the Late Jurassic–Neocomian by the collision of the Paleozoic amphibolite-greenschist rocks are known also in the Chukchi Chukchi–Alaska plate (including Hyperborea) with Siberia, shelf south of the Barrow arch, in gneiss domes of East and the related closure of the Alazeya–South Anyui–An- Chukchi, in North Alaska, and in the Seward Peninsula (e.g, gayucham Ocean. The zone central part is occupied by the Klemperer et al., 2002; Natalin et al., 1999). Dispersed Verkhoyansk–Kolyma collisional system lying along the Mid- fragments of mafic-ultramafic assemblages and jaspers were dle Cretaceous collisional suture. The system borders the found among Cambrian terrigenous rocks of the Cretaceous deformed margins of a Paleozoic plate (with Precambrian Kolyma Loop suture (Parfenov and Kuz’min, 2001). fragments), known as the New Siberian–Chukchi–Brooks fold The continents of Laurentia, Hyperborea, Baltica, and belt in the northeast, and the Siberian craton (Verkhoyansk Siberia rifted off again at the Neoproterozoic–Cambrian orogenic system) in the southwest (Bogdanov and Tilman, boundary during opening of the Iapetus (Lawver et al., 2002). 1993; Filatova and Khain, 2008; Parfenov et al., 2003; etc.). Terrigenous and carbonate shelf deposition in Hyperborea The Verkhoyansk–Kolyma collisional system, the core of continued in the Early Paleozoic, when ocean occupied (in the the zone, has a complex fold-thrust structure with juxtaposed present frame of reference) the western Canadian Arctic sheets of marine, shelf, and island arc rocks (ophiolite, archipelago, including Ellesmere Island with the thrust Ordo- volcanics, jasper, and terrigenous rocks) of different ages from vician Peary ophiolite (Klemperer et al., 2002), and western the Cambrian to the Neocomian. The bordering Middle Alaska, as well as the present southern periphery of the Cretaceous collisional suture includes the Kolyma Loop and 336 V.E. Khain et al. / Russian Geology and Geophysics 50 (2009) 334–345

South Anyui segments separated by a system of left-lateral Hope Basins. The fold-thrust belts of the zone are composed strike-slip faults, with the Kobuk segment on its eastern of Proterozoic–Paleozoic terrigenous and carbonate sediments extension (Fig. 1). The suture involves deformed rocks of the deposited on the Paleozoic plate, as well as Triassic and closed ocean (Bondarenko, 2004; Kuz’michev et al., 2005; Jurassic rift-related terrigenous or less often igneous rocks. Sokolov et al., 2001). The upper structural position in the thrust system belongs to The New Siberian–Chukchi–Brooks fold-thrust system con- Paleozoic and Mesozoic marine, shelf, and island arc facies sists of foreland basin strata subject to thin-skin deformation thrust from the Verkhoyansk–Kolyma collisional system. The with mostly north trending folds and imbricate thrusts. The Chukchi zone underwent Late Jurassic–Early Cretaceous deformation becomes weaker away from the Verkhoyansk– granulite-amphibolite metamorphism in a belt involving a Chukchi collisional system, and, according to this criterion, chain of gneiss domes (Akinin and Calvert, 2002; Amato et al., the New Siberian–Chukchi–Brooks system is divided into the 2002; Gelman, 1995a,b, 1996; Natalin et al., 1999) and outer and inner zones (Fig. 1). numerous syncollisional plutons. The inner Chukchi zone lies right along the suture, on the The outer (frontal) New Siberian–Wrangel–Herald–Lis- modern continental margin and the adjacent Chukchi Sea burne–Brooks thrust zone (Fig. 1) extends in the NW direction shelf, making the basement of the Cenozoic South Chukchi– and covers mainly the East Arctic shelves. In the southwest V.E. Khain et al. / Russian Geology and Geophysics 50 (2009) 334–345 337

Fig. 1. Tectonic framework of East Arctic. 1, 2. Early Paleozoic continental plate (East Arctic part of Laurussia), consisting of (1) Hyperborean craton (Grenvillian and fragments of Baikalian orogens) with weakly deformed Neoproterozoic–Mesozoic sedimentary cover on East Siberian and Chukchi shelves (a), Mendeleev Ridge (b), Chukchi Ridge (c), and with sediments intruded by De Long arch (d), and (2) fragments of Early Paleozoic orogens. 3–11. Late Jurassic–Middle Cretaceous Verkhoyansk–Chukchi–North Alaska tectonic zone, with New Siberian–Chukchi–Brooks orogenic system (deformed Paleozoic plate margin) (3–5): inner Chukchi zone with strongly deformed sedimentary cover of continental plate and gneiss domes, known onshore (a) and inferred on Arctic shelves (b) (3), Cretaceous gneiss domes (A, Alarmaut; K, Koolen; Ku, Kuekvun; N, Neshkan; S, Senyavin) (4), New Siberian–Wrangel–Herald–Lisburne–Brooks zone with deformed sedimentary cover of continental plate (5), known onshore (a) and inferred on Arctic shelves (b); 6, 7, Verkhoyansk–Kolyma collisional system and Middle Cretaceous suture along its border: fold-thrust belts in Paleozoic–Neocomian island arc, shelf, and marine facies (6), segments of Middle Cretaceous suture (7), including Kolyma Loop (a), South Anyui, with that buried under Cretaceous–Cenozoic sediments (b), Kobuk (c); 8. Verkhoyansk orogenic system (deformed margin of Siberian craton); 9–11, igneous rocks and basins produced by Middle Cretaceous (Late Kimmerian) orogeny: Early Cretaceous syncollisional granites (9), syn- and post-thrusting basins and pull-apart basins with turbidite and terrigenous-olistostrome fill (10), basins along thrust front with imbricate Cretaceous terrigenous fill (11). 