STRUCTURE AND HYDROCARBON POTENTIAL OF EAST BARENTS AND NORTH CHUKCHI BASINS| Structure and hydrocarbon potential of east Barents and North Chukchi basins |
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VERZHBITSKY V.E., FRANTZEN E.M., LITTLE A., SOKOLOV S.D., TUCHKOVA M.I. Here we present the synthesis of available geological-geophysical data on two largest depocenters of Russian Arctic shelf - East Barents and North Chukchi basins.They are filled by thick piles of the sediments - up to 18 km and even more. It is believed, that the sedimentation in both of the basins has started in Paleozoic (Late Devonian) time. The giant hydrocarbon potential is proven for East Barents and widely proposed for the North Chukchi basin. This study is based on the newly acquired seismic data (2005, 2006), available published material and some results of field-work observations on Northern Chukotka and Wrangel Island (2006). Our research of East Barents Sea region is mostly based on the TGS/New Geoprojects seismic data (2005) and TGS/VBPR Geophysical Atlas of Barents Sea (2004). Eastern Barents Sea region is one of the most hydrocarbon-rich sectors of Russian Arctic shelf, where the well-known gas and gas-condensate fields were discovered: Shtokman (one of the largest discoveries in the world), Ledovoe, Ludlov, etc. (e.g., Gramberg et al., 2002; Shipilov, Tarasov, 1998). The majority of the regional HC fields and potential structures are localized within the prominent NNE-trending East Barents rift (trough), filled by up to 18-20 km thick Late(?) Paleozoic – Mesozoic sequences (Bogdanov, Khain, 1996). The trough is traced to the west from Pay-Khoy - Novaya Zemlya Early Kimmerian fold belt (and its’ foredeep) and consists of two different sedimentary basins (Southeast and Northeast), separated by Ludlov Saddle. The timing of initiation of the trough formation is still a matter of discussion. It is proposed, that the Khibiny-Kontozero extensional fault zone on the Kola Peninsula, with most extensive alkaline magmatism event in the Late Devonian (timing of rifting initiation?), ~360-380 Ma (Sharkov, 2002), represents the SW onshore propagation of East Barents rift (Baluev, 2006; Leonov et al., 2007). Major subsidence of the trough took place during Late Permian and, mostly, Triassic, accompanied by voluminous terrigenous sedimentation (Shipilov, Tarasov, 1998; Shipilov, 2003). It is believed, that two major magmatic events influenced the East Barents trough in Mesozoic. First, Permian/Triassic, is considered to be synchronous with Siberian traps magmatism and second, dated in Ludlov offshore well, corresponds to the Late Jurassic-Early Cretaceous time (Shipilov, Tarasov, 1998). These events caused the intrusion of numerous basaltic sills and dikes, intruded mostly into Upper Permian-Triassic strata and recognizable on seismic records as high-amplitude prominent anomalies. Large vertical motions, probably associated with Late Jurassic-Early Cretaceous magmatism, likely formed the regional base Cretaceous unconformity (BCU), overlain by progradational sequences (Geophysical Atlas…, 2004). The main hydrocarbon accumulations are related to gently folded Triassic, Jurassic and, to a lesser extent, Cretaceous units. It is interesting to note, that the 3D geothermal modeling has revealed the positive dome-shaped thermal anomaly, associated with the main regional hydrocarbon fields in the Southwestern Barents Sea (Podgornykh, Khutorskoy, 2000). The origin of the fold structures is still controversial. Shipilov (1998) related the folding to magmatic sills intrusion expanding influence on the above strata. According to another point of view, formation of the contractional domes was caused by far-field effect of Paleogene right-lateral transpressional movements between Greenland and Svalbard during opening of Nowegian-Greenland sector of North Atlantic (Geophysical Atlas…, 2004). The U.S. Arctic Alaska region is famous for large oil and gas discoveries (Prudhoe Bay, Kuparuk River, Burger, etc), whereas, the geologically equivalent region in Russian territory is still poorly explored (e.g., Orudzheva et al., 1999; Mazarovich, Sokolov, 2003; Burlin, Shipelkevich, 2006; Khain, Polyakova, 2007). The Russian sector of the Chukchi Sea includes several regional tectonic subdivisions, from south to north: Chukotka (Late Kimmerian) fold belt, South Chukchi Basin (K2(?)