In West Siberia the oil is concentrated in Jurassic and Lower Cretaceous rocks of the central region, and the gas largely in Aptian-Cenomanian rocks of the north. Total effects of recent tectonic movements and Late Cenozoic glaciation on these petroleum deposits are examined on a basis of three factors:
1. Changes in volume of traps and time of hydrocarbon migration into pools. Closure on structures in the gas-prone north of West Siberia increased by 50-70 m during Neogene-Quaternary time. Since these structurtes are largely filled, 20-30 percent of the volume of the gas pools here can be attributed to Late Cenozoic tectonic movements.
In the oil-prone Middle Ob region closure on the Cenozoic strata generally does not exceed 10-20 m. Closure on strata lower in the section is generally not more than 40-80 m. Consequently, Cenozoic movements account for about 20 percent of the closure. Since the traps are largely filled, a significant amount of oil must have entered the traps during the Cenozoic.
Increase in monoclinal dip along the margins of the West Siberian platform during the Cenozoic due to uplift of the border areas has emptied the large structural traps there.
2. Opening of earlier fractures and development of new ones for migration of hydrocarbons. In the southern part of the platform oil has migrated along vertical fractures from Jurassic source beds into Lower Cretaceous reservoirs. The extent of fracturing in Mesozoic clays in the Middle Ob region decreases upward from the Jurassic to the Upper Cretaceous rocks. In all areas where oil pools are present in the Neocomian the rocks are clearly fractured. Oils are similar to one another in successive horizons of a single region but then vary from region to region.
The oil-gas stage is the distance from the base of the sedimentary cover to the highest petroleum pool. Where Cenozoic growth of closure is not more than 10 percent, average stage is 0.5 km; with 20 percent it is 0.7 km; with 30 percent it is 1.9 km; and with more than 30 percent it is 2.8 km. This sharp increase is due to breaking of clay seals.
3. Changes in formation pressure. In the north of the West Siberian platform where Neogene-Quaternary uplift was 200 m and more, the resulting drop in formation pressure reached 20-25 atm. In the Middle Ob and southeastern regions this drop did not exceed 5-10 atm. Eustatic lowering of sea level in the Late Cenozoic in connection with formation of the continental ice sheets also caused drops in formation pressure. This effect was uneven, however. In the late Pliocene when the level in the Polar Basin dropped 250-300 m in comparison with the present, the decrease in formation pressure in the north of West Siberia was 25-30 atm, whereas in the Middle Ob region it was 2-3 atm. Periodic increases and decreases in formation pressure and temperature were accompanied by de-gassing of the formation waters in the north of West Siberia. (Taken from Kuzin, 1983; digested in Petroleum Geology, vol. 20, no. 8, p. 358-360)
Copyright 2000 James Clarke. You are encouraged to print out this News Letter and to forward it to others. Earlier News Letters available at: http://geocities.com/internetgeology/ This News Letter is distributed by e-mail without charge in the interest of our science. To be added to the distribution list, please send your e-mail address to: jamesclarke@erols.com