Oil shows were noted on the Siberian craton (East Siberian oil-gas province) more than 200 years ago. Shallow wells were drilled at the beginning of the 20th Century, and the first deep wells were in Yakutia in the Thirties. The first flows of oil and gas were recovered at the beginning of the Fifties.
The stratigraphic range of pay zones on the craton is from Riphean in the Proterozoic to Mesozoic. A total of 23 oil-gas reservoirs have been found. They are all complex and do not have consistent properties.
An important feature of the Siberian craton is high intensity of neotectonic movements and anomalously low formation temperature and pressure in several of its basins. Magnitude of these movements ranges from -1000 m in the Ust'-Aldan depression to +1000-1500 m in parts of the Putoran anteklize. Such movements may have led to extensive re-ordering of oil and gas pools.
The neotectonic activity took place at a time during which permafrost formed over extensive parts of the craton. In Central Yakutia permafrost began its development about 600,000 years ago. Much attention has been given to the formation of gas hydrates in Siberia. This, however, is but one part of the action of permafrost on hydrocarbon accumulations. The geothermal effect of the cryolitic zone extends far beyond the depth of hydrate formation. For example, in the Vendian clastic reservoirs in Srednebotuobinsk field temperature does not exceed 15-18 degrees C at depth of 1900 m. On Nepa arch temperature at the surface of the basement is not more than 15-20 degrees C.
Hydrocarbons that accumulated in traps before the regional cooling acquired completely different migrational capacity under conditions of low formation temperature. This resulted in a definite increase in capillary pressure. A consequence of this increase in capillary pressure is that it inhibits penetration of hydrocarbons through water-saturated rocks. Under the previous geothermal conditions this was not an insurmountable obstacle.
The result of re-ordering of hydrocarbon pools by neotectonic activity may be formation of pools in non-anticlinal traps, as found on the Nepa arch. The cooling action of the permafrost practically doubles the resistivity to migration of hydrocarbons. High capillary pressures in the sandstone reservoirs in conjunction with a very gentle attitude of the regional oil-gas strata hinder movement of hydrocarbons toward the crests of highs. In spite of strong forces of buoyancy of the large volume of migrating gas, the oil and gas were held on the southeast flank of the arch.
Migrating gas was concentrated on the new Putoran and inherited Aldan anteklizes, Yakutsk arch, and others. At low formation temperatures it was hardly possible for hydrocarbons to reach the crests of anteklizes and arches. Most of the oil and gas was retained on flanks by capillary seals, which consist of water-saturated rock with medium to low permeability.
Resistance to migration of hydrocarbons on the Siberian craton limits effectiveness of searching for anticlines. In Lena-Tunguska province 80-85 percent of the anticlines disclosed by geophysics turned out non-productive. Exploration for non-anticlinal traps should not assume that the oil and gas have reached the heads of pinch-out strata because capillary pressure may have prevented the hydrocarbons from reaching the distal parts of permeable beds.
The sedimentary section of the Siberian craton has a very high hydrocarbon potential, and a significant portion of this is oil. Most of this will probably be in non-anticlinal traps. (Taken from Bol'shakov, 1989; digested in Petroleum Geology, vol. 34, no 4, 2000, in press)
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