12, Albian–Late Cretaceous Okhotsk–Chukchi continental-margin magmatic belt. 13–17. Paleozoic–Cenozoic continental rift basins on Arctic shelves and attendant magmatism: Devonian rift basins filled with volcanic-sedimentary rocks (13), Cretaceous arc-shaped and radiating troughs of De Long arch with terrigenous and volcanic fill (14), Cretaceous–Cenozoic rift and pull-apart basins within shelves and islands of East Siberian and Chukchi Seas and continental margin (a) and Cenozoic basins within Mendeleev and Chukchi Ridges (b); open spaces are onshore Neogene–Quaternary sediments (15), Cretaceous–Cenozoic basins of continental slopes and rise (16), within-plate basalt and bimodal volcanic assemblages: undifferentiated Early Cretaceous and Cenozoic in islands (a) and on shelves, inferred from magnetic data (b) (17). 18, 19. Amerasia Basin as part of modern Arctic Ocean: Canada Basin with Late Jurassic–Middle Cretaceous oceanic crust under Late Cretaceous–Cenozoic terrigenous sediments (a) and zone of Northwind underthrust, hypothetical zone of Mesozoic failed subduction (b) (18), Podvodnikov Basin with Early–Middle Cretaceous (?) suboceanic crust under Late Cretaceous–Cenozoic sediments (19). 20. Boundary between oceanic and continental crust (hachure towards Amerasia Basin). 21. Known (a) and inferred (b) boundaries of lithological units. 22. Inferred fault boundaries between fragments of Early Paleozoic orogens. 23. Known (a) and inferred, from magnetic data (b) frontal thrusts of Kolyma Loop and South Anyui–Kobuk sutures. 24. Known (a) and inferred (b) frontal thrusts of New Siberian–Chukchi–Brooks orogenic system (abbreviations stand for names of its segments: NS, New Siberian; Wr, Wrangel; He, Herald; Li, Lisburne; Br, Brooks). 25. Northwind thrust. 26. Known (a) and inferred, from magnetic data (b) left-lateral strike-slip faults of South Anyui system; 27. Other strike-slip faults, with direction of motion (dashed line shows inferred faults). 28. Normal faults. 29. Thrusts (a) and normal faults (b) in Cretaceous–Cenozoic basins. 30. Arc-shaped faults bordering Cretaceous–Cenozoic De Long arch. 31. Undifferentiated faults. 32. Inferred sediment thicknesses (km) in Cretaceous–Cenozoic basins. 33. Sea depths (m). Names of basins are abbreviated as: Vi, Vilkitsky; Co, Colville; NO, Northwind; NC, North Chukchi; Ha, Hanna; Ho, Hope; Ch, Charlie; SC, South Chukchi; PM and CN stand for Pevek–Mendeleev and Chukchi–Northwind faults, respectively. Inset is location map of study area, with national frontier between Russia and US. it borders the Chukchi zone or immediately the collisional New Siberian segment lying farther northwest (Fig. 1) is suture, and in the northeast it thrusts over weakly deformed exposed in islands of the East Siberian Sea. Rocks of the sediments deposited upon the continental plate. The frontal whole Paleozoic to Valanginian section of Kotel’nyi Island thrust zone is exposed only in Alaska and in sea islands, its were subject to Middle Cretaceous deformation and make NW bound being traceable as a tortuous beaded gravity high cut folds combined with SW and NE thrusts (Parfenov and into segments by NE transversal faults (Fig. 1). The segments Kuz’min, 2001). were often interpreted as separate terranes, but purposeful The frontal thrust zone is associated with a chain of correlation (Filatova and Khain, 2007) showed a uniform foreland basins which formed during the thrusting event to structure from the Brooks Range in the southeast to the New make up a foredeep similar to the Verkhoyansk Foredeep in Siberian Islands in the northwest. its age and structure. The Middle Cretaceous East Chukchi The Brooks Range segment, a typical example of thin-skin foredeep borders the Barrow arch (Moore et al., 2002) in the tectonics in the frontal zone, consists of several north trending north, is extended with a chain of basins before the front of thrust sheets with shelf facies topped by ophiolite fragments; the Lisburne, Herald, and Wrangel thrusts (Fig. 1), and may the ophiolite roots are located farther to the south, in the continue northwestward into the New Siberian archipelago. Kobuk suture (trace of the Middle Cretaceous Angayucham The foredeep includes the Colville foreland basin adjacent Ocean closure). The imbricate thrust structures consist of to the Brooks frontal thrust (Fig. 1), which lies unconformably Paleozoic–Late Jurassic oceanic and island arc rocks. All other upon the Ellesmerian orogenic structures and upon the N–S sheets in the Brooks segment are northward thrusts of the Hanna trough sealed with Pennsylvanian–Mesozoic (including southern Alaskan edge of the continental plate (Klemperer Early Neocomian) sediments (Sherwood et al., 2002). The et al., 2002; Patrick and Lieberman, 1988; Toro et al., 2002). subsiding basin received molasse from the concurrently grow- The same thin-skin tectonic structure shows up in the Lis- ing Brooks mountains. It was mainly Aptian–Early Albian fine burne, Herald (Moore et al., 2005), and Wrangel segments. clastics from oldest low uplifts and then coarser sediments in The Wrangel segment exposed within Wrangel Island is a the latest Albian–Late Cretaceous. The sediment fill of the package of NE trending thrusts and folds overturned in the Colville Basin totals a thickness of 5–8 km decreasing to NE direction (Kos’ko et al., 1993) which are composed of 1–2 km in the west. The sediments are traditionally interpreted Proterozoic and Paleozoic–Mesozoic shelf sediments of the jointly as the Early Cretaceous–Neogene Brookian sequence continental plate cover. The deformation took place between divided by a pre-Paleocene structural unconformity (Fig. 2). 150 and 115 Ma ago (Kos’ko and Ushakov, 2003). The However, the deposits of the Colville Basin proper span an fold-thrust structure of the Herald and Wrangel segments is interval from the Aptian to the earliest Late Cretaceous. As well pronounced in seismic profiles (Burlin and Shipelkevich, the frontal thrusts of the Brooks segment were rising and 2006), where the NE thrusts involve rocks of the Franklinian thrusting northward, the Colville Basin fill propagated in the acoustic basement and Devonian–Early Mesozoic rocks. The same direction, whereby duplex thrusting involved Aptian-Al- 338 V.E. Khain et al. / Russian Geology and Geophysics 50 (2009) 334–345

