MARGULIS L.S., MARGULIS E.A. VNIGRI, Russia

Phanerozoic history of formation of Batemts sea region is intricate and include not less than five cardinal reconstructions of tectonic conditions. At a modem level of knowledge the following structural and historic stages of Barents sea region can be emphasized: lower Paleozoic (pre-middle-upper Devonian), middle Paleozoic (middle-upper Devonian - lower Permian). Paleozoic - early Mesozoic (lower Permian - Triassic), Mesozoic (Jurassic -Cretaceous) and Cenozoic. The lower Paleozoic (pre-middle Devonian) history of region's development is lesser studied, there are numerous contradictory ideas /1, 4, 6/. However, apparently, during the Silurian - early Devonian Barents sea region was a part of vast passive margin of Euro-American continent, on which terrigenous-carbonaceous plate complex was been accumulated. Its thickness is increased up to 1-2 km toward pericratonic submersion by east and north-east. An accretion, folding and orogeny of Scandinavian active margin took place during the Silurian - middle Devonian. The zone of joint of Baikalian and Caledonian geoblocks is traced under sedimentary cover from the peninsula Varanger to northern end of Novaya Zemlia. To middle Devonian - the beginning of late Devonian Barents sea region already represented a part of new continent. Its further development till the Triassic was determined by evolution of the Ural ocean and origin of orogen

The beginning of middle Paleozoic (middle-upper Devonian - lower Permian) stage is marked by split of the consolidated crust and origin of suboceanic rift. The rift represented a wedge separation and existed en form of ramifications of Ural paleoocean, extending from Kara Sea through Novaya Zemlia to South Barents depression. South Barents fault zone was the southern limit of the rift during all middle Paleozoic stage; the northern limit is presumably traced on a sutural zone between Baikalian and Caledonian blocks. Intensive stage of rift spreading, as well as of other similar structures of eastern border of Euroamerica, account for Frasnian stage. At this stage the rift represented an oceanic depression characterized by heightened seismicity, depth of sea, close to a level of carbonaceous compensation and basic submarine volcanism. The axial part of Murmansk rift is latitudinally extended through Medvezhiy bay (northern peninsula of Novaya Zemlia) /3/. Here a shallow-water limestones of the lower Devonian is overlied by black carbonaceous shales of the middle Devonian (about 200 m) with nektonic fauna, layers of basalts and numerous sliding prisms of shelf carbonates. A sequence of clayey-siliceous shales alternating with layers of underwater effusive rocks of basic and ultrabasic composition and pelagic limestones (middle Devonian - Frasnian) places above. A top of a section is composed of ophthanites, enriched by layers with phosphorite concretions; carbonaceous and sandy turbidites are present too. The thickness of a section is 0.9 km.

During the Carboniferous - lower Permian the deep-water fine-grained deposits - silicites, siliceous and bituminous argillites were accumulated within the limits of depression in conditions of "sediment's dearth" completing a rifting stage of development of Murmansk rift.

Tectonic and depositional zonation during the late Paleozoic - early Mesozoic stage also is largely conditioned by location of Murmansk deep-water depression, which from the Kugurian age is progradately filled by a detrital material from glowing Ural orogen 151. The filling of Murmansk depression starts with formation of terrigenous and avalanche-sliding deposits in foot of southern continental slope of the shelf These were described by V.I. Ustitskiy and Povysheva in the sections of Novaya Zemlia as Kamiakulian series with thickness above 2500 m. Similar horizons of olistoliths occur on continental slops of actual passive oceanic margins during the periods of seismic activity with the angle of slope more than 1.5-2°. The upper part of Permian section in the limits of prograding front of clinoforms is composed of shelf rhythmic terrigenous deposits with infrequent remnants of bivalves and colonial corals and is described on Novaya Zemlia as Britvinskian series with thickness about 2 km. The total thickness of the Permian section in the sea reaches 5 km.

During the Permian a detrital material from Ural compensated only southern part of Murmansk deep-water depression which include actual Kurentsovskaya structural zone and a south of Southern Barents depression.

The conditions of "sediment's dearth" were remained at the north of deep-water depression and here the Permian section is composed of aleuro-argillaceous sequence with thickness 2-3 km. The section does not contain any paleontological remnants, is characterized by fine horizontal stratification and intensive piritization. This sequence is studied by wells Admiralteiskaya-1 and Krestovskaya-1 on Admiralteisky arch. The shelf fades occur only durina the Tatarian age; they are concordantly overlied by Triassic sediments. The existence of Murmansk rift depression was terminated during the Triassic. At that time it was a depocentre of sedimentation and "trap" of avalanche solid run off. which was bent under a weight of accumulated sediments. The sedimentation occurred in an unstable tectonic conditions and periodic overbalance. Here there was an area of Triassic volcanism.

