Wide occurrence of Proterozoic sedimentary rocks and of regional oil and gas 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. Interest in the hydrocarbon potential of the Pachelma aulacogen dates back to the Fifties. Results of current litho-facies, paleotectonic, and geochemical investigations confirm the favorability of this region.
The Pachelma aulacogen has a northwest trend, extending to the southeast from the Moscow area to the North Caspian depression. Its length is some 700 km, and width is more or less 75 km. The Vorona high divides it into two segments. The Upper Proterozoic deposits of this aulacogen are 1-4 km thick and consist of normal sedimentary rocks. These are largely clastics, among which are oil-source beds and reservoirs.
At the base of the sedimentary section 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. These sediments were deposited largely under continental conditions with appearance of local episodic shallow-water marine or lagoonal conditions. Porosities are up to 12-20 percent.
Topping out the Riphean are the Irgiz and Peresypkino Formations. The Irgiz is sandstone that carries some siltstone, shale, and films of carbonized organic matter. The Peresypkino consists of interbedded gray dolomite, shale, and sandstone.
The Sekretarkin Formation is the transgressive part of the Lower Vendian. It is largely red clayey limestone, shale, and fine-grained sandstone. The overlying Vedenyapin Formation consists of interbedded limestone, shale, siltstone, and sandstone, colors of which are gray-green to black. Deposition was under reducing conditions. The Vedenyapin is regarded as the most probable oil source of all the Proterozoic rocks of the Pachelma aulacogen.
Next in the section are the Vorona, Krasnoozer, Sosed, Toropets, Yartsev, Redkin, Makar'yev, and Kotlin Formations. The Redkin Formation consists of alternating gray to black sandstone along with some shale and siltstone. Content of organic matter is 1.5-3.0 percent. An oil shale 20 m thick is present in the middle of the Redkin. This rock is sapropelic and has been referred to as the "Vendian Domanik". Deposition was in an epi-continental marine basin under reducing conditions.
Geologic conditions are persuasive that commercial oil and gas are to be found in the Pachelma aulacogen. (Taken from Postnikova, Bazhenova, and Kotsareva, 1998, one map and one stratigraphic correlation chart; digested in Petroleum Geology. vol. 32, no. 3, 1998) 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 distributed without charge in the interest of our science of petroleum geology. To be added to the mailing list, please send your e-mail address to jamesclarke@erols.com