KAZANIN G.S., ZAYATS I.V., SHKARUBO S.I., PAVLOV S.P.
JSC «Marine Arctic Geological Expedition», Russia

Undoubtedly the Arctic shelf of Russia is the main reserve of oil-and-gas industry of the country.

The Arctic Sea subsoil contains about 80% of initial total hydrocarbon resources of the whole Russian shelf. Poor geological — geophysical study of water areas is one of the problems hindering hydrocarbon potential development of shelf sedimentary basins, resulted in unpreparedness of HC reserves.

The main scope of seismic work was carried out till the 90s of the last century, but its quality do not comply with modern requirements. CDP reflection shooting was carried out using 24- and 48-fold coverage, the streamer of up to 3000 m in length and amplitude frequency characteristics of sources couldn't provide enough propagation distance and detailed geological layering.

The average seismic grid density on the Barents Sea shelf (including the Pechora Sea) is 0,3 km/sq km. Most seismic surveys are carried out in the Pechora Sea and in the south of the Barents Sea. However, on the whole the seismic grid density is not more than 0,1 km/sq km in the 90% of the Arctic Sea area. Until recently the northern areas of the Barents and Kara Seas were studied by means of single reconnaissance lines. In order to finish the regional stage of work the density of seismic grid should be more than 0,5 km/sq km (Yu.P. Trutnev, Mineral Resources of the Arctic Shelf, 2006).

Therefore, the critical goal to reproduce mineral resources base of the country is an intensification of geological study and preparation of hydrocarbon resources base of the Russian continental shelf. Realizing this problem the Ministry of Natural Resources and Federal Agency for Subsoil Use promoted regional geological exploration for oil and gas in the Arctic seas since 2004 within the frame of “A long-term government program of Russian mineral resources base reproduction...”

Marine Arctic Geological Expedition (MAGE) is one of the Russian companies which carries out seismic surveys of geological study of the shelf by tender. Specialized scientific-research vessels of MAGE performing 2D seismic surveys (RV “Geolog Dmitriy Nalivkin” and RV “Professor Kurentsov”) are rigged with up to the minute equipment: digital seismic streamers of up to 8000 m in length, array air guns, positioning systems and onboard processing centers.

For the period since 2004 vessels of JSC MAGE carried out seismic surveys in potential areas of the Barents, Kara and Laptev Seas, the scope of these works exceeded 35000 km.

In 2004 there was specified a structure of the sedimentary cover and folded basement in Obruchevsky swell area and found out new hydrocarbon traps by means of seismic works carried out on Priyamalsky shelf of the Kara Sea. The growth of localized forecast resources due to defined local highs was 675 million tones of oil equivalent. Additional growth of forecast resources can be received due to non-anticline HC traps in the Cretaceous part of the section (T.A. Kirillova-Pokrovskaya, A.B. Dyachenko, 2007).

In 2005 MAGE carried out CDP reflection shooting at the southern pericline of Admiralteysky swell. There were revealed “bright spot” anomalies to the north-east of Ledovoe field in Cretaceous deposits of potential oil-and-gas complex. Previously there weren’t defined any HC accumulations in that complex. A structural nose complicated with faults forms a combined trap along the top of Albian deposits. A similar trap is found out to the south of Luninskaya structure. The area of traps along the horizon Г1 (Albian – Cenomanian) is 650 sq km (Pavlov S.P., 2007).

In 2005 the RV “Professor Kurentsov” carried out 3000 km of seismic lines in complex with gravity and magnetic surveys in the southern part of the Laptev Sea within Ust Lenskiy rift and adjoining highs. Ust Lenskiy rift is an extensional structure – a fault-segmented system of grabens, its length is about 600 km. A large thickness of deposits in the axial part of the rift results from the combination of tectonic factor (extension crust thinning) with a great number of sedimentary material. Maximum activity of the first stage of extension is shown by consediment faults which have the highest amplitude in the lower part of the section. The upper complex overlapping all structural elements of the rift system is disturbed only by separate faults.

