The Volga-Ural oil-gas province has an area of 500,000 sq km (200 sq mi). It is located on
the Russian Plain between the Volga River on the west, the Ural Mountains on the east, and the
North Caspian depression on the south. On the north it merges with the Timan-Pechora oil-gas
province. The region extends from cold, humid taiga on the north to hot, dry deserts on the
south (Peterson and Clarke, 1983).
The Volga-Ural oil-gas province is in general coincident with the Volga-Ural regional high
(anteklize), a broad upwarp in the east-central part of the East European craton. In the central
part of this regional high is the broad Tatar arch, on which are located the major oil fields of the
province. Then peripheral to this arch are troughs of the Kama-Kinel system of uncompensated
downwarps (News Letter no. 11, September 20).
Numerous channel deposits have been mapped during the last decade in the Volga-Ural
oil-gas province. They are present along practically the entire sedimentary section (Middle
Devonian to Upper Permian) but are best developed in the Lower and Middle Carboniferous
of the Middle Volga region of Tatarstan, Bashkortostan, Udmurtia, Samara, and Ul'yanov.
During the Early Carboniferous the broad shelf area between the Kama-Kinel downwarps
experienced uplift. At the same time the downwarps were at their maximum subsidence.
Relief of the sea floor of the marine basin was thus at maximum. Individual parts of the shelf
areas were raised so high that sedimentation was interrupted. Canyon-like features were cut
into the uplifted areas to empty into the Kama-Kinel downwarps.
Subsequent regression, still in the Early Carboniferous, closed out the Kama-Kinel downwarps
by clastic deposition. The river systems thus had a higher base level, and their channels became
choaked with alluvium. Deltas formed at the mouths of the rivers. Analogs are said to be found
in the Maykop region and also in eastern Kansas.
The alluvial channel fills have been conduits for lateral migration of hydrocarbons from the
Kama-Kinel downwarps into the higher shelf areas, supplying oil to both the clastic fills and
stratigraphically lower Tournaisian carbonates and possibly also Upper Devonian reservoirs.
The channels have also served as conduits for vertical migration of hydrocarbons from the
underlying Devonian, where source beds comprise up to 200 m of clastics and 80-100 m of
Domanik bituminous shaly carbonate. This oil is now represented by pools in the Carboniferous
and bitumen in the Permian sections. >P>
In spite of numerous shows in these channel fills, commercial discoveries have been small.
The width of these channels is measured in only hundreds of meters, and consequently they are
missed where the well net is thin. There is also a problem with seals - poor quality and thin. >P>
Recognition of these channel deposits broadens the oil prospects for the Kama-Kinel system
of downwarps. Where rivers debouched into the downwarps, thick piles of sand and silt
accumulated. One such area, designated the Tiinsk-Sakhcha structural-facies zone, is 90-100 km
long and 15-20 km wide. It is favorable for new discoveries (Petroleum Geology, vol. 32, no. 2,
p. 107, 1998).
Copyright 1999 James Clarke. You are encouraged to print out copies of this News Letter and
to forward it to others.