Fig. 2. Generalized stratigraphy of North Alaska and adjacent Arctic shelves, after (Houseknecht and Bird, 2005). See text for details of lithology. bian sediments which originally were thrust by the Brooks The Amerasia Basin in the eastern Arctic Ocean includes Range. The northern edge of the East Chukchi foredeep may the Canada and Podvodnikov basins separated by the Men- be located at base of the Cenozoic North Chukchi Basin deleev and Chukchi–Northwind Ridges (Fig. 1). Both ridges (Fig. 1). The Middle Cretaceous Brooks Range being a are blocks of continental crust (Lebedeva-Ivanova et al., 2006) mountain area up to present, the terrigenous input to the that extend the Hyperborean craton. Oceanic crust production Colville Basin has continued through the Cenozoic. was generally synchronous in the Canada and Podvodnikov– Note that the structures of the Verhoyansk–Chukchi tec- Makarov basins and had completed by the Late Cretaceous. tonic zone formed as perioceanic basins relative to the present Later the two basins were the rapidly growing terrigenous Arctic fringing seas, and are coeval with the opening of the depocenters which became surrounded by continental slope Amerasia Basin within the Arctic Ocean. basins (Fig. 1). The Late Devonian–Early Jurassic Ellesmerian terrigenous- The best documented Canada Basin is contoured by the carbonate sequence distinguished within the East Arctic shelf 3000 m isobath and its maximum sea depth is about 4000 m. and onshore areas (Fig. 2) turned out to be heterogeneous. According to gravity and seismic profiling data (e.g., Gurevich Typical shelf deposition upon the East Arctic Early Paleozoic et al., 2006), the basin lies on 4–8 km thick oceanic crust and plate lasted through the Late Permian and was followed in is filled with 6–8 km of sediments. The southern Chukchi– Triassic–Late Jurassic time by synrift deposition (Beaufortian Alaska continental border of the basin is cut by a series of sequence). It was continental rifting, precursor to the Amerasia listric faults and troughs (Grantz et al., 1998; Klemperer et al., spreading, with deposition of thick turbidite and shale, punc- 2002). The border with the Northwind Ridge runs along a tuated with intraplate magmatism, possibly an echo of the large zone of over- and under-thrusting (Grantz et al., 1998) Siberian trap magmatism. The Amerasian Basin opened in the (Fig. 1) mirrored from the side of the basin by a linear negative Late Jurassic–Early Albian by a mechanism which remains a gravity anomaly up to –100 mGal, which may be signature of subject of discussion. For instance, the model of Grantz et al. a Cretaceous failed subduction beneath the Chukchi Ridge. (1981, 1990a,b, 1998) attributes it to dispersal and rotation The Podvodnikov Basin is the southern element of the (with transform faulting) of the Chukchi–Alaska plate. The Podvodnikov–Makarov Basin system between the Lomonosov collision of this plate with Siberia resulted in closure of the and Alfa–Mendeleev Ridges. The crust thickness estimated Alazeya–South Anyui–Angayucham Ocean and created the from multichannel seismic profiling data (along profiles Middle Cretaceous Verkhoyansk–Chukchi orogen. SLO-8991 and Arktika-2000) decreases from 18–22 km in the V.E. Khain et al. / Russian Geology and Geophysics 50 (2009) 334–345 339

Podvodnikov Basin to 8–12 km in the Makarov Basin separate, in particular, the Mendeleev and Chukchi–Northwind (Gramberg et al., 1997; Lebedeva-Ivanova et al., 2006; Ridges from the shelf. The same system of strike-slip faults Zamanskii et al., 1999). Seismic sections image a large zone bounds the North Chukchi and South Chukchi–Hope pull-apart of closely spaced faults along the border of the Podvodnikov basins (Fig. 1). The North Chukchi Basin, like the South Basin with the continental shelf and sharp crust thickening Chukchi–Hope Basin, initiated at the Cretaceous–Paleogene from 32 to 40 km south of the zone. Heat flow data collected boundary. Seismic profiling data clearly show a terrigenous by Russian and Canadian scientists from drifting ice and used Cenozoic sediment fill of the basin lying discordantly over to infer lithospheric temperatures and heat flow density in East older structures, which are either Jurassic–Neocomian rifts on Arctic (Khutorskoi et al., 2006) indicate the lack of recent the margin of the opening Canada Basin (Sherwood et al., tectonic activity (with quite a low background of 60– 2002) or Cretaceous pull-apart basins concurrent with strike- 2 2 70 mW/m in the Podvodnikov Basin and to 80 mW/m slip faulting (like the Colville foreland basin). beneath the Mendeleev Ridge), which is consistent with The NW faults are crosscut by younger NE strike-slip faults seismic quiescence (Gramberg et al., 1997). The temperature that traverse the shelf and continue into the Central Arctic at the sediment base in the Podvodnikov Basin decreases domain, judging by gravity maps (Fig. 1). Along the