SENIN B.V., LEONTCHIK M.I., SENIN T.B.
OAO «Soyuzmorgeo», Russia, OAO «Severneftegaz», Russia

Gas presence in Triassic deposits is proved by gas field discoveries in the Murmansky and North Kildinsky Structures and some structures of the Timan-Pechora Provinces in the East Kolguev and Sorokin oil-and-gas-bearing regions (OGR) (the Korovinsky, Kumzhinsky, Taravei, Varandei fields). Regional oil-bearing nature of Triassic deposits is confirmed by oil pools in Lower Triassic deposits on the Kolguev Island, in the Barents Sea Norwegian sector (the Sn?hvit, South Goliaf, Nukula fields) and by oil shows on the Spitsbergen Archipelago and in the east of Franz-Josef Land.

So, oil and gas shows of different scale are found in the Barents Region in Triassic complexes of different age both on land and offshore.

A large amount of geological and geophysical data was collected, analyzed, and generalized and paleo-geographical, sedimentation environments, and thickness schemes for three Triassic sub-periods were produced to reveal and describe elements of the Triassic petroleum system. We also studied data on oil and organic matter (OM) composition and analyzed seismic sections to reveal the zones of hydrocarbon trap forming for different types of traps.

The studies resulted in a generalized scheme of Triassic petroleum system development in the Barents Sea region. The scheme represents: the area of the Triassic paleobasin; regional thickness distribution for Triassic deposits; generalized contours of the supposed Triassic source rock area; areas with regional Upper Jurassic cap rocks, fields and oil shows revealed by drilling and geological survey on the adjacent land.

The main specific feature of the Triassic paleobasin is large thickness (up to 7-8 km) of deposits in the South Barents Basin (SBB). The stratum forming took place in avalanche sedimentation environments at sedimentation rate exceeding 250 – 300 mm / 1000 years /Senin, Levitin,1999/. Judging from thickness information and the chronostratigraphic scheme based on drilling, seismic and geological data, the avalanche sedimentation environments existed there from the Late Permian second half till the beginning of Late Triassic.

Granting that accumulation of sedimentary strata in avalanche sedimentation environments is mainly typical of sea basins /Lisitsyn, 1988; Koronovsky, 2006/, one may expect forming of deposits enriched with organic matter of sapropelic or mixed type in Early and Middle Triassic. These deposits can generate liquid HC at favorable thermobaric conditions. According to published data, potential source rocks in the Barents Sea Region may generate oil at the depth 5.5 – 2.5 km. As “inversion” of catagenetic zonality i.e. decrease of organic matter maturity with depth is highly probable in depressions with a thick (20-25 km) sedimentary cover /Verba et al., 1999/, the lower boundary of the oil window in some areas of the Barents Sea may be as deep as 6-8 km. The large zone of anomaly high formation pressure in the Middle – Upper Triassic deposits up to 2 km thick revealed by seismic studies in the SBB central part may contribute to this phenomenon. In all probability, the zone may affect geometry of the sedimentary stratum, oil and gas generation environments and HC migration paths in Triassic deposits. Considering the above stated reasons and according to our structural schemes Triassic deposits are in the main oil window now most likely.

The generalized contour of a large area where Triassic source rocks may be developed is drawn on the base of the formational – paleogeographic analysis we fulfilled for Triassic sub-periods.

Lower Triassic oil potential is proved by the Pestchanoozersky and Tarsky fields discovered in the Charkobozhsky Suite of the Kolguev Island. Many researchers of this region believe that these oil pools are secondary. At that, two possible paths of migration are supposed. The first one is migration from underlying Permian – Carboniferous deposits /Bordovskaya et al.,1993; Ivanova, Suprunenko,1996; Suprunenko, Bro,1994/, and the second – from Mesozoic deposits of the South Barents Basin /Shimansky et al.,1996, Lebedev et al., 2001/. Anyway, both groups of experts agree that the share of clayey rocks of sea origin enriched with sapropelic organic matter and having higher generation potential increases to the north of Kolguev towards the SBB. A SBB-confined Early Triassic sea basin with avalanche sedimentation is reveled by paleosedimentation, seismic facies and thickness analyses.

Besides, two accumulation areas are forecasted in the sedimentary section: one of mainly sapropelic and humus-sapropelic OM, and the other of mainly humus and sapropelic-humus OM.

According to seismic data, early Triassic deposits in the central part of the South Barents Basin are at the depth up to 7 250 m corresponding to the МК3-МК4 catagenetic stage (completion of oil generation). Lower Triassic oils on the Kolguev Island are of a high catagenetic stage.

