IGNATENKO E.A, LEONTCHIK V.I., SENIN B.V.
OAO “Soyuzmorgeo”, Russia

Formational-paleogeographic schemes, lithologic sections and well correlation panels demonstrating special features of the region’s burial history were produced basing on complex analysis of seismic data, results of deep offshore drilling and other information; reservoir models were constructed for some promising areas of the water bodies.

Basin widening and differentiation by sedimentation environments, up to deep water ones, at sedimentation rate up to 30-50 mm/1000 years [1] were generally typical of the Barents region from Ordovician, and particularly in Silurian and beginning of Early Devonian. Large regressions, forming of intraplatform highs, and denudation occurred in Middle Ordovician and Late Silurian.

In Middle Devonian, tectonic conditions were quieter and carbonate platform forming began at shallow marine, coastal and lagoon sedimentation environments and low sedimentation rate. Reefs including barrier ones and halokinesis are detected. The Baltic Shield and the Timan as well as the Svalbard and Franz Josef Land archipelagos in the north were the source areas.

At the Middle – Late Devonian boundary, rifting is detected that gave the onset of the South Barents Basin (SBB) and the North Barents Basin (NBB) forming. Accumulation of the Domanik formation took place at the same time.

Stabilization and accumulation of mainly carbonate rocks, ones of reef origin among them, were typical for Late Devonian – Carboniferous. Reef buildings are revealed by drilling on the Pechora Platform and along its northern margin. Occurrence of carbonate banks is supposed on the Kola and Murmansk Monoclines, the Korginsky and Kurentsovsky Steps, Prednovozemelsky Foreland (PF), east and north-east slopes on the Admiraltejstva Ridge, near the Fedynsky, Demidovsky, Fersman Highs on the base of geophysical data. Supposed carbonate banks around the East Barents Trough (EBT) may be represented by several reef buildings. One may suppose that the banks are elements of a Frasnian – Lower Permian barrier reef. Development of the barrier reef systems took place at the boundary of platform shelf and relatively deep water sedimentation zones most likely. Two branches of reef buildings mark transition from the inner shelf to the outer one. Moderate subsiding (up to 20 mm / 1000 years) of the entire region took place in Early Carboniferous. Sedimentation environments were shallow marine [2].

Unlike Barents sediments, Carboniferous and Lower Permian formations in the South Kara Basin (SKB) are mainly terrigenous. They are mapped in a narrow zone along the Pai-Hoi – Novaya Zemlya folded system and supposedly dip in SKB in the east.

Late Permian – Early Triassic time is characterized by tectonic activation expressed in orogenic belt forming, faulting, rifting, basalt effusions and acid intrusions, development of terrigenous – effusive strata and basin forming at “avalanche” sedimentation. Sedimentation rate in the East Barents Trough was maximal (up to 150 mm / 1000 years) in the beginning of Triassic [3, 4]. Clinoforms developed on the trough boards in Late Permian. Angular unconformities observed in seismic lines show that non-compensated downwarping took place intermittently in the SKB.

Paleogeographic environment began to change by the end of Early Triassic when downwarping became slower gradually. Sedimentation was terrigenous. Structural reorganization started in Middle Triassic. The united Barents Basin began to split into the East and West ones. Most of the SBB and NBB were a transitional area from shelf to deep water sea bed. High sedimentation rate (up to 100 mm / 1000 years) in the east and south of the Barents Sea at that time was caused by differentiated subsiding and active erosion of adjacent land. Accumulation took place in quickly alternating environments of deep shelf troughs, shallow water, and occasionally flooded lowland. Onland and underwater estuary complexes were widespread and formed large delta and delta-front valleys with lenticular sand bodies in them. The bodies are represented by crossbedded and clinoform seismic facies in the central part of the East Barents Trough.

Specific rootless structures, diapiric and cryptodiapiric supposedly detected in the west of the Prednovozemelsky Foreland (Sedov Trough) are connected with flow of plastic Permian and Triassic clays.

In the Pechora Basin, accumulation of sandy-clayey coastal-lagoon deposits occurred.

