The study area includes the Volga-Ural and North Caspian oil-gas
provinces. Source beds are present in ten parts of the section:
Middle-Upper Riphean, Vendian-Lower Cambrian, Middle and
Upper Cambrian, Ordovician, Silurian-Lower Devonian, Eifelian
(Middle Devonian),Givetian (Middle Devonian), Visean (Lower
Carboniferous), Middle Carboniferous, and Lower Permian.

The main structures of the region formed by intersection of the
East European, Siberian, and Kazak plates and the West Siberian
micro-continents. This activity was in three main cycles: 1) ancient
(Riphean-Cambrian), intermediate (Ordovician-Early Devonian),
and late (Middle Devonian-Cenozoic). The main structures of the
Riphean-Cambrian cycle are rifts with Riphean fills, passive
continental margins, and supra-rift sags. The Ordovician-Early
Devonian cycle was a time of rifting. A passive margin developed
in Late Devonian and Early Carboniferous time, and plate collision
marked Permian-Early Triassic time.

In Riphean time source beds were deposited in continental rifts
and at the suture between the continent and the ocean to the east.
In Vendian-Early Cambrian time deposition was in broad supra-rift
sags, and in the Middle and Late Cambrian only on the eastern
margin of the craton.

At the beginning of the Ordovician-Early Devonian cycle, source
beds collected in small rift-related depressions and on the shelf of the
passive margin on the east. No source beds were deposited in the
Early Devonian on the stable continental interior. The area of
deposition broadened along the passive margin but was limited by
collision with island arcs.

Divergent conditions prevailed in Middle Devonian-early Visean
time. In the Middle Devonian source beds were deposited in the North
Caspian depression and in the eastern part of the Volga-Ural passive
margin. In the Late Devonian-Tournaisian the area of source-bed
deposition broadened substantially on the eastern margin of the East
European craton, in the Dnieper-Donets aulacogen, and on micro-
continents and oceanic areas to the east. The oceanic zone of source-
bed deposition was greatly reduced at the end of the Visean.

Late Visean-Early Triassic was marked by convergence. In the late
Visean-Middle Carboniferous stage source beds collected in the North
Caspian depression and southern part of the East European craton.
The oceanic zone was gradually closed out by docking of continents.

During the Early Permian source beds were deposited only in the
Cis-Ural foredeep and in the center, south, and east of the North
Caspian depression.

The Riphean-Cambrian rocks have a low potential for hydrocarbon
generation. The Ordovician-Lower Devonian rocks have low to medium
potential. The Upper Devonian-Tournaisian divergent rocks have
medium to high potential, and the upper Visean-Lower Permian
convergent rocks have low to medium potential (Petroleum Geology,
vol. 32, no. 2, p. 172-183, 1998; 3 maps and one cross section - from
Larskaya and Shein, 1997).

Copyright 2000 James Clarke. You are encouraged to print out this
News Letter and to forward it to others. Earlier News Letters are
available at http://geocities.com/internetgeology/