At concluding Triassic age of late Paleozoic-early Mesozoic stage Barents sea region was developed under influence of important geologic phenomena: a development of Paleopacific margin and formation of Polar Ural. It becomes a part of large sedimentary basin, which occupied eastern Greenland. Barents sea, southern part of Kara Sea and Timan-Pechora region. The basin was characterized by considerable volumes of coming terrigenous material, broad spectrum of sedimentary conditions - from submontane to deep-marine, differentiated conditions of submission (30-200 m / million of years). Post-Triassic tectonic movements disturbed the unity of basin, and its western (Greenland) part was practically eroded completely. Deep-water area of Triassic basin with uncompensated sedimentation was placed within the limits of the platform and open in direction of Paleopacific. The thickness of Triassic deposits in deep-water part of basin is minimum - 0.2-1.0 km. These sediments are characterized in the lower part of section (the lower-middle Triassic) fine clastic rocks with rich fauna of ammonites, brachiopods and bivalves and widespread occurrence of bituminous argillites of predominantly middle Triassic age with phosphorite concretions and rich plankton. In a direction to central parts of deep-water area a volume of these argillites in section and their age range is increasing. Eastern (external) limit of deep-water area is expressed by an edge of continental paleoshelf and clinoforms for foot of paleoslope, fixing movement of detrital material from east and southeast. The area of paleoshelf covers on Barents sea a northern and southwestern parts of actual East-Barents trough and the east of West-Barents trough. Grey-colored sandy-aleurolitic rocks with various marine fauna prevail in the section of Triassic deposits of shelf area. The thickness of section is up to 3.5-4 km. The wedges of horizons of fine clastic bituminous rocks, proper for adjacent deep-water area, are observed too. The southern part of Barents sea region during the Triassic was an area of lagoon-continental and continental sedimentation; the area of continental molassoid deposits "nestles up" to Ural. Here the greatest thickness of Triassic deposits (up to 5-6 km) is connected with a depocentre, inherited a position of Murmansk suboceanic depression. Probably a location of future Barents megatrough is outlined during the late Triassic. It is fixed by a broad band (about 250 km) of intensive development of trappean magmatism. The first manifestations of magmatic activity are found out in the boundary Permian - early Triassic layers in a zone of intensive compensatory sumission. The same zone was an area of Triassic volcanism.

At the Mesozoic (Jurassic-Cretaceous) stage Barents region has experienced a cardinal tectonic reconstruction. On a boundary of the Triassic and Jurassic there is a formation of Novaya Zemlia kimmerids and Barents megatrough. They orthogonally overlapped more ancient structures. Folded Novaya Zemlia orogen and megatrough, adjacent to it, are members of the largest geodynamic system, their formation was accompanied by an intrusion of hypogene material of a planet and, probably, essential processing of earth crust within the limits of a megatrough. The formation of these structures was accompanied by magmatism: in folded area - granitoids, in a megatrough - sills of diabases. Increasing orogen displaced sedimentary depocentres to west. Jurassic and Cretaceous deposits form vast plate cover overlying different horizons of more ancient formations. The scale of pre-Jurassuc discordance is reduced to depocentre of basin at expense of appearance of more and more ancient horizons of the Jurassic. Jurassic deposits, as a rule, consist of two sequences: lower-middle Jurassic -predominantly sandy with thickness 0.3-1.5 km and upper Jurassic -essentially clayey with thickness of 0,02-1,5 km. Practically all area of Barents megatrough and Svalbard platform during the Jurassic represented a large deltaic systems progressed to enough deep-water basin from Ural and Novaya Zemlia orogens from a southeast and east and from Svalbard from northwest 111. The most deep-water conditions existed during the Kimmeridgian-Volgian in the west of South Barents depression in the area of wells Severo-Murmanskaya-1, 2 and Arcticheskaya-1. The Volgian age was the time of the greatest distribution of relatively deep-water bituminous clays, practically everywhere developed in Barents megatrough. The upper Volgian clayey horizon overlaps transgressively with distinct discordance a more ancient Jurassic rocks in eastern marginal part of Barents megatrough, and lies over different horizons of the Triassic in Prinovozemelskaya zone of dislocations. Probably late Jurassic transgression eroded just Paleozoic deposits of Novaya Zemlia. where the numerous blocks with late Jurassic fauna are met. By discordances between lower and Jurassic, Jurassic and Cretaceous in Prinovayazemlia, Novozemelskiy orogen sequentially grown during all Jurassic-Cretaceous stage, forming eastern flank of Barents megatrough.