In Late Triassic, previous sedimentation environments at somewhat lower rate (40-50 mm / 1000 years in most basins) partly remained in the Barents Sea. Continental or coastal more seldom conditions prevailed with forming of sandy-clayey coal-bearing strata. So, Triassic was completed by sea regression. That is why large sandy bodies of fans and paleodeltas are typical for Upper Triassic deposits. They are detected on the Kola and Murmansk Monoclines, the Kurentsovsky Step, in the south of the Prednovozemelsky Foreland. Widespread river network of Late Triassic is represented in seismic sections by seismic facies of paleovaleys, paleochannels, beach ridges, and bars. This situation could cause forming of a thick terrigenous stratum rich with organic matter and able to generate large amounts of HC in the SBB central part.

In the SKB, transgression took place in the end of Triassic. It was maximal in Tampeiskian when sedimentation compensated downwarping. Judging by modern thickness of the complex (3.5 – 4 km), the depocenter corresponded to the Pukhutchansky Depression. The South Kara and North Kara Basins were separated by the North Sibirian Step which played the role of an interbasin barrier.

In Early Jurassic, the shelf sea mainly existed in the NBB, and in the SBB partly. Sedimentation environments were continental, coastal and lagoon. A low plain corresponded to the Central Barents Highs. The Baltic Shield, Spitsbergen (east) and Pai-Hoi – Novaya Zemlya were sources areas. It was the time of maximal leveling of the surrounding land and stopping of the Barents Basin subsiding.

In Middle Jurassic, a new (Jurassic – Neocomian) transgressive phase with a moderate sedimentation rate (up to 35 mm / 1000 years) began in the Eastern Barents Sea. Sedimentary material was transported from the east, south, and west and became more clayey. Most of the sediments in the SBB arrived from the south and east. Coastal and sandy-clayey lagoon deposits were also accumulated. Paleochannels formed delta and delta-front complexes up to 1000 m thick.

Transgression continued in Late Jurassic, and subsiding occurred everywhere. Land area decreased significantly and marine sedimentation environments prevailed.

Orogen reactivation happened in Jurassic and Neocomian and a chain of arch highs was formed that separated sedimentary basins. Material arrived from border orogens and interbasin barriers. In the Kara Sea, the Jurassic transgression combined Triassic basins, the whole Yamal peninsula, and the north part of the Gydansky Peninsula into a united area of downwarping and sedimentation with the rate 20 – 25 mm / 1000 years. Generally, Early – Middle Jurassic deposits were formed in shallow water environments and Late Jurassic deposits – in relatively deep water (overdeep shelf) ones. Relatively deep water conditions remained in Neocomian. Short-term undercompensation happened in the basin beginning from Volgian. It facilitated forming of bazhenites i.e. black bituminous clays, as well as clinoforms. By the end of Neocomian, sedimentation environments were shallow water and shelf.

New activation of tectonic processes occurred in post-Neocomian. In Aptian, sedimentation environments varied from shallow water in the west of the South Kara shelf to stably deep water ones in its east part. North of the SKB and the North Siberian Step formed a delta plain.

Mainly shelf, shallow water, shallow shelf, and continental environments were typical of Late Cretaceous. Delta facies, some regressions and a large transgression in Kuznetsovsky time (Turonian - Coniacian) are noted. In the Barents Sea, the Late Cretaceous regression was completed by deep erosion of underlying deposits and lack of sedimentation on a large part of the region.

Cenozoic was the time of ocean forming and differentiated movements in the near ocean zone in the Arctic. Uplifting and erosion were generally typical of the Barents-Kara Region in Eocene – Paleocene. Sedimentation took place in the SKB, and also in the south of the SBB in Paleogene, Oligocene, and Miocene. All the rest of the region was a transitional zone. See transgression with synchronous reactivation of the Pai-Hoi – Novaya Zemlya, Ural, and Timan Orogens happened in Pliocene.

Sedimentation history analysis enabled us to construct models of reservoirs and hydrocarbon migration paths for the Kola Monocline, Ludlov Saddle, Central Barents Step, Admiraltejstva Ridge, and Pechora Platform – Kurentsovsky Step.