Middle Triassic deposits are represented by clays of sea origin with organic matter of a mixed sapropelic-humus type /Stupakova, 2001/. Continental and lagoon-continental sedimentation environments were most typical of the Barents Sea south (Kolguev) in Middle Triassic. The share of offshore sediments increases to the north and north-west. Stably marine sedimentation environments existed in the northern part of the region (the Frantz-Josef Archipelago). Thus, possibility of source rock forming increases sharply to the north of Kolguev. Two accumulation areas are predicted in the Middle Triassic paleogeographic scheme for sediments enriched with organic matter: one of mainly sapropelic and humus-sapropelic OM, and the other of mainly humus and sapropelic-humus OM. These areas are significantly smaller than the analogous ones in Lower Triassic i.e. the area of sapropelic deposits decreased and shifted to the north, but most favorable conditions for such strata forming remain in the north of SBB and in the NBB. The depth of Middle Triassic top is 4400-5 330 m in the center of SBB that corresponds to the main oil window.

In Late Triassic, the deep water accumulation areas decreased significantly in comparison with Early – Middle Triassic time. Elements of buried paleovalleys well expressed in the wave field form a specific feature of Late Triassic. According to our Late Triassic formational – paleogeographic scheme, the system of paleovalleys is situated within the continental area where sedimentation environments changed successively from height to plane and lowland. We believe that the discharge area (delta front) where the thickness of Upper Triassic deposits increases up to 1 500-2 500 m was located in a basin of a shallow water epicontinental shelf sea similar to the modern Caspian or Aral Seas. In all probability, accumulation of Late Triassic strata of sapropelic–humus type in the South Barents Basin central part, and of humus-sapropelic type and great thickness in its deepest part could also occur, but the spread area of these deposits is limited.

Thus, the paleosedimentation analysis enabled us to outline an area where possible oil source rocks developed during the entire Triassic in the deepest part of the Barents Sea Region and where the Triassic section is most complete.

Migration of liquid hydrocarbons at Triassic OM entrance into the main stage of oil and gas generation in the central part and on the boards of the South Barents Basin could be both vertical and lateral. HC could migrate vertically along regional fault zones and crushing zones which cut the Paleozoic – Mesozoic section and also along local dislocations formed within Triassic and Jurassic strata only as a result of possible intraformational deformations caused by tectonic activation during the Cimmerian tectonic stage (Triassic – Early Jurassic) and also by intrastratal intrusions of different age (up to Carboniferous).

Lateral migration could occur along bed rise. At that, main migration channels could be formed by pore space of sandy layers and fissures on bed surfaces. Paleovalley channels could also provide favorable migration paths. HC moved in the direction opposite to valley dip and up along the slopes (boards) of the SBB and filled existing anticlinal and non-anticlinal traps in both Triassic and overlying Jurassic – Cretaceous deposits. Elements of the paleovalley system were revealed by seismic survey in Lower and Upper Triassic deposits near the Kurentsovsky Structure /Skobelskaya, 1988/, on the offshore continuation of the Malozemelsky-Kolguev Monocline, Kola-Kanin Monocline and on the north offshore continuation of the Timan-Pechora Provinces. Oil accumulations in Lower Triassic deposits on the Kolguev Island could be formed as a result of migration from the south board zone of SBB along Early Triassic river channels.

Accumulative sandy-siltstone and sandy-pebble formations of valley and coastal origin (bars, beaches, underwater deltas, beach ridges and others) in combination with the structural factor or independently could be considered as most typical promising traps in Triassic deposits of the Barents Sea.
Thus, results of the analysis indicate rather high probability to discover new oil accumulations in Triassic deposits of the Barents Sea, particularly in its board zones and uplifted zones of deep water troughs. The accumulations could be confined to sandy, sandy-siltstone, and sandy-pebble reservoirs in lithologic and structural-lithologic traps. But detailed elaboration of these ideas demands well-directed efforts in geological and geophysical study of the complexes mentioned above.

References
Evdokimova N.K., Yashin D.S., Kim B.I. Hydrocarbon potential of sedimentary cover deposits on the shelves of Russian East Arctic Seas. // Geology of Oil and Gas № 2, 2008.
Lisitcin A.P. Avalanche sedimentation and accumulation brakes in seas and oceans. Moscow, Nauka, 1988, 309 p.
Stupakova A.V. Development of the Barents Sea basins and their hydrocarbon potential. Abstract of doctoral thesis. Moscow, 2001, 41 p.
Senin B.V., Levitan M.A. Phanerozoic evolution of sedimentation rates and the effect of the Late Hercynian event on geological structure of the Barents-Kara Region. Russian Geophysical Journal № 13-14, 1999, p. 80-88.

Oil and gas of Arctic shelf 2008