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Airborne gravity and magnetic surveys method of creating the basis Печать E-mail

FGUNPP "Geologorazvedka", Russia, GNPP "Sevmorgeo", Russia, OIFZ, Moscow, Russia

The main outlook for development of the Russian gas-and-oil producing industry in the 21st century is soundly associated with shelves of the Arctic and Far-Eastern seas. For most of them, the regional stage of mineral exploration is not completed as yet. Because of poorly explored subsurface, the problems of the reliability of a total estimation of hydrocarbon (HC) resources and the zonal prediction of oil and gas-bearing potential still remain unsolved. The HC resources of the Barents Sea subsoil are the targets to be developed in the nearest future. There, on the open sea and in the Pechora shelf some large and unique deposits have been detected, a great number of highly promising targets were revealed. For the water areas of this region, the priority trends are:

1) search for large oil fields in the Pechora shelf in the gas-and-oil accumulation zones (GOAZ) of the north-eastern slope of the Bolshezemelsky vault, in the continental and adjacent offshore parts of which some oil fields have been discovered;

2) development of gas resources of the Shtokmanovsky-Luninsky sill:

3) the «land - sea» transition region, being of particular interest because of its prospects and relative accessibility for development, search for new deposits and the ones continued from land.

The first-priority targets in the near-term outlook are defined as follow:
• the water area of the Korotaikhinskaya depression, where widely-spread overthrust dislocations may occur;
• the Southern and Northem-Prinovozemelsky shelves - mainly, oil and mixed GOAZ;
• the Murmansk shelf- thick (5-10km) strata of Riphean deposits.

Engineering and technological difficulties in geological and geophysical studies of Arctic and subarctic water areas, as well as (in all places) «transition» zone, predetermine the paramount role of remote geophysical methods, among which integrated airborne gravity and magnetic surveys have a special place.

The state-of-the-art remote geophysical surveys imply a highly sensitive instrumentation, equipped with means for information recording, acquisition and storage, a satellite system of aeronavigation, new (in conformity with the world's standards) technologies for data processing and analysis of the structural-morphological features of geophysical fields. The FGUNPP «Geologorazvedka» has arranged production of domestic highly sensitive instruments such as an AKM-01 airborne magnetometer, superior to foreign analogs in many parameters: has developed engineering- technological and methodical solutions for carrying out integrated surveys. The airborne geophysical system was used in shelves (and adjacent land) of the Caspian Sea and the Sea of Azov, in the Volga-Ural oil and gas-bearing province, etc. Under a contract with the Ministry of Natural Resources of the Russian Federation, in 2002-2003, the FGUNPP «Geologorazvedka» has performed tests using the airborne gravity systems (MAG-1, CHEKAN-AM) in the West Siberian oil and gas-bearing province, including shallow waters of the Ob and Taz Bays. The root-mean-square error of Ag field measurements using the MAG-1 (±0.34 mGal) allows to generate free-air reduction maps with a 1 mGal isoline section, which corresponds to a 1:100 000 survey scale. The practical result of the surveys was the detection of several small (4-6km) local anomalies of Ag field (for example, in the water areas of the Taz Bay to the south-east of the Yurkharovsky deposit), similar by amplitude (1-2 mGal) and morphology to the anomalies, reflecting the known deposits. The results of the tests had predetermined the tasks for the integrated airborne gravity and magnetic production survey, performed in the eastern Kamchatka shelf (in the Karaginsky and Olyutorsky Bays) of the Bering Sea.

The created system and its software support allow to highly reliably record the weak anomalies of geophysical fields, obtain their (geophysical fields) measurements practically undistorted by inertia and, therefore, enhance the capabilities of airborne geophysical methods in solving the following major tasks:

1. Prediction of the basement's matter and structure heterogeneity, the hypsometry of erosional-tectonic relief of the basement, the kinematics (dynamics) of paleotectonic processes and the effects of the basement's faults on the structure of the sedimentary cover;

2. Assessment of the thickness and matter-and-structure heterogeneities of the sedimentary cover, based on the regional regularities of layer-lateral variations in the magnetic properties of rocks and the diagnostic indications of different morphological-genetic types of heterogeneities in the magnetic field, reflecting the local targets, explored in detail, including HC fields.

3. Studies of «thin» structures of the magnetic and gravity fields as media of information about the spatial confinement and morphology of heterogeneities, including those arising due to the effects of fluids from a deposit on enclosing rocks. Anomalous changes in the petrophysical parameters in the hydrocarbon saturation zone have a zonal-circular structure. A decrease in the magnetic properties of rocks within the dispersion areal in the above-deposit space of the fluid components was noted over several HC fields in the Timan-Pechora, Volga-Ural. West Siberian and other provinces of the world.

Thus, the main actual data for oil and gas-bearing basin modeling, estimation of its hydrocarbon potential and HC resources (basement's structure and matter composition, determination of cover's differentiated thickness and its zoning according to the physical properties of rocks, prediction of potential gas-and-oil accumulation zones and detection of local heterogeneities of different morphological-genetic types) can be derived from the materials of integrated airborne geophysical surveys, characterized by high economy, efficiency and accessibility. The results of airborne gravity and magnetic surveys show objectively the possibility of their efficient application not only in solving regional geological structure matters for territories and water areas, but also at the oil-prospecting stage of mineral exploration, especially in regions with complicated physical-geographical conditions («transit» shallow water, Arctic and Far-Eastern seas, etc.).

On the whole, 1: 100000 - 1:200 000 integrated airborne gravity and magnetic surveys may be recommended to be carried out in the shelves of Arctic and Far-Eastern seas, including the Barents Sea. i.e. in the regions, where 1:200 000 gravity surveys have not been practically conducted.



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