Separate prognosis and formation of hydrocarbon systems in west Siberia

The west Siberian oil and gas province occupies an area of more than 2.5 million km[sup 2]. More than 1900 oil and gas pools have been discovered there. A study of the vertical and regional distribution of hydrocarbons suggests there is a regular zonation of the various phase state. Within the Mesozoic to Cenozoic series, five major zones have been identified. The first zone of Aptian, Albian, and Cenomanian rocks contains all principal gas accumulations of the province (95% of west Siberian gas) and a number of super giant and giant dry-gas fields have been discovered. Reservoir temperature varies from 9 to 20[degrees]C. The second zone includes Valanginian through lower Aptian rocks and contains gas and oil-gas condensate pools. A group of wet-gas fields is located between the Nadym and Taz rivers, and also on the Yamal and Gydan peninsulas. The total thickness of the Neocomian is 1000 m. Reservoir temperature varies from 30 to 75[degrees]C. The third zone contains mainly oil pools. The zone embraces Valanginian-Hauterivian and Berriasian-Valanginian productive horizons of the middle Ob region. Reservoir temperature is between 70 and 100[degrees]C. The fourth zone comprises Middle and Upper Jurassic rocks and contains oil and gas condensate pools, oilmore » with associated gas-bearing Upper and Middle Jurassic sandstones in the Salym and Krasnolensk region. The oil is lower density than the Neocomian oil and production is smaller. Reservoir temperature reaches 120 to 140[degrees]C. The fifth zone is in the deeply buried rocks of the Achimov Formation and Lower-Middle Jurassic strata of the Urengoy field in the northern part of the basin. The zone contains gas condensate and the reservoir temperature is more than 140[degrees]C. Specific features of the West Siberian basin is the genetic character of the vertical and areal zonation; the distribution and type of hydrocarbons is a function of primary kerogen type and in situ generation and entrapment.« less