Cenozoic Tectonics and Hydrocarbon Phase Differentiation, West Siberia
Internet Geology News Lertter No. 40, April 10, 2000
Global tectonic events during the Mesozoic included break up of
Pangea II Super-Continent with formation of the Atlantic, Indian, and
Arctic Oceans. Opening of the Arctic Ocean had its effect on West
Siberia. In late Eocene time the northern part of the West Siberian
Basin experienced uplift, and the southern part subsidence. These
events led to broad Eocene regression of the sea, and in Oligocene
time the West Siberian Lowland was a lacustrine-alluvial plain.
The top of the middle Eocene Lyulinvor Horizon ia taken as the
zero surface for calculating tectonic movements. Thicknesses of the
Paleogene-Neogene sediments indicate that this datum subsided
700 m during Paleogene-Neogene time in the south of the basin and
was uplifted by this amount in the north. The axis of these movements
was located along the modern east-west course of the Ob River.
During all of the Mesozoic this axial area was a zone of high stand of
the basement and served as a barrier to southward penetration of the
sea during times of transgression. The sea did cross over this zone
from time to time along rift basins that cut this barrier.
The oil and gas pools of West Siberia occur in five plays: Lower-Middle Jurassic, Upper Jurassic, Neocomian, Hauterivian-Barremian, and Aptian-Cenomanian. Each of the lower three
is hydrodynamically closed over a large part of the basin, as indicated by high gas content in the oils and by high formation pressures. The Hauterivian-Barremian and Aptian-Cenomanian plays are a single hydrodynamic system in the north, but to the south they are divided into two parts by a lower Aptian clay seal. The Hauterivian-Cenomanian hydrodynamic system is overlain throughout the basin by a Turonian-Paleogene clay seal.
Nesterov estimates that the formation waters of the Mesozoic of
West Siberia contain 500 trillion cubic meters (17,500 tcf) of dissolved
gas, and Kortsenshteyn estimates that the Cretaceous alone contains
440 trillion cubic meters (15,400 tcf). Kruglikov and others conclude that
formation water moves from south to north from areas of high pressure
to low pressure. According to Kortsenshteyn the lower formation pressure
in the north provided the conditions for discharge of dissolved gas into the
free state in the Nadym-Taz region, leading to formation of gigantic pools.
The radius of the area of gathering of gas for a 1 km thickness of
Cenomanian is estimated at 400 km. (Taken from Surkov and Smirnov, 1994; digested in PETROLEUM GEOLOGY, vol 29, no. 9/10, 1994,
5 paleogeographic maps, 2 cross sections, and 1 Cenozoic stratigraphic
chart)
Copyright 2000 James Clarke. You are encouraged to print out this
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