faults northward from 250 do 150 °C (Khutorskoi et al., 2006), and there are coeval pull-apart basins filled with 5–6 km thick the sediment thickness decreases in the same direction. The sediments (Franke et al., 2004). A rift trough of this kind crustal base likewise becomes colder (from 750 to 700 °C) divides the Mendeleev and Chukchi Ridges (Fig. 1) and and shallower to the north. extends into the Canada Basin. The age of rifting and spreading is recorded in Early Cretaceous (or, possibly Late Cretaceous) within-plate basaltic and bimodal magmatism known in the Barents, Kara, East Known and potential petroleum basins Siberian, and Chukchi shelves (e.g., Shipilov, 2004; Silantiev et al., 2004). The Early Cretaceous magmatic event within the The tectonic map of East Arctic (Fig. 1) and the map of East Siberian Sea produced the De Long arch (Fig. 1) which Fig. 3 show sedimentary basins prospective for petroleum was interpreted (Shipilov, 2004) as a volcanic swell. In gravity exploration. They are basins on the Alaska North Slope, the and seismic profiling images, the arch appears as a concentric South Chukchi and New Siberian–North Chukchi Basins, and horst-graben system split by arc-shaped and radiating faults, the East Siberian foreland basin. Their history and the which includes, among others, the New Siberian and Vilkitsky respective seismic facies were discussed in the previous sedimentary basins (Fig. 1). The arch central part, uplifted the section. In this section we provide more details of tectonics, highest by diapirism, is occupied by a condensed section of lithology, stratigraphy, and, where possibly, chemistry of basin Cretaceous–Cenozoic sediments and alkali basalts erupted in sediments for appraisal of the hydrocarbon potential. So far Middle Cretaceous and Miocene–Pliocene time. The volcanics petroleum plays have been identified and evaluated only in are of within-plate affinity and may be related to a mantle the Alaska North Slope. plume (e.g., Silantiev et al., 2004.). The existence of a large The section of the Alaska North Slope basin consists of magma source is consistent with the pattern of magnetic Cretaceous–Cenozoic strata of the Colville foreland basin, the anomalies; the distribution of smaller chambers (and basaltic Triassic–Jurassic sediments in the Beaufort rift basin, the fields) is controlled by arch-shaped peripheral faults (Maz- Carboniferous–Permian Ellesmere shelf deposits, and the Late arovich and Sokolov, 2003; Taylor et al., 1981). Devonian–Early Carboniferous Endicott Group in the Hanna Note that the opening of the Amerasia Ocean was concur- trough, all lying over the Franklinian sequence of metamorphic rent with subsidence of troughs bounded by listric faults in its and igneous rocks making an acoustic basement (Fig. 2). The continental surroundings. These troughs are known, for in- basin is bounded on the south by the Brooks frontal thrust and stance, in the southern periphery of the Canada Basin (Grantz extends into the shelf as far as the southern bounds of the et al., 1998) and may exist also in the basement beneath the Canada Basin; its western boundary coincides with the front North Chukchi Basin. In Late Cretaceous and Cenozoic time, of the Herald and Lisburne thrusts, and the eastern border is the East Siberian and Chukchi shelves underwent general tentative. subsidence along faults. The faults (whatever be their geome- The basin hosts the US largest Prudhoe Bay field producing try) are marked by gravity field gradients and the related mostly from Late Devonian–Early Cretaceous Ellesmerian and basins along them correspond to linear or less often isometric Beaufortian reservoirs (Houseknecht and Bird, 2005; Sher- gravity lows. wood et al., 2002) (Fig. 2). The field was originally estimated The basins within the East Siberian and Chukchi shelves to hold about 10 billion barrel of oil but the inplace resources are mostly Cretaceous–Cenozoic NW pull-apart and rift were later reassessed to be as high as 14 BBOE (Durham, basins which are coeval with and run along NW right-lateral 2006). The Ellesmerian and Beaufortian sequences bear about strike-slip faults. The faults originated on the shelf during the 90% of oil in the Arctic Alaska Petroleum Province, with, final stage of Late Cretaceous compression (Bondarenko, besides Prudhoe Bay, medium-size and large fields of Endi- 2004) but most of them were related to opening of the cott, Lisburne, Liberty (Tern), Lisburne Pool, Northstar, Sag Cenozoic Eurasia Basin. The NW strike-slip faults border the River, Barrow, Alpine, Kuparuk, and Thomson. Smaller continental shelf edge and delineate narrow rift troughs that accumulations of Gubik, Umiat, Fish Creek, Simpson, West 340 V.E. Khain et al. / Russian Geology and Geophysics 50 (2009) 334–345

Fig. 3. Cretaceous–Cenozoic East Arctic basins with known and potential hydrocarbon resources. 1, limits of potential petroleum basins; 2, isopachs, km; 3–6, faults, of uncertain geometry (3), normal faults (4), thrust and reverse faults (5), strike-slip faults (6); 7, hypothetical fans; 8, prospective zones with uncertain petroleum potential. Roman numerals stand for names of known (I) and inferred (II–IV) petroleum basins: Alaska North Slope (I), South Chukchi (II), New Siberian–North Chukchi (III), Fore-East Siberian (IV). Abbreviations stand for names of basins in South Chukchi petroleum basin (Lo for De Long, NSc for North Schmidt, SSc for South Schmidt, Ho for Hope) and New Siberian–North Chukchi petroleum basins (NS for New Siberian, Vi for Vilkitsky, NCh for North Chukchi).

