The wide occurrence of Proterozoic sedimentary rocks and of
regional oil shows associated with them are persuasive that the
Riphean and Vendian rocks of the central parts of the East European
craton as well as the Paleozoic rocks are favorable for discovery of
oil and gas.
The Pachelma aulacogen extends some 600 km southeastward
from the Moscow area to the Saratov area. It is in general 50 to 100 km
wide. It is bounded on the northeast by the Tokmov arch and on the
southwest by the Voronezh massif. Interest in the petroleum potential
of this feature dates from the Fifties.
The Upper Proterozoic deposits of Pachelma aulacogen are 1-4 km
thick and consist of normal sedimentary rocks, largely clastics, among
which are oil-source beds and reservoirs. Four structural-facies zones
are recognized: Kaverino on the northwest, Pachelma in the central
part, and Saratov and Pugachev on the southeast. These differ in
composition and thickness of sediments of the same age.
At the base of the sedimentary cover is a complex of red sandstones
of the Tatishchevo Formation of the Lower Riphean, Rtishchev Formation
of the Middle Riphean, and Tsnin Formation of the Upper Riphean. This
redbed complex was deposited under continental conditions with local
episodic shallow-water marine or lagoonal conditions. Next overlying is
the Upper Riphean Ingiz Formation, which consists of sandstone, siltstone,
and shale. Present here are films of organic matter. At the top of the
Riphean is the Peresypkino Formation - interbedded gray dolomite,
shale, and sandstone.
The Lower Vendian Sekretarkin Formation is transgressive onto the
Upper Riphean. It consists of clayey limestone, shale, and fine-grained
sandstone, deposited largely under oxidizing conditions, giving way from
time to time to weakly reducing. Resting conformably on the Sekretarkin
is the Vedenyapin Formation - interbedded clayey limestone, shale,
siltstone, and sandstone gray-green to black in color. Deposition was
in a shallow marine basin under reducing conditions. The Vedenyapin
is regarded as the most probable oil source of all the Proterozoic rocks
of Pachelma aulacogen. Reservoir rock with porosity of 2 to 18 percent
is present also.
The next overlying Vorona Formation is largely well sorted arkosic
sandstones. These are the main reservoirs of the Proterozoic of the
Pachelma aulacogen, and they have yielded oil and gas shows in several
wells. Their porosity is 12-21 percent. Resting on an erosion surface on
the Vorona is the Krasnoozer Formation, which consists of greenish black
shale, siltstone, and fine-grained sandstone. It is possibly both oil
source and seal. Younger formations of the Vendian not only fill
the aulacogen but also extend out onto crystalline basment.
Geologic conditions are persuasive that commercial oil and gas are to
be found in the Pachelma aulacogen.
Taken from Postnikova, Bazhenova, and Kotsareva (1998); digested in
Petroleum Geology, vol. 32, no. 3, 1998; one structure map and three
stratigraphic sections.
Copyright 2001 James Clarke. You are encouraged to print out this News
Letter and to forward it to others. Earlier News Letters are available at
our web page: http://geocities.com/internetgeology/
This News Letter is published without charge in the interest of the science
of petroleum geology. To be added to the miling list please send your
e-mail address to: jamesclarke@erols.com
For information on the journal Petroleum Geology, please telephone or Fax
us at 703 759 3754