SELEZNEV V.S., SOLOVIEV V.M., SYSOEV A.P., BABUSHKIN S.M., KASHUN V.N., BRYKSIN A.A., LISEIKIN A.V.
Geophysical Survey SB RAS, Russia, JSC «Sibneftegeofizika», Russia

The east of Russia, including territories of Eastern Siberia and the Far East, occupies more than half of area of the Russian Federation.

About 2.0 million sq.km. make water areas of the adjacent East Arctic and Far East seas. The region has huge economic, geopolitical value and is an advanced post of the country in Asian-Pacific region (APR). Within region, including shelves, about 140 fields of oil and gas are opened, opening about 50 large oil fields and over 170 gas fields is predicted; more than quarter of world's reserves of hydrocarbons under modern forecasts are located in a zone of the Arctic shelf.

At the same time oil and gas resources of the East of Russia are distributed and studied extremely non-uniformly.

The considerable part of territory practically is not studied by drilling and seismic prospecting. Despite increasing volume of seismic researches in the given territory there are “white stains” in ten thousand of square kilometers, not captured by seismic researches. In the Eastern Siberia these areas are adjoining to such rivers as Lena, Yenisei, Khatanga, Olenek, to their channels and inflows (Nizhnaya and Podkamennaya Tunguska, Angara, Vilyui, Aldan, etc.) in which limits there are ecological restrictions and the technical difficulties, not allowing to execute seismic prospecting in usual way. One of ways of the permission of this situation is statement of “ecologically pure” CDP seismic works on water areas of the rivers with pneumatic sources of excitation.

Geophysical Survey of the Siberian Branch of the Russian Academy of Science by request of the Ministry of Natural Resources of Russia Federation fulfils for the last few years a number of seismoprospecting profiles on water areas of the rivers Ob, Vakh, Yenisei, Lena, etc. by the general extent more than 2500 running km. [Seleznev and other, 2004, 2006, 2007]. Considering a combination of midget and big depths investigations had difficult character and at their carrying out the variety of specialised modern radiating and a data-acquisition equipment was applied. In the course of works the technological completion of rented ships which have not been adapted for carrying out of seismoprospecting works was spent: installation of seisphone cable assembly winches for descent and lifting, installation of the crane for descent to water of airguns, placing of compressors, the rigging etc.

A number of methodical questions, such as selection of optimum airgun combination for excitation of signals on small depths, a choice of optimum system of the working off dared also, allowing to solve problems of tracing of target horizons in a sedimentary cover on depths in 2-5 km, etc. and as a whole thicknesses of the top part of earth crust to depths of 10 and more km. It is necessary to notice, that at working off the seismoprospecting profiles on the river Lena (in headwaters) there were essential difficulties which were not earlier. Except small depth of the river Lena on a site of works (reaching to 2-3 meters even on a waterway) and a strong current (up to a 10 kph around rifts) there has been registered the huge noise of microseism (up to 1500 microvolt), more than hundred times exceeding noise in a mouth of the river Yenisei. It is clear, that it was impossible to receive the conditioned material within the limits of already approved earlier “river technology”. Along with technical difficulties at the area of works the difficulties in the geological plan have appeared. Extremely non-uniform top part of earth crust is visible already in sections of coastal adjournment. The top part of a cut (the environment of excitation of elastic vibrations) in the riverbed of the river of Lena is presented by sandy-gravel and pebble alluvial formations of thickness to first tens metres.

Their substratum, slopes and watersheds in a valley of the river of Lena everywhere develop the carbonate thickness dismembered on a number of original measures, deployed in a alternating series of anticlines and synclines with amplitudes up to 300-400 meters complicated by longitudinal breaks with subvertical displacement to several hundreds of meters.

As a result the technology of river seismoprospecting works with reference to conditions of the rivers of Eastern Siberia (with presence of thresholds, a fast current, midget (less than a meter) depths, etc.) has been modified . The essence of the modified technology of river seismoprospecting researches consists in the following. As well as earlier on river works the two-ship technology of working off was applied: one vessel – an excitation source, other vessel – layer of a seisphone cable assembly. On sites with strong noise where it is impossible to receive good data using hydrophones (such sites as a rule are near to thresholds with a fast watercourse, and also sites with a difficult relief of a bottom, presence of the big blockages, strong change and reduction of depth of the river), stations “Baikal-AS” are placed on a coast. Experience of works on Lena shows that presence of clips, underwater ledges around a coastal line does not allow to apply long land seisphone cable assembly, therefore effective there were three-channel mobile digital stations “Baikal-AS” (elaboration of GS SB RAS).