Sag, etc. were discovered in the Cretaceous–Cenozoic rift east of the study area. Hydrocarbons migrated along the Brookian sequence. About 60 oil and gas accumulations occur border faults of the rift and along the Early Cretaceous within the Barrow arch (Bird, 2001). unconformity. Additional influx of hydrocarbons was from the The lower Ellesmerian strata in the Alaska North Slope Shublik shale deposited in the depocenter of the today’s Basin (Fig. 2) are Late Devonian–Permian continental-shelf Chukchi Sea shelf (Sherwood et al., 1998). Oils in the Prudhoe deposits that came from a source within the present-day Arctic Bay and other associated fields in the Ellesmerian and Ocean and accumulated on a south-facing passive margin Beaufortian sequences have high sulfur contents (1–2%), low (Houseknecht and Bird, 2005; Sherwood et al., 2002). It was gas/oil ratios (less than 1500 tcf/BOE), V enrichment over Ni, a complex amalgamation of onshore deltaic and submarine fan and low saturated/aromatic HC ratios (0.9–1.7). facies of a coastal-plain at the land–sea transition. The The Ellesmerian and Beaufortian sequences are discor- Ellesmerian shelf deposits (Lisburne and Sadlerochit Forma- dantly overlain (along the Lower Cretaceous Unconformity, tions), as well as the Endicott Group in the Hanna trough, LCU) by the Colville Basin sediments (Cretaceous–Cenozoic contain both source and reservoir rocks (Bird and Jordan, Brookian sequence). The Brookian sequence contains the Hue 1977), the Endicott source rocks being only gas prone below radioactive shale source rocks at its base lying under the 10 km. gas-prone Torok–Nanushuk and Canning–Sagavanirktok For- The lower section of the Triassic–Jurassic Beaufortian mations. Gas producing are the Colville, Nanushuk, and Upper sequence consists of synrift deposits and includes the source Torok formations which bear coal beds and have medium formations of Shublik and Kingak black shales (Bayliss and thermal maturity with a vitrinite reflectance of R0 = 0.5–0.6% Magoon, 1988) with sandstone beds (e.g., Sag River) between (Claypool and Magoon, 1988). The largest oil-generating and inside them. The shales host the Prudhoe Bay, South depocenters are located in the shelf part of the basin where Barrow and other oil fields. The Shublik and Kingak shale the Brookian strata are 5–8 km thick. formations locally bear high contents of types I and II organic Oils in the Brookian sequence have low sulfur contents matter (according to Van Krevelen’s diagrams) of up to 3–5% (less than 1%), relatively high gas/oil ratios (1500– Corg and 0.03–0.3% hydrocarbons, with up to 60–90% col- 7000 cf/BOE), greater concentrations of saturated hydrocar- loalginite in the north of the basin. The rocks are thermally bons (saturated/aromatic HC ratios 1.7–3.5) and low N and V mature (vitrinite reflectance R0 = 0.6–1.3%, corresponding to (Ni higher than V). Oils from the lower and upper parts of the main zone of oil generation) and generate oil and the sequence obviously differ in physicochemical and hydro- condensate (Magoon, 1994). The upper section of the Beaufor- carbon compositions and apparently belong to two different tian sequence (Upper Jurassic–Neocomian) consists of pebble petroleum systems with a Paleozoic–Jurassic and a Cretaceous shale deposited during a later rifting stage related to Am- sources. This idea is consistent with the C isotope composition erasian spreading. and biomarkers (Magoon and Claypool, 1981). Oil accumulations in the Barrow arch are derived from the Recent data from the so-called 1002 area (Montgomery, Shublik and Kingak shales, including those in the Beaufort 2005) in coastal plains between the Prudhoe Bay field to the V.E. Khain et al. / Russian Geology and Geophysics 50 (2009) 334–345 341 west and the Mackenzie delta to the east predict inplace The New Siberian–North Chukchi Basin, the largest in the resources of 11.6 to 31.5 BBOE of oil, with technically East Arctic shelf, has attracted attention of many Russian recoverable resources in a range from 4.3 to 11.8 bln bbl. The geologists who distinguish it in different configurations and area has not been drilled so far as it remains off limits to under various names (e.g., Filatova and Khain, 2007; Ivanova, exploration being part of the Arctic National Wildlife Refuge 2004; Khain and Polyakova, 2006; Kleschev and Shein, 2007; (ANWR). Kos’ko, 1988; Kravchenko, 1996; Mazarovich and Sokolov, Correlation of well sections in the Chukchi shelf and the 2003; Vinogradov et al., 2004). The petroleum basin has its Alaska North Slope Basin, according to faunal and geophysi- limits partly coinciding with large fault zones of the respective cal data, showed that the two areas shared the same Phanero- directions (Figs. 1 and 3). zoic geological history (Klemperer et al., 2002). A well in the The New Siberian–North Chukchi Basin is bounded by the Burger field stripped Hauterivian–Barremian sandstone strati- New Siberian–Wrangel–Herald thrust front on the west and graphically equivalent to the C sandstone of the Kuparuk south and by the Chukchi Ridge (borderland) and the De Long Formation in the Beaufortian sequence which is a high-quality arch in the north, and the North Chukchi Ridge of the Barrow reservoir for the onshore Kuparuk petroleum field. A sand dome in the east. The basin formed in the Aptian–Cenozoic reservoir discordantly overlain by the Pebble and Hue shales upon the East Arctic continental plate, on its segments hosts the Burger gas and condensate accumulation with 9.5 tcf deformed in the Middle Cretaceous and on undeformed of gas and 489 mln bbl of natural gas liquids (Houseknecht sediments (Filatova and Khain, 2007). The basement of the and Bird, 2005). basin consists of rifts filled with Carboniferous–Neocomian Total known oil and gas resources of the Arctic