Late Kimmeridgian phase of tectonic activity has conditioned an uplift of almost all large positive structural elements contributing a preservation and deepening of Volgian deep-water depression filled during the Neocomian by thick detrital cone.

The Jurassic-Cretaceous deposits of Barents Sea are generically and spatially connected with the covers of young plates of Western Siberia, Norwegian and Northern seas. In all these regions Jurassic-Cretaceous plate cover begins with various lower-middle Jurassic formations. The upper Jurassic clayey sequence, bound with a vast transgression, is also characteristic for them. In the upper part of Jurassic deposits the formation of black bituminous clays and combustible shales (Bazhenovskaya formation of Western Siberia. Kimmeridgian-Volgian clays of Barents sea. Oxfordian-Volgian combustible shales of Northern sea) is observed. The Neocomian sequences of lateral increasing and filling of these depressions occupy the vast spaces of Barents, Kara Seas and Western Siberia. Postneocomian Cretaceous sequences already completely compensate a submersion.

It is necessary to emphasize two features of region: 1) the alignments of structural elements of various ages of cover's subdivisions are various; the Alpine East-Barents megatrough orthogonally overlies the structures of the middle Paleozoic - early Mesozoic; 2) the Devonian - Cretaceous history is divided into the periods of: formation (D2-C1) and filling (C1-T1) of deep-water depression, initial stage and formation of East-Barents megatrough (T:?-Kz). Sedimentary covers of these periods are specific and have an essential autonomy of hydrocarbon generation and accumulation and correspond to concept of hydrocarbon (naphthid) systems.

It is possible to mark out 3 hydrocarbon systems in the cover of Barents sea region: early Paleozoic. Devonian - early Triassic and middle Triassic - Cretaceous. The early Paleozoic hydrocarbon system is largely broken by subsequent tectogenesis. The deposits of hydrocarbon can be found in it on peripherals of South-Barents depression. The Devonian -early Triassic hydrocarbon system has thick (up to 1.5-2 km) upper Devonian - Permian source formation accumulated in Murmansk trough. For extension it is replaced by shelf carbonates with reefs, and in upper Permian interval - by terrigenous deposits. Lower Triassic molassoids end the section. By the data of simulation depressional black shales and domanikoids have fully realized its potential of oil generation (gradation MC4_5). Apparently Kimmeridgian tectonic reconstruction has resulted in considerable dispersion of accumulated oil. Southern board of Paleozoic depression (actual Kuretsovskaya step), Kola monocline, Fedymskiy arch was not seriously reconstructed, it is possible to expect oil pools here. The inflows of oil are obtained from Paleozoic carbonates on the Lopp. Norsel uplifts, in Hammerfest basin and Finnmark plate. The middle Triassic - Cretaceous hydrocarbon system has the highest hydrocarbon potential determined as by own generative capabilities, as by fluid transport from more ancient hydrocarbon systems. Middle Triassic and upper Jurassic rocks are major source formations. The middle Triassic mature (gradation MC1.2) bituminous-clayey formation occurs in the west of region, its thin wedges split more shallow-water formations in the east. A field of development of middle Triassic depressional sequence and adjacent regions are perspective for oil discoveries. They have large structural traps and thick layers of sandy reservoirs of the upper Triassic - Jurassic (northwest board of South-Barents depression, North-Barents depression). At the date only one oil pool is found in Hammerfest trough, but the oils, bound with Triassic source rocks are widespread on arctic margin of Paleopacific. The upper Jurassic sequence (150-350 м) is composed of bituminous clays and occurs on a considerable area. It is poorly changed (gradation PC-MCi) and basically generate a gas.

Thick (up to 3 km) coal-bearing sequences of the upper Triassic and lower-middle Jurassic are essentially gas-bearing too. The upper Jurassic sandstones are major reservoirs of the largest oil-gas condensate pools of the region.

The analysis of sedimentary structures of hydrocarbon systems, first of all distribution of rich source rocks in tectonic depressions and nature of filling of these depressions, is one of basic ways of solution of problem of Barents sea - forecast of petroliferous areas.


ARCTIC SHELF OIL AND GAS CONFERENCE 2004