V.Verzhbitsky and T.Savostina - TGS-NOPEC Geophysical Company Moscow, Russian Federation
E.Frantzen - TGS-NOPEC Geophysical Company ASA, Asker, Norway
A.Little - TGS Geological Products and Services, Stavanger, Norway
S.D.Sokolov and M.I.Tuchkova - Geological Institute, Russian Academy of Sciences, Moscow, Russian Federation
During the summer of 2006 TGS-NOPEC Geophysical Company conjointly with "Geophysical Solutions Integrator" acquired new seismic data in the Russian part of the Chukchi Sea (total volume approximately 3600 km). Due to the absence of offshore wells in the Russian sector of Chukchi Sea, the interpretation of acquired seismic data and definition of probable hydrocarbon potential must be based on the comparison with the US sector of the Chukchi Sea and the Alaska North Slope, as well as on the geology of Chukotka Peninsula and Wrangel Island (figure). In particular, the preliminary results of International Russian-Sweeden-USA geological expedition-2006 on Wrangel Island and Northern Chukotka (Sokolov et al., 2007; Pease et al., 2007; Verzhbitsky and Miller, 2007) are quite important as well. The famous HC discoveries are known for US Arctic Alaska and Chukchi shelf (Prudhoe Bay, Kuparuk, Burger, etc), whereas Russian part is still poorly explored by present-day geological and geophysical methods. Nevertheless, the widely proposed similarities in tectonic history and depositional settings of both sectors, point to the significant hydrocarbon potential of Russian Chukchi shelf.
The studied area of Chukchi Sea includes several regional tectonic subdivisions (from South to North): Chukotka fold belt area, South-Chukchi sedimentary basin, Wrangel (Wrangel-Herald) Arch, North-Chukchi sedimentary basin (figure). Southernmost area, nearest to Chukotka Peninsula, is a part of Late Kimmerian New Siberian-Chukotka fold-thrust fault belt, formed in Neocomian (pre-Aptian/Albian) as a result of closure of South-Anyui paleooceanic basin, subsequent collision between Eurasia and New Siberian-Chukotka microplate and the formation of narrow, highly deformed South-Anyui suture zone (e.g. Bondarenko, 2004; Sokolov et al., 2001, 2002).
More to the North, between Chukotka and Late Kimmerian Wrangel-Herald arch, the South Chukchi sedimentary basin was outlined by several researchers [e.g. Shipilov et al., 1989; Mazarovich and Sokolov, 2003]. The time of its formation is unclear, but as it is superimposed on Late Kimmerian fold belt, the age of its sediment filling must be not earlier, than Aptian-Albian - Late Cretaceous. According to the drilled cores in US part of Chukchi Sea, on the proposed E-SE continuation of the South Chukchi basin - Hope basin, its age is Cenozoic (Tolson, 1987).
Wrangel Arch and North Chukchi Basin
It is commonly believed, that Wrangel arch represents the NW continuation of Herald thrust - Cape Lisburne and Brooks Range fold-thrust fault structure. The time of the arch formation may corresponds also to the Late Kimmerian, Neocomian, but its reactivation might took place later, in Cenozoic (Moore et al., 2002; O'Sullivan et al., 1997). The structures of Wrangel arch are overthrusted on the North-Chukchi sedimentary basin, which is underlain by Mid-Palaeozoic (Franklinian) basement, Ellesmerian deformation [Grantz et al., 1990; Khain, 2001]. It is proposed, that the lowest part of the sedimentary cover must contain Upper Devonian -Carboniferous sediments, as it proposed also for the Hanna trough (Sherwood et al., 2002). The maximum Pz-Mz-Cz sediment thickness of the North Chukchi basin exceeds 16 km.
