The Bashkir paleo-arch on the clastic Devonian of the eastern passive margin of the Russian platform is located within the region of the modern Yuryuzano-Sylven depression and much of the Bashkir arch. This ancient structural feature lost its morphological expression in later structure and is recognized only from thickness and facies of sediments of Devonian age. At the end of the Early Hercynian and beginning of the Middle Hercynian tectonism the Bashkir paleo-arch was a system of island-block highs that rose above sea level and exerted considerable influence on the evolution of the Late Devonian marine basin and the character of deposition.
The Orlov Beds of the Frasnian Stage of the lower part of the Upper Devonian were deposited along the west margin of the Bashkir paleo-arch. These beds rest with erosion on rocks from pre-Devonian to late Middle Devonian in age. This formation is subdivided into a lower sandstone-siltstone member and an upper shale member. The sandstone-siltstone member consists largely of fine- to medium-grained porous gray quartz sandstone, which in places is dense, limy, or silty. Thickness is 2-14 m. Porosity is up to 20 percent, and permeability is up to 0.835 darcies. The shale member consists of gray to greenish deposits that carry numerous plant remains. Thickness is 0 to 44 m. Maximum thickness of Orlov rocks is localized in a belt 6-12 km wide and some 200 km long.
The following model is presented to explain the formation of the Orlov Beds. Subsidence of the crest of the Bashkir paleo-arch to below sea level as a result of broad Late Devonian transgression led to development of submarine currents that flowed around elevations on the sea floor. The general direction of flow was from south to north along the regional slope. The largest and most stable of these currents flowed along faults that bounded blocks and steps of the Bashkir paleo-arch. Middle Devonian Mendym carbonates, which formed the sea floor, had been eroded unevenly, and small residual carbonate "hills" remained. A commercial flow of oil has been recovered from one such buried hill; the pool actually occurs in surrounding sandstone of the Orlov.
When the Bashkir paleo-arch stood above sea level in the initial stage of the Middle Hercynian, conditions were favorable for growth of reefs along the coast line. With subsequent subsidence the paleo-islands were converted into banks, and conditions were favorable for growth of bioherms.
To the west of the belt of Orlov rocks the oil-gas potential of the Upper Devonian Famennian and Lower and Middle Carboniferous is associated largely with drape structures over organic buildups in the lower part of the carbonate Devonian.
In summary, new discoveries are possible in various kinds of traps in the Frasnian sediments of the study area.
Taken from Masagutov, 1988; digested in Petroleum Geology,
vol. 30, no. 3, p, 265-269, 1996, one map, one cross section.
Copyright 2002 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/
This News Letter is distributed without charge in the interest of
our science. To be added to the mailing list, please send your
e-mail address to: jamesclarke@erols.com
For information of the journal Petroleum Geology please FAX
703 759-3754 or telephone 703 759-4487.
A man of knowledge used words with restraint, and a man of
understanding is even-tempered. Proverbs 17-27.