Late Precambrian rifts in the Timan-Pechora oil-gas basin are filled by unmetamorphosed sedimentary rocks more than 10 km thick. Their upper parts have potential for commercial gas. The Phanerozoic basin fill is mildly deformed in the western and central parts into broad depressions and more narrow intervening highs. The eastern part is strongly deformed by Late Paleozoic thrusting from the rising Urals. Eight oil-gas plays are recognized in the basin.
(1) The Ordovician-Lower Devonian play consists of limestone, dolomite, and clastics up to 2.5 km thick. The reservoir rock is fracture-pore limestone and dolomite, and porosity is 5-10 percent. Only local seals are present. This play has accounted for well over half the increase in petroleum reserves of recent years.
(2) The Middle Devonian-lower Frasnian ("Clastic Devonian") play is generally 200-500 m thick and contains several sandstone reservoirs. This play accounts for most of the reserves of Timan-Pechora basin. The oil at Yarega field is very viscous, and the sand grains of the reservoir rock are anatase rather than quartz.
(3) The rocks of the upper Frasnian-Tournaisian play are largely shelf facies, grading to the east and south into open sea facies. Deposition was during a time of successive transgression and regression, reflecting perhaps global sea level changes as a result of glaciation in the Southern Hemisphere. Three main reef-building cycles are recognized for late Frasnian time. The lowermost strati- graphic unit of this play is generally accepted as the Domanik Formation, which is largely a bituminous limestone and appears to be the source for much of the oil of this play. Limestone is the principal reservoir rock.
(4) The Lower Carboniferous play consists of lower and middle Visean rocks. The rocks of the previous play were eroded extensively in pre-Visean time from the Timan-Pechora basin southward through the Volga-Ural basin, and in the Dnieper-Donets depression. Visean clastics fill lows on this erosion surface. Thickness is generally 0 to 40 m. Reservoir properties are good, but pools are small due to lack of a regional seal.
(5) The Upper Visean-Lower Permian play consists of shallow-water marine carbonates 1.0-1.5 km thick, overlain by Kungurian evaporites. Reflecting orogenic movements in the rising Urals, the clastic component in the Lower Pemian increased with time.
(6) The Lower Permian play consists of limestones 100-350 m thick in the western and central parts of the basin and clastics up to 2 km thick in the eastern part.
(7) The Upper Permian rocks are near-shore marine, alluvial, and lake- swamp coal-bearing deposits. Thickness ranges from 100-800 m on the craton to 3-4 km in the Cis-Ural foredeep. Reservoirs are excellent, but seals are limited.
(8) The Triassic section consists of variegated siltstone, shale, and sandstone. Thickness ranges from 150 m to 4 km.
Back in the Sixties Soviet planners held high expectations for the Timan-Pechora basin. During the last three decades, however, there have been more disappointments than successes. Nevertheless, there are structuresthat have not been tested, deep plays that have not been explored, Upper Devonian and Lower Carboniferous pinchouts that have not been examined, reef buildups that have not been drilled, and overthrust zones that offer great potential. (Taken from Clarke, International Geology Review, Vol. 36, p. 314, 1994)
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/ This News Letter is distributed without charge by e-mail in the interest of our science. To receive it, please send your e-mail address to jamesclarke@erols.com