In the Northern part of Wrangel Arch, close to its junction with North Chukchi basin, the double (both North- and South-vergent) "positive flower" and "push-up" structures are widely distributed in the basement (earlier) and upper part of sedimentary cover (latest) with vertical offset up to the several hundreds of meters (fig. 4). This structural pattern point to the combined compressional/strike-slip (transpressional) kinematics of the Wrangel arch front. The observation is in a good agreement with previous result, obtained for the NW part of East Siberian Sea: Drachev et al.  also pointed to the transpressional structural pattern of the Late Kimmerian fold-thrust fault northern front with Hyperborean (epi-Ellesmerian) massif.
The rather uniform fold-thrust fault-nappe structure of Wrangel arch, gently dipping to the south, is well-documented in on-land outcrops of Precambrian-Paleozoic-Triassic rocks of Wrangel Island [e.g. Kos'ko et al., 1993]. If so, transpressional structural pattern of northern front of Late Kimmerides, visible on the obtained seismic profiles, may reflect the latest stages of compressional collision-related deformation, occurred in basement (1st) and in upper part of overlying sediments (2nd). So, at least two or three main compressional events occurred during the formation of Wrangel arch structure. It is interesting to note, that within the South-Anyui suture zone two main stages of collisional deformation were recognized: first - compressional, formed the North-vergent fold and thrust fault fabric, and, second - right-lateral transpressional, formed subvertical and double vergent (both North and South) structural pattern, superimposed on the earlier structure. Both these stages were occurred before the Albian time [Sokolov et al., 2001, 2002; Bondarenko, 2004]. From the other (South-Eastern) side of Wrangel-Herald arch, in Brooks Range and North Slope area two main stages of compression were recognized -Jurassic-Early Cretaceous (1) and Late Cretaceous/Early Cenozoic (2) with intermediate Mid-Cretaceous extensional stage [Miller and Hudson, 1991]. Slightly inverted small half-grabens, superimposed on the Wrangel arch basement, also may point to the intermediate stage of extension between two compressional/transpressional events. Although the time of compressional/transpressional/extensional stages in studied area are controversial, their existence provide us a good opportunity to correlate the established structure in Eastern Chukchi Sea region with NE East Siberian Sea, Northern Chukotka and, probably, Brooks Range areas.
Three main angular unconformities were recognized at the seismic lines. The lowermost unconformity (LCU) may corresponds to the main collisional stage completion in Late Neocomian and the beginning of Brooks orogen-derived clastic molasse sedimentation (Brookian sequences) of Aptian-Albian - Cenozoic. We propose, that this boundary corresponds to the well-known pre-Aptian unconformity of northeastern Eurasia related to the South-Anyui paleooceanic basin closure time. In particular, according to the last precise U-Pb (SHRIMP) dating of postcollisional plutons in the Northern Chukotka (made in Stanford University, CA, USA), the oldest granites are as old as Late Early Cretaceous (Aptian - 117 Ma) (Katkov et al., 2007), giving us the upper time limit for the age of collisional deformation. Below this surface, the Ellesmerian (Upper Devonian-Upper Jurassic) and Rift (Upper Jurassic - Early Cretaceous, related to the rift episode on the Northern Chukchi shelf) (Sherwood et al., 2002). The most obvious unconformity in the upper part of sediment may corresponds to Mid-Brookian, MBU (~K/Cz), reported earlier for this area by A.Grantz et al. (1990) and Burlin, Shipelkevich (2006). The uppermost angular unconformity is not very well-known here. We speculate, that it may be as old as 24 Ma compressional/cooling event (latest Late Oligocene), reported for Brooks Range and Colville basin (O'Sullivan et al., 1997; Moore et al., 2002), but not known for Chukotka/Wrangel area. It doesn't contradict with the structure of South-Chukchi basin, where some pre-Miocene inversion (transpression) structures are also occur.