This fact denotes a significant retardation of rifting at the newest stage of the Laptev Sea development (Kazanin G.S., Kirillova-Pokrovskaya T.A., 2006).

Main tectonic elements of Ust Lenskiy and West Laptev riftogene zone represent a complicated group of conjugate horsts and grabens; anticline highs are localized and prerequisites for non-anticline fault-screened trap formation are created there. Within the research area there was thoroughly studied the structure of 7 local highs which were known before and found out 13 new ones, their total area is more than 14000 sq km. Forecast HC resources by Д2 category are about 1.5 billion tones in oil equivalent. An increase of localized resources of 600 million tones is received due to six delineated anticline highs.

In 2007 the studied area of the Laptev Sea shelf was extended westward. A new seismic grid (2000 km) allowed to delineate a few projected highs and find out one more large anticline structure.

Works which were carried out in the south-western part of the basin provided a new material; it will help to understand many debating points on the geology of the Laptev Sea. New data will allow to specify the type of conjunction of the Siberian Platform and edging fold systems with rift troughs of the shelf area. Besides, they will also help to argue a stratigraphic volume of the sedimentary cover basing on the character of tracked unconformities and structural stiles of seismic complexes.

Nowadays, poorly studied areas of the north of the Arctic shelf, in particular the Barents Sea are one of few areas in the Russian part of the northern seas where can be discovered large and very large anticline highs and consequently hydrocarbon fields.

Validity of this thesis was confirmed by integrated geophysical works which were carried out by JSC MAGE at the eastern side of North Barents depression in 2006 – 2007. Previously, prior to performing mentioned researches 2D seismic grid density was 0.02 km/sq km. It allowed to find out only one large anticline structure – Orlovskoe high (about 1000 sq km in area) in the total area of 150000 sq km.

2D seismic surveys within the scope of 7000 km at the line grid of 25 x 35 km increased observation density up to 0.05 km/sq km, on the whole it corresponds to a reconnaissance stage of geological study of the region. Nevertheless, performed works allowed to find out 20 new local anticline objects 26890 sq km in total area. Varnekskoe, Gidrografov and Salmskoe anticline highs are the largest ones. Gidrografov and Salmskoe highs are limited to a water area of the arched uplift of Franz Josef Land. Structures are found out in Triassic deposits, oil-and-gas prospects are highly estimated. Areas of Gidrografov and Salmskoe highs exceed 1000 sq km.

Varnekskoe anticline high which represents a brachyanticline of the north-eastern strike is limited to Prednovozemelskaya structural area (Zhelaniya Cape high) and its dimensions are comparable to dimensions of Admiralteysky megaswell high. Upper Devonian – Middle Triassic deposits with a total thickness of about 7000 m are reachable for drilling. Lithological – facies composition of Paleozoic deposits of Varnekskaya structure was formed due to progradational sedimentation, it considerably increases its oil-and-gas prospects in comparison with Admiralteysky megaswell objects.

Thus, marine regional seismic works carried out at the modern technological level show their high efficiency providing a significant growth of new potential oil-and-gas objects.

Literature

Trutnev Yu.P. On efficiency enhancement of hydrocarbon resources development of the continental shelf of the Russian Federation. – Mineral Resources of Russia. Economics and management. (Special Issue: Mineral Resources of the Russian shelf, 2006, p. 4 – 11).
Kirillova-Pokrovskaya T.A., Dyachenko A.B. Prerequisites for the development of oil-and-gas potential of the Kara Sea. – Razvedka I okhrana nedr, No. 9, 2007, p. 69 – 78.
Pavlov S.P. Oil-and-gas potential of Triassic and Upper Cretaceous deposits of the southern pericline of Admiralteysky swell. – Razvedka I okhrana nedr, No. 9, p. 52 – 55.
Kazanin G.S., Kirillova-Pokrovskaya T.A. Some results of CDP reflection shooting in the Laptev Sea. – Thesis of report at the conference “Oil and gas of the Arctic shelf”, Murmansk, 2006.

Oil and gas of Arctic shelf 2008