With an area of 3.5 million sq km (1.3 million sq mi) West Siberia is the largest flat area
on the Earth. The platform cover consists largely of Jurassic and Cretaceous clastics,
which are host to the huge oil and gas pools of the region. Beneath these clastics is a variety
of Precambrian, Paleozoic, and Triassic rocks, some of which may carry commercially
significant hydrocarbons.
The seismic reflection methods used in the Sixties and Seventies in West Siberia were
limited to tracing only marker reflecting horizons M (top of Koshay Member of lower Aptian)
and B (Bazhenov Formation of the Upper Jurassic). These data were quite adequate for the
very successful initial exploration. The Lower Cretaceous Neocomian rocks between these
two seismic markers were subdivided into a lower Migeon Formation and an overlying
Vartov Formation. Achimov Beds (sandstones) were recognized within the Megion.
After assembly of an enormous amount of drilling and common depth point seismic data
the sedimentary structure of the region became clearer. Most significant was recognition of
clinoforms. These features did not show up on the earlier reflection profiles. There is no
mention of such features in our first report on West Siberia (Clarke, Girard, Peterson, and
Rachlin, U.S.G.S. Open File Report 77-871, 136 p, 1977).
The clinoforms are huge lenses of sand 30-150 km wide in an eastwest direction and more
than 1000 km long in a north-south direction. Several models have been proposed. One is that
these features developed on the margin of a relatively deep-water basin, where uncompensated
sedimentation predominated. According to another model deposition was along the eastern
margin of a relatively shallow sea.
It has now been recognized that seismic horizon B coincides with a rise in sea level
associated with tectonic subsidence and formation of a deep-water intra-continental marine basin.
During the course of Volgian-Barremian time this basin was filled with clastics. Horizon M
corresponds with an important rise in sea level, which coincides with a stage of relative tectonic
calm. Clayey sediments of the Alym Formation accumulated.
On east-west regional profiles the clinoforms slope from east to west in the eastern part of the
basin and from west to east in the western part. Pinching out on their margins, their mid-section
thickness exceeds 200-300 m. They overlap one another as shingles on a roof on down to the
Bazhenov Formation. Their elongation parallel with the ancient shore line does not allow them
to be called deltaic or turbidite deposits. These clinoforms are designated as lanarclines -
bodies of lateral accretion along a paleo-shore line controlled by long-shore currents.
Some 33 west-sloping lanarclines and 20 east-sloping lanarclines are provisionally recognized.
At the top of this section are 7 clinoforms that show no east or west slope. Since the length
of the Neocomian is estimated at about 20 million years, each clinoform corresponds with some
0.5-1 million years. Recognition of these sand bodies opens new directions for exploration
(Petroleum Geology, vol. 34, no. 1, p. 94-106, 2000; Peterson and Clarke, 1991, AAPG Studies
in Geology #32, 96 p.; Oil and Gas Resources in the West Siberian Basin, Russia: Energy
Information Administration, DOE/EIA-0617 - available at http://www.eia.doe.gov).
Copyright 1999, James Clarke. You are encouraged to print out these News Letters and to
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