Alaska terrigenous and carbonate sediments coeval to the Ellese- Petroleum Province, which encompasses the Alaska North merian and Beaufortian strata. The basin is a large and Slope Basin and the neighbor Chukchi and Beaufort shelves, complex NW rift zone bordered in the north and south by were estimated as 80 BBOE of inplace oil and about 230 tcf steep faults of which the southern one has a prominent of natural gas (Houseknecht and Bird, 2005). The greatest right-lateral strike-slip component. Franke et al. (2004) and potential is expected from structural and stratigraphic traps of Sherwood et al. (2002) interpret it as a main transform fault the Beaufortian and Brookian (rift and fold-thrust) sequences. responsible for the detachment of the northern block of the The South Chukchi Basin is located in the southern Hanna trough and northeastward deepening of the East Chukchi Sea and in the southeastern East Siberian Sea (Fig. 1), Siberian Sea shelf. A large transversal uplift and a structural between the Wrangel–Herald–Lisburne frontal thrust zone in closure divide the basin into three segments: one with the the north and the basement uplift in the onshore Chukchi area North Chukchi Basin in the east, another one with the in the southeast. The basin, extended in the NW direction, has Vilkitsky trough in the west, and the northwestern one with a been shaped up in its present geometry and structure through narrow “bicorn” New Siberian trough confined between the the Cenozoic under the effect of right-lateral strike-slip De Long arch and the Kotel’nyi–Svyatoi Nos ridge. faulting (Klemperer et al., 2002) and is of a rift origin. The North Chukchi Basin lies upon a very deep rift trough The eastern part of the basin, with the Hope Basins located marked by a sharp sediment thickness difference. According mostly on the American shelf, has been better documented. to CMP data (Grantz et al., 1990a, b), the basin section The basins have a system of half-grabens at their base, of includes the Lower and Upper Brookian subsequences sepa- which two largest ones are filled with 4–6 km thick sediments rated by the Middle Brookian unconformity. The Lower (Klemperer et al., 2002; Tolson, 1987). The Cenozoic basin Brookian sequence, 11 km of total thickness, consists of three fill includes three seismic facies of Middle Eocene–Oligocene, seismic facies of which the lower one may correspond to Early Miocene, and Late Miocene ages correlated with drill Aptian–Albian molasse (3 km thick) originated at the front of sections in the Kotzebue Basin south of the study area. The the Wrangel–Herald thrust and two other facies are Late lowermost seismic facies consists of nonmarine deltaic sand Cretaceous silt and clay interbedded with sandstone. The and volcanic tuff with coal interbeds and few lava flows, the Cenozoic section of the Upper Brookian sequence is tenta- middle one corresponds to shelf sand and silt, and the upper tively divided by unconformities into three facies of Paleo- facies is composed of marine and lacustrine clayey silt with cene–Eocene, Oligocene–Miocene, and Pliocene–Pleistocene sand and coal interbeds. Each facies contains both source and (over 3 km thick). The total thickness of Late Cretaceous–Ce- reservoir formations. nozoic sediments above the basin axis exceeds 14 km but is The greater western part of the basin is occupied by the as low as 4–2 km on the basin sides. The whole sedimentary Russian shelf where the top of the Cretaceous section of the cover, deposited possibly since the Jurassic, may be 18–22 km Lower Brookian sequence (acoustic basement) lies at depths thick in total (Kim et al., 2007). of 1–4 km, deepening in the northward direction. The The Brookian sequence is pierced by large diapirs, which plane-bedded Upper Brookian sequence consists of fluvial and have Paleozoic roots and are thought to be either salt (Thurston estuarine terrigenous deposits with their thicknesses of 2 km and Theiss, 1987) or argillic (Grantz et al., 1990a,b). The to 4 km in the De Long and North and South Schmidt Basins North Chukchi Basin fill is densely faulted, especially in its arranged from west to east (Fig. 3). The basin sediments may Vilkitsky and New Siberian sectors adjacent to the De Long be gas prone, but the deeper Hope Basins in the American arch. The most strongly faulted places likely underwent rapid shelf appear to have better prospects. subsidence. 342 V.E. Khain et al. / Russian Geology and Geophysics 50 (2009) 334–345

There are two deposition centers in the Vilkitsky trough in bidite with volcanic interbeds and one Cenozoic facies the western sector of the New Siberian–North Chukchi Basin likewise composed of marine turbidite but without volcanic and three small but rather deep depocenters in the northwest- rocks. ern sector with the narrow bicorn New Siberian trough; all The available data from the East Arctic shelf and slope depocenters are separated by structural closures where the basins are too scarce to estimate their hydrocarbon potential sedimentary section misses Cretaceous strata and the sediment but one may use their analogy with better explored petroleum thickness reduces to 2–4 km, as in the transversal uplift basins that exist in similar passive margin settings. The New between the Vilkitsky and North Chukchi troughs. The total Siberian–Chukchi rift zone located in the today’s outer shelf thickness of Upper Cretaceous–Cenozoic sediments in the appears to be similar to the Beaufort rift east of it, in which depocenters reaches 10 km (Fig. 3). The section includes a the sediments were sources for many oil and gas fields of the Cretaceous, a Paleogene–Miocene, and a Pliocene–Quaternary Barrow arch. Thus, one may expect that the New Siberian– seismic facies (Franke and Hinz, 1999) which correlate with Chukchi rifting was favorable for