| Structural-tectonic zoning and oil-and-gas prospects of Spitsbergen continental margin |
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KAZANIN G.S., SHLYKOVA V.V., FEDUKHINA T.YA., PAVLOV S.P., SUPRUNENKO O.I., ZUYKOVA O.N., USTINOV N.V. JSC MAGE has already carried out geophysical researches for more than 30 years in the water area of the Spitsbergen continental margin.At first these researches were performed along single lines and had a reconnaissance character and it allowed to find out and characterize basic structural zones, such as the West Arctic platform, a continent-ocean transition zone, representing a chain of perioceanic troughs, and the Norwegian-Greenland oceanic basin. Since 2002 there have been carried out systematic researches in the south-western sector of the Spitsbergen continental margin. The set of research methods remains the same: reflection shooting, refraction shooting (on separate lines), high resolution seismic, gravity and magnetic surveys and sea-bottom sampling. There has changed the quality of these researches, there was significantly increased equipment resolution, horizontal accuracy of surveying points, and software. In 2005 there was added gashydrochemical survey; its methods and technology were developed by employees of JSC "Yuzhmorgeoshelf". The complex character of researches allowed to cover a wide spectrum of questions connected with the geological structure of the Spitsbergen continental margin from deep earth crust horizons to a water layer where the subject matter is the concentration of hydrocarbon gases dissolved in water.The depth structure of studied area is shown in physical fields and first of all in the gravity field. Joint analysis of the gravity field and refraction – DSS data allowed to find out a regressive relation between occurrence depth of Moho and gravity field, there was constructed a crust thickness map with its use. There were recorded two intensive gravity anomalies of oval shape in the continent - ocean transition zone, connected with mantle plumes. Such gravity anomalies are also registered in other areas of passive continental margins (Pogrebitsky and others, 2001). The present geological and tectonic structure of the Spitsbergen continental margin was defined by reorganization of plate movement causing the subsequent forming of a stretching zone in Late Oligocene Age along the West Barents Sea margin, and then formation of the spreading centre of the Knipovich ridge. Pomorsky perioceanic trough was formed as a result of the Knipovich ridge development and its moving away from the origin margin and it was laid on the original Palaeocene-Eocene basin (Shipilov and others, 2007). Due to tectonic reorganization of the territory in Neogene Age the uplift of the Barents Sea margin and increasing of the erosion influence on the Barents Sea shelf provided an input of a large volume of sediments filled perioceanic troughs and even overfilled them. The complex of these global processes generated the present tectonic plan of the Spitsbergen continental margin, and it resulted in heterogeneity of Pre-Cenozoic basement and lateral differentiation of the sedimentary cover. On the base of integrated geological - geophysical researches carried out in 2008 there were found out smaller structural elements and thoroughly studied the structure of large structural zones, such as the zone of perioceanic troughs, the Norwegian – Spitsbergen step zone and the West Arctic platform represented by the orogenic system of West Spitsbergen and Svalbard plate. The sedimentary cover is represented by Cenozoic and Late Paleozoic – Mesozoic(?) sediments. The Cenozoic sediment complexes completely fill a deep-water Norwegian – Greenland oceanic basin, continent – ocean transition zone represented by perioceanic troughs and the Norwegian – Spitsbergen step zone. Late Paleozoic – Mesozoic(?) sediments are developed in graben-like troughs of the transition zone and they form the sedimentary cover of Svalbard plate. The conjunction zone of continental and oceanic basement is limited to depocentral areas in Atka and Pomorsky troughs. The destruction belt of continental crust is located between that zone and the Norwegian – Spitsbergen step zone. In the axial part of troughs the depth of heterogeneous basement occurrence reaches 12 km. Tectonic zoning is based on the top of heterogeneous Pre-Cenozoic basement (reflector Fok-Fk) with analysis of gravity anomaly distribution. The structure of East flank of the Knipovich ridge and Orogenic system of West Spitsbergen is thoroughly studied in works of E.A. Gusev (2001, 2005), E.V. Shipilov (2005, 2007), S.I. Shkarubo (1993, 1997, 2007), J.J. Livshitsa (1973) etc., therefore the basic attention will be devoted to the structure of the perioceanic trough zone and the Norwegian-Spitsbergen step zone. The Norwegian-Spitsbergen step zone is a system of horsts and grabens defined along the heterogeneous basement which presumably consists of Middle – Upper Riphean (?) rocks. Within this zone there was defined an underwater continuation of the Prince Karl horst, the Forlandsunnet graben, the Saffolk graben and the Torrel horst separated by sublatitudinal disjunctive fractures. The Prince Karl horst and Forlandsunnet graben are characterized by