Reservoir and HC migration models for the Kola Monocline. Mesozoic traps of the Kola Monocline are mainly of lithologic type. Their cap rocks are represented by Upper Triassic clays; source rocks are represented by Permian –Triassic formations of the SBB, at that Domanic (Upper Devonian) deposits form oil generating layers, and clayey Permian formations are gas generating. New accumulations can be discovered in: Neocomian clinoforms – Jurassic sandy lenses at existence of reliable seals; Permian porous reservoirs; Lower Permian, Carboniferous, and Devonian carbonate facies traced on the Kola Monocline. The Murmansky gas field located on the Kola Monocline is discovered in porous Lower-Middle Triassic reservoirs. Risk factor is connected with deposit outcrops on the sea bottom in the south part of the monocline.

Reservoir and HC migration models for the Ludlove Saddle – Central Barents Step and the Admiraltejstva Ridge. Characteristics of the section’s Paleozoic part are typical of the entire Pechora-Barents Region: carbonate and carbonate-terrigenous deposits; occurrence of Permian, Carboniferous, and Devonian formation of reef origin; Devonian Domanic formations that generate liquid hydrocarbons; good sealing of the source rocks.

Mesozoic part of the section has the following special features: terrigenous composition of the rocks; Permian – Triassic deposits on the Admiraltejstva Ridge consist of terrigenous material from the Novaya Zemlya Orogen; Permian – Triassic strata could generate liquid hydrocarbons within the Sedov Trough adjacent to the Admiraltejstva Ridge and thus traps on the eastern slope of the ridge may contain oil; gas mainly migrated from the west, from the SBB; traps of the East Barents Trough western margin may contain gas and gas condensate generated by Permian – Triassic source rocks of the South and North Barents Basins; magmatic sills occurring in the SBB could promote gas generation and faulting could help to sealing as it is proved by data on the Shtokman – Ludlov Step; Permian clastic, Lower Permian carbonate, and Devonian carbonate and clastic reservoirs are hydrocarbon promising. Risk factor - Upper Jurassic regional screen can be fragmentarily eroded.

Reservoir and HC migration models for the Pechora Platform and Kurentsovsky Step. Devonian – Mesozoic complex of the Varandei-Adzvinsky structural zone western part, the Pomorsky-Kolvinsky Ridge and the Kurentsovsky Step is considered. Paleozoic deposits are carbonate, carbonate-terrigenous and terrigenous; terrigenous section is typical of Devonian deposits’ middle part, carbonate one – for Lower Permian – Carboniferous. Reefs and bioherms are widespread; sedimentation environments were shallow marine, coastal, and lagoon; main oil source rocks are represented by Domanic (Upper Devonian) for traps in Upper Paleozoic and overlaying complexes, and Silurian – Lower Devonian clayey carbonates for Lower and Middle Devonian reservoirs. Both formations are oil-prone; generation of hydrocarbons took place in all depressions with following upwards migration; fault tectonics was one of the effects that controlled hydrocarbon migration. Hydrocarbons in Silurian – Devonian deposits of the Khoreiversky Depression and of the Varandei-Adzvinsky zone partly may be connected with the East Pechora kitchen area. Then, offshore accumulations may be larger than land ones.

The Upper Permian – Mesozoic section is terrigenous, accumulated in mainly continental and sub-continental sedimentation environments. Fans and paleo-deltas are typical of bottom sediments. Therefore, facial substitution, fan and lens forming is typical; clayey Permian – Triassic deposits with humus organic matter may be gas source rocks; accumulation potential of Mesozoic (Triassic) deposits of the Pechora Platform may be connected with lithologic traps.

References
Senin B.V. Special features of sedimentation and distribution of oil-bearing zones in the Timan – Pechora Provinces. // Depth exploration and protection, 1999, no. 7-8, pp 15-20.
Suprunenko O.I., Korago E.A., Viskunova K.G. In the book: Geology and Minerals of Russia. Arctic and Far East seas, 5, vol. 1. Ed. by Gramberg I.S. et al. Spb., VSEGEI pub., 2004, pp 161 – 190.
Ignatenko E.A. Structural-tectonic zonation and evolution history of the West Arctic Megaplatform, Russian Barents Sea // Petroleum Exploration and Exploitation in Norway, ed. by S. Hanslien, NPF Special Publication 4, Elseiver, Amsterdam, 1995, pp 305-320.
Senin B.V., Levitan M.A. Correlation of geological events in sedimentary basins of the Barents Sea: 5th Zonenshain Conference on Plate Tectonics. Moscow, Nov. 22-25, 1995, p. 118-119.

Oil and gas of Arctic shelf 2008