oil and gas generation, both the Laptev Sea seismic facies and correspond to global in sediments and in basement uplifts, within the East Siberian geological events. The eastern depocenter includes pull-apart continental slope transition which faced hydrocarbon flows. sub-basins, with <6 km thick sediments, formed through the The East Arctic basins share some features of similarity Oligocene–Miocene (Franke et al., 2004). Cretaceous deposits with the basin of the Mackenzie delta on the Beaufort slope are discontinuous, the Paleogene–Miocene facies has a high where over 70 small and medium-size oil and gas accumula- thickness in sags while the more uniformly thick Pliocene- tions were discovered (Skipper, 2001), the largest being the Quaternary facies levels out the previous tectonic pattern. Taglu, Parsons Lake, and Niglintgak gas and Amauligak oil The Upper Cretaceous–Cenozoic sedimentary lens is com- discoveries. The recoverable resources in Eocene and Oligo- posed of clay-siltstone and silt-clay rocks, mainly above the cene reservoirs exceed 3.5 BBOE of oil and 35 tcf of gas trough floor, whereas siltstone, sand, and conglomerate are (Dixon et al., 1994). rare and restricted to the basin sides. The section is locally Prospective basins like those in the East Arctic shelf and coaliferous, and the Neogene sequence is injected by lavas slope are known on other continents as well. They are, for along the basin northern side known in Zhokhov and Vilkitsky instance, those that generated large Akata, Landana, Malembo, Islands. The Cretaceous and Paleogene regional unconformi- and other fields in thick fans on the West African shelf and ties are erosive (Gramberg et al., 2004). Fluvial, deltaic, fan, slope (Khain and Polyakova, 2004, 2008; Zabanbark and and turbidite deposits accumulated in frequently alternating Konyukhov, 2005) and high-quality reservoirs in sand-filled coastal, tidal, and shelf environments. Deepwater environ- channels and canyons in the slope steep part. The combination ments may have occurred during transgressions above the axes of favorable conditions for hydrocarbon generation and en- of the Vilkitsky and North Chukchi troughs, which were trapment created the large fields of Bonga, Erha, Agbami, favorable for deposition of mudrocks rich in water-borne Dalia, Girassol, etc. A number of deepwater discoveries were organic matter. The uplifted borders of the New Siberian– reported in Eocene sand fans at the Gulf of Mexico continental Chukchi Basin were subject to erosion during regressions rise (Durham, 2006). attendant with coarser deposition along the basin sides. Thus, similar oil source rocks may be expected also in the Judging by the sea bottom topography, the continental margin New Siberian–North Chukchi and East Siberian Basins in the may have grown at the account of submarine fans between East Arctic outer shelf. They may be similar in lithology and the De Long arch and the Chukchi Ridge. facies to the Brookian sequence in the Alaska North Slope The East Siberian foreland basin located along the and, by this analogy, are expected to generate mostly natural continental slope adjacent to the De Long and Mendeleev gas and less paraffin oil in fringes of gas and condensate fields. Ridges (Fig. 1, 3) extends into the continental rise and may Large thicknesses (10–14 km) and seismic velocity variations partly cover the Podvodnikov Basin. The major basin received (from 1.75 to 4.0 km/s) (Lazurkin and Pavlov, 2005) suggest sediments shed from the shelf that emerged during several that the sedimentary section may include all zones of thermal lowstand events associated with Late Cenozoic glaciation. maturity and hydrocarbon generation. Hydrocarbons may Depending on the sea level, the deltas and fans of northern accumulate in structural or depositional traps of channel-levee paleo-rivers migrated between the land and the continental systems and in traps associated with diapirs. slope or moved hundreds of kilometers along the shelf. The Oil and gas generation in the East Arctic shelf and slope became cut by syndepositional faults that created a shelf-slope basins appears to be well explained in terms of a stepped profile. Sediment thicknesses in places of continuous known model of phase zoning according to chemistry, tecton- deposition at the continental rise reach 10 km or more ics, and lithology of formations. This is a succession of gas, (Lazurkin and Pavlov, 2005). The basin section consists of oil, and bitumen zones arranged away from the depocenter Cretaceous–Cenozoic sediments over a Precambrian–Meso- toward uplifts with pinching out sand beds. Patterns of this zoic folded basement, with oceanic or suboceanic crust in kind were reported from the modern continental slope, as well deepwater parts, as one may expect by analogy with the slope as in the West Siberian, Timan–Pechora, East Barents, North basins west of the area (Sekretov, 2002). Sediments belong to Alaska, and other basins where deep-seated high-temperature three seismic facies: two Cretaceous ones (Aptian–Santonian gas failed to accumulate in situ in low-porosity reservoirs. and Campanian–Lower Paleocene) of marine sand-clay tur- Thus it was expulsed along faults through zones of abnormal V.E. Khain et al. / Russian Geology and Geophysics 50 (2009) 334–345 343 pressure to medium basin depths, into the zone of oil the available data on tectonics and petroleum potential of the generation at lower temperatures and pressures, and forced oil underexplored East Arctic area. We investigated the ages, up into highs and slopes; gas accumulation occurred mostly history, and settings of the major tectonic units and imaged in the basin central parts. them in a generalized tectonic map. The petroleum potential According to this model, high-quality reservoirs of the New was studied