the north-western orientation peculiar to all Caledonian structures of this region. The Saffolk graben and Torrel horst located along the south-western part of Spitsbergen coast were identified for the first time. The main strike of specified elements differs from already known Prince Karl horst and Forlandsunnet graben. The Saffolk graben and Torrel horst are characterized by a north-eastern trending. The step zone is separated from structures of the West Arctic platform and the Norwegian-Greenland basin by Hornsund fault zone which takes a north-western direction. The zone of perioceanic troughs comprises Atka and Pomorsky troughs. They are underlain by heterogeneous basement. The western side of troughs consists of basalts of the East flank of the Knipovich ridge, whereas the east one is represented by blocks of breaking continental basement. Atka and Pomorsky troughs are divided by the Sorkapp saddle from each other. The Sorkapp saddle is a chain of upfaulted oceanic basement blocks separated by faults with north-western and north-eastern strikes. The north-western trending faults are elements of mid-ocean Knipovich ridge system (transforms). The block-faulted basement structure forms structural elements of higher order. Within the Atka trough there can be clearly seen Is-Forden and Atka steps and Atka depression. The Isfjorden step represents a system of continental basement blocks probably composed of Middle-Upper Riphean rocks, generated as a result of high-amplitude fault movements and represented as a single structural zone with northwest strike. The throws of basement blocks reach 2.0 km relative to each other. The Atka step represents an oceanic basement high limited to an isohypse of 5.0 km along the top of heterogeneous basement. The Atka depression is generated by the downfaulted motion with throw of up to 5.0 km, where glide planes of faults are inclined into depression depocentre. The Atka depression is represented as a narrow stretched zone of a north-western strike segmented in blocks by tectonic deformations, they move north-westward along these deformations. The Pomorsky trough is divided into the West Bear shelf and Pomorsky steps forming its east continental and western oceanic sides. The central position in the trough is occupied by the Pomorsky depression. The West Spitsbergen shelf step is a system of continental basement blocks composed of different age rocks and downfaulted by listric faults. The Pomorsky step is a high of an oceanic basement jointed to the north with the Sorkapp saddle by a north-western trending transform fault. The basement surface is gently dipped southward reaching the maximum depths (up to 11.0 km) in the southern part of the Pomorsky trough in the contact zone of oceanic and continental basements. On the base of reflection data interpretation in the structure of Cenozoic sedimentary cover there was defined a set of complexes represented bottom-up: Late Oligocene – Early Miocene(?), Middle – Upper Miocene, Upper Pliocene, Eopleistocene and Neopleistocene – Holocene deposits. Two lower complexes formed at stages of laying and initial growth of oceanic crust in the spreading center of the Knipovich ridge. A thick progradational wedge of Pliocene – Holocene deposits formed at the margin due to influence of glaciers on the Spitsbergen and Barents Sea shelf. Structural traps are defined along the top of the reflector R7 connected with the bottom of Upper Pliocene deposits. The largest anticline structures were given specific names – Tampenflaket and Salpunten. Mentioned structures are located in the north-western part of Atka trough (Kazanin G.S. and others, 2007) and limited to Isfjorden and Atka steps bounding Atka depression, and from that area hydrocarbons (HC) could migrate updip. The increased thermal stream and considerable lying depths of sediment material must provide maturing of the organic substance (OS) and hydrocarbon generation (Crane, 1988). An indirect sign of hydrocarbon presence within the Cenozoic succession are “torch” anomalies according to echo-sounding data acquired in vicinity of the anticline local high “Tampenflaket”, and numerous “bright spot” anomalies at CDP reflection sections as a whole along the sedimentary section. Hydrocarbon prospects connected with anticline highs which were defined in Late Paleozoic – Mesozoic deposit complexes of the Norwegian – Spitsbergen step zone should be estimated more cautiously as the zone has a very complicated tectonic structure. Prospects connected with local structures which were defined within Pomorsky trough are also estimated composedly as they are characterized by small amplitudes and rather steep upwarping of trough sides and it means HC migration to structural zones which are located upwards. Thus, the block-faulted basement structure, considerable thickness of the Cenozoic sedimentary cover, its lithologic variety, raised values of thermal stream, and also high concentration of the dissolved hydrocarbon gases in water registered by gashydrochemical survey show that the Spitsbergen continental margin represents a prospective feature for hydrocarbon searches. The listed factors can provide tectonic and lithologic screening of possible hydrocarbon reservoirs at various stratigraphic levels. Literature |