Following airguns are applied for excitation of vibrations: sources which have well proved earlier “Puls-6” with volume from 1.5 to 3 liters and sources “Malysh” with volume from 0.2 litres to 0.5 litres necessary at working off sites with small water. Airguns unite in groups: sources “Puls-6” and “Malysh” are formed of 6 and 20 units of devices accordingly. Development work have shown, that the minimum depths of immersing of airguns at which can be received conditioned records with preservation of reflexions from the main index horizons are 1.5 and 0.6 m for sources “Puls-6” and “Malysh” accordingly [Seleznev and other, 2007].

It is necessary to notice, that possibility of updating the technology of river seismoprospecting works became possible in many respects thanks to occurrence of a high-sensitivity digital data-acquisition equipment “Baikal-AS” and “Reftek” with a wide dynamic and frequency range. The equipment is tight and can be established in the water sated sand, has the built in memory up to 4 gigabytes, an independent portable power source, can be programmed for some days of work on the schedule or in a continuous mode of record till for 40 hours.

Except oil and gas industrial deposits of alluvial gold, tin, diamonds, platinoids are also found out on shelves of the Arctic seas. In this connection, interest to revealing of borders of a continental shelf is shown now by many states, and not only subarctic. For definition of external border of underwater suburb of continent of the coastal state are used bathymetrical (that is maps of a relief of a sea-bottom with marks of depths) and geological criteria. With the first all is simple: position of bottom of a continental slope is defined on the maximum change of a bias taking into account position of the 2500-meter isobaths. More difficult situation is with revealing of geological criteria. They include data about structure and a thickness of a sedimentary cover (the adjournment covering a shelf), the nature of earth crust within a shelf and geology-geophysical proofs of position of continental bottom.

Capacity of a sedimentary layer within external border of underwater suburb of continent should make 1 % (and more) the shortest distance from bottom of a continental slope. For definition of parameters of geological model of a structure of continental shelves it is necessary to increase volumes of sea seismoprospecting researches. The technology of river seismic prospecting developed by Geophysical Survey of the Siberian Branch of the Russian Academy of Science, can be applied and on continental shelves. Field river seismic works on profiles in Yenisei gulf in 2006 were approached on conditions to the coastal sea: depth of a bottom reached several tens meters, installation of equipment was carried out on vessels of a sea-going class. Time sevtions of high quality, comparable with data of land seismic prospecting in adjacent territories [Seleznev and other, 2006] are as a result received.

Offered by Geophysical Survey of the Siberian Branch of the Russian Academy of Science the technique of research of the Arctic shelf consists in the following. Seismoprospecting profiles begin in the large rivers running into Arctic ocean (Ob, Yenisei, Lena, etc.) approximately about 300 km from a mouth for time sections construction within the continent. Further profiles proceed deep into ocean approximately on 500 km for research of a zone of transition a land-sea, structures of a continental shelf and its proof of outfit to continent by geological criteria. The technique of field works is supposed similar to works in Yenisei gulf. Experience of works in a river mouth of Enisei has shown that the received time section shines practically all earth crust. As sources it is offered to use group of airguns “Puls-6”, lowered on depth nearby 6 meters for achievement of the maximum capacity of a radiated signal; a data-acquisition equipment – a seisphone cable assembly with a seismic station “Intromarin-L2” and telemeter system “MarshLine”, allowing to carry out researches at immersing on depth up to 100 meters.

Literature

Seleznev V. S, Soloviev V.M., Sysoev A.P., Babushkin S.M., Emanov A.F., Bryksin A.A. Seismic researches on water areas: problems, results, prospects. Ways of realisation of oil and gas potential of Khanty-Mansiysk autonomous region: Materials of the seventh scientifically-practical conference. Volume 1, Khanty-Mansiysk, The publishing house «IzdatNaukaServis», 2004, p.204-212.
Seleznev V. S, Soloviev V.M., Sysoev A.P., Babushkin S.M.,Bryksin A.A. Detailed seismic researches on water areas and in transit (water-land) zones. Ways of increase of efficiency of geologo-prospecting works on oil and gas in the Eastern Siberia and Republic Sakha (Yakutia): Materials of the all-Russia conference, SSRIGGMP, Novosibirsk, 2006, p.212-215.
Seleznev V. S, Soloviev V.M., Sysoev A.P., Babushkin S.M., Kashun V. N, Bryksin A.A., Liseikin A.V. River seismic prospecting in the east of Russia. Prospects of development of an oil and gas extraction complex of Krasnoyarsk region: The collection of materials of scientifically-practical conference. Krasnoyarsk, KSRIGGMP, 2007, p.143-146.

OIL AND GAS OF ARCTIC SHELF 2008