especially for numerous small Cretaceous–Ceno- Siberian–North Chukchi Basin (Khain and Polyakova, 2007) zoic basins on the East Siberian and Chukchi shelves and may exist in submarine fans hypothetically associated with slope, and likewise sketched in the respective map. least deformed sediments on the northern basin side between 1. The East Arctic area consists of several tectonic units of the Chukchi Ridge and the De Long arch (Fig. 3). In the south different types and ages. Its polar sector is occupied by the a prospective area with an uncertain potential is inferred in Amerasia Basin, which is now part of the Arctic Ocean. The the side of the North Chukchi Basin. No less prospective are adjacent continental margin of Eurasia and North America wedge-shaped zones along the edges of the large transversal involves fragments of the Hyperborean craton and Early uplift between the Vilkitsky and North Chukchi Basins and Paleozoic to Middle Cretaceous orogens. The orogens were along structural closures. The New Siberian trough on the produced by closure of oceans of the respective ages which northwestern extension of the major basin may be producing led to successive amalgamation of cratonic blocks into in its western part within the uplifted side and a high dividing supercontinents. Of great importance was the Ellesmerian the trough into two arms. phase of the Early Paleozoic orogeny responsible for the Sandstone or weathering residue reservoirs in small pull- assembly of Laurussia in the south of the study area. At the apart basins and transversal uplifts at medium and shallow Devonian–Carboniferous boundary, Laurussia underwent rift- depths above the large base of the New Siberian–North ing and spreading with opening of the Alazeya–South Anyui– Chukchi Basin may contain gas accumulations. Angayucham Ocean. Closure of the latter, ensuing from Late The relatively high gradient of the continental slope in the Jurassic–Early Cretaceous spreading in the Amerasia Basin, East Siberian foreland basin prompts the existence of fan gave rise to the Verkhoyansk–Chukchi tectonic zone with the systems with channels and canyons prominent in satellite Verkhoyansk–Kolyma collisional system along the deformed images. The repeatedly rejuvenated canyons at the steep steps continental margins, and syncollisional foredeeps. The Am- of the faulted slope, as well as sand-filled channels, may trap erasian spreading (Canada and Podvodnikov–Makarov Basins) oil and gas. Voluminous finer sediments in the distal ends of turbidite systems in the continental rise may be active oil and forerun by Triassic–Jurassic rifting was the initial event in the gas producers supplying hydrocarbons into the upper section modern history of the Arctic Ocean. Later Cretaceous and and into traps on the stepped slope. Cenozoic pull-apart and rift basins that arose on shelves and The main East Arctic known and potential oil and gas continental slopes of the fringing seas during the opening of discoveries (in the New Siberian Islands, in the Alaska North the Amerasia and Eurasia Oceans had completed the tectonic Slope, in the Beaufort–Mackenzie area, and in the onshore framework of the today’s East Arctic. part of the Verkhoyansk–Chukchi fold-thrust system) span a 2. The Mesozoic–Cenozoic shelf (South Chukchi and New large stratigraphic range from the Upper Devonian to the Siberian–North Chukchi) and continental slope (East Siberian Cenozoic. The largest accumulations are held by the Elles- on the Podvodnikov Basin periphery) basins on the East Arctic merian and Beaufortian sequences (Upper Devonian–Neo- margin may be expected to have high petroleum potential, by comian). The Aptian–Cenozoic Brookian sequence contains analogy with basins of the Arctic and Atlantic passive margins 1/4 oil and about 1/2 gas relative to the resources of older where economic oil and gas fields were discovered. reservoirs. The reason may be in a greater thickness of Oil and gas in shelf basins may be trapped in zones of Devonian–Neocomian deposits in onshore basins. pinching out sand beds where they meet transversal uplifts, Taking into account the stratigraphic range of the shelf and structural closures, and slopes. Reservoirs in slope basins may slope basins, one may expect a greater hydrocarbon potential be associated with the sand fill of canyons and channels on from Cretaceous–Cenozoic reservoirs in the New Siberian– the stepped continental slope. North Chukchi Basin and from Cenozoic reservoirs in the Most of oil and gas fields may occur in Cretaceous and South Chukchi and Fore-East Siberian Basin. Older reservoirs Mesozoic basins. Natural gas entrapment may occur in synrift in basement troughs may be prospective in the northwestern sediments of two stratigraphic levels: Late Devonian–Early side of the major basin because Devonian, Carboniferous, and Carboniferous strata (equivalent of the terrigenous Endicott Permian clayey limestone and Lower–Middle Triassic shale Group in the Hanna trough) and especially Triassic–Jurassic source formations were discovered in the neighbor New formations (equivalent to thick shale and turbidite formations Siberian Islands and in their extension of a submerged of the Beaufort and other basins). Late Cretaceous–Mesozoic pericline. basins appear to be mostly gas prone with less significant oil and bitumen resources. Conclusions The study was carried out as part of Projects 1 and 14 of an ESD RAS Program and was supported by grants 08-05- The reported study has been performed as part of an IPY 00748 from the Russian Foundation for Basic Research and (International Polar Year) project with the aim to synthesize SS-651.2008.5 from the President of the Russian Federation. 344 V.E. Khain et al. / Russian Geology and Geophysics 50 (2009) 334–345

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