-Cz), Wrangel (Wrangel-Herald) Arch Kimmerian, and the North Chukchi Basin. Wrangel Island, the uplifted block of the arch, is mostly composed of intensively deformed Precambrian metamorphic rocks and Paleozoic-Triassic sedimentary sequences, probably subjected to Early Carboniferous magmatism (Kos’ko et al., 1993; 2003). Thus, the Island is the key area for onshore investigation of the geological structure of Russian Chukchi Sea shelf. During summer of 2006, field works by International Russian-USA-Swedish expedition (S.Sokolov, M.Tuchkova, E.Miller, V.Pease and V.Verzhbitsky) were carried out in the Central, Western and Southern parts of the Island (Pease et al., 2007; Sokolov et al., 2007). The geological complexes of Wrangel arch are overthrusted to the North on the much lesser deformed North-Chukchi sedimentary basin, which is underlain by Mid-Paleozoic (Franklinian) basement (Khain, Polyakova, 2007). It is likely, that the lowest part of the sedimentary cover contains Upper Devonian (?)-Carboniferous sediments, as it proposed for the Hanna trough (Sherwood et al., 2002). The maximum Pz-Mz-Cz sediment thickness of the North Chukchi basin exceeds 16 km. During the autumn of 2006, TGS conjointly with “Geophysical Solutions Integrator”, acquired new seismic data in the Russian part of the Chukchi Sea. Due to the absence of offshore wells in the Russian sector, interpretation of the seismic data is quite speculative. Several major angular unconformities were recognized on the seismic lines. These are used to delineate the tectonic mega-sequences, compare to the U.S. sector and onshore Chukotka and Wrangel Island. The deepest unconformity (LCU), identifiable in the North Chukchi basin, may correspond to the main collisional stage completion in Late Neocomian, and the beginning of orogen-derived clastic molasse sedimentation equivalent to Brookian sequences of Aptian-Albian – Cenozoic age. This boundary also corresponds to the well-known pre-Aptian unconformity of northeastern Eurasia, related to Eurasia/Chukotka collision (e.g. Sokolov et al., 2002; Katkov et al., 2007). The Ellesmerian (D3-J3) and Rift mega-sequences (J3-K1) (Sherwood et al., 2002) may be indentified below this boundary. The most obvious unconformity in the upper part of sediments may correspond to the Mid-Brookian, MBU (~K/Cz), reported earlier for this area by (Grantz et al., 1990) and (Burlin, Shipelkevich, 2006). The shallowest identifiable angular unconformity is not very well-known here. We speculate, that it may be as old as 24 Ma related to a compressional event (latest Late Oligocene), reported for Brooks Range and Colville basin (O'Sullivan et al., 1997; Moore et al., 2002), but unknown for Chukotka/Wrangel area. The proposed age for this unconformity doesn’t contradict with pre-Miocene inversion (transpression?) occurring in the South-Chukchi basin. The wide-spread anticline structures in Paleozoic, Mesozoic and, to a lesser extent, Cenozoic sequences, provide ample structural traps. Wedge-outs, unconformities, deltaic/progradational units and potential fault traps, are also abundant. These features are often associated with “bright spot” seismic anomalies and gas chimneys, point to the significant regional hydrocarbon prospectivity (Verzhbitsky et al., 2008). According to the lithological-geochemical investigations on Wrangel Island, Carboniferous and Triassic sedimentary successions were activated as oil-generating units in the geological past. The Carbonate formation of Carboniferous age, appears to be similar in lithology with the Lisburne formation in Alaska, where it represents one of the producing units of the Prudhoe Bay oil field (Khain, Polyakova, 2007). It is likely, that Ellesmerian strata and, probably, Jurassic-Lower Cretaceous sediments immediately to the north from Wrangel Arch are moderately deformed, and relatively shallow. Thus, the buried northern slope of Wrangel Arch may be quite prospective, as mentioned above sequences represent main producing units of Arctic Alaska. In The participants of the International geological expedition-2006 are very grateful to Director and scientific staff of Wrangel Island National Reserve for their help on organization and carrying out of the field works. The Arctic research of the authors from Geological Institute RAS was supported by RFBR # 08-05-00547 grant and ONZ RAS. Literature Oil and gas of Arctic shelf 2008
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