The wide-spread anticline structures in Paleozoic, Mesozoic and, to a lesser extent, Cenozoic sequences and "bright spot" seismic anomalies, proposed gas chimneys, etc point to the present-day active hydrocarbon systems. According to the lithological-geochemical investigations, Carboniferous and Triassic sedimentary successions, exposed on Wrangel Island, were acted as oil-generating units. Carbonate formation of Carboniferous age, appears to be similar in lithology with Lisburne formation on Alaska, where it represents one of the producing units of Prudhoe Bay oil field (Khain, Polyakova, 2007). It is likely, that Ellesmerian strata (Carboniferous, Permian and Triassic) and, probably, younger sediments of Rift sequences to the north of Wrangel Arch are not very deep-subsided (2-4 seconds, twt) and moderately deformed. So, the northern slope of Wrangel Arch appears to be quite prospective, as the mentioned above sequences represent main HC producing units of Arctic Alaska. The Early Tertiary (visible above the MBU unconformity) deltaic/progradational sequences are quite interesting as well.
The South Chukchi Basin
The South Chukchi sedimentary rifted basin separates the Late Kimmerian Chukotka Fold Belt and the Wrangel Arch, and represents the NW continuation of well-known Hope Basin of the Chukchi Sea (US sector), filled by Cenozoic nonmarine, marine and lacustrine rocks (Tolson, 1987). The sediment thickness above acoustic basement seldom exceeds 3-4 km, but can locally reach 5-6 km. The geometry of the faults indicates an extensional/transtensional setting of the SCB evolution (similar with the Hope basin in US part of Chukchi Sea), although compressional structures (folds and reverse faults) also occur. The low-angle pre-rift thrust faults are recognized at the base of the South Chukchi Basin. Most probably, they represent the highly deformed Triassic sequences overthrusting the much less deformed Upper Jurassic-Lower Cretaceous strata, as shown earlier for Central Northern Chukotka (Baranov, 1995). The main stages of SCB development are comparable with those of the Hope basin (Tolson, 1987; Elswick, Toro, 2003). It was concluded earlier, that the Hope basin hydrocarbon potential is quite low, as it contains thermally immature nonmarine gas-prone deposits (Tolson, 1987). Nevertheless, it was also noted, that if organic-rich oil-prone source rocks and reservoir rocks are present in most subsided parts of the basins (2,5-5,0 km), it may lead to significant hydrocarbon accumulations occurrence (Tolson, 1987). Previously, we already have noticed, that the changes in phase or polarity in upper parts of the sedimentary cover, listric fault planes in the pre-rift sequences, associated with areas of reduced reflectivity in the upper sediments may point to a gas presence. The Upper Jurassic - Lower Cretaceous syn-orogenic clastic sequence, which is exposed in the Central Northern Chukotka (figure), contains visible plants remnants and coal beds (Klubov, Semenov, 1973). It is clear from regional geological maps, that this organic rich unit is continuing offshore to East Siberian (Chauna bay) and, probably, to Chukchi shelf (Verzhbitsky et al., 2008). Thus, we propose, that the J3-K1 pre-rift sequence may represent regional gas source rock, at least for the South Chukchi basin. Although the two drill wells at the SE termination of U.S. Hope basin have recovered Mid-Eocene and younger sediments, the analysis of onshore data from Chukotka and Wrangel Island (Natal'in, 1999; Kos'ko et al., 1993) points to the beginning of rift-related sedimentation from Late Cretaceous-Paleocene time. So, additional source rocks may corresponds to Azolla strata, deposited during Early Eocene warming event and containing large amounts of buried organic material (Moran et al., 2006). The analysis of bottom sediments in this area has revealed anomalous concentrations of migrated hydrocarbon gases, suggesting significant HC prospectivity of the South Chukchi (Hope) basin (Yashin, Kim, 2007) as well.
The estimated total recoverable resources of East Siberian and Chukchi Seas constitutes more than 8 billions of tons in oil equivalent (Geology and Mineral resources, 2004). According to the another opinion (Arctic Today..., 2006), taking into the consideration the geological correlation with Arctic Alaska, the estimates must be at least 2 times higher.