The non-salt rocks of the Upper Devonian Famennian salt complex of Belorussia are oil-bearing in several areas. The first such flow of oil was recovered in 1953. Commercial flows or strong shows have been found in several places.
The upper Famennian salt complex is present over a wide area of Pripyat downwarp. Thickness ranges from 32-100 to 2000-3500 m. Average is 500-600 m. This range is due to variations in the section and to salt tectonics. The top of this complex is at depths that range from 365 to 4715 m. Members of pure salt 100 m thick and more alternate with non-salt members.
Several rhythm-members are recognized. At the base of each is a non-salt member and then a salt member. In the border areas of the downwarp the halite complex undergoes facies change to dolomite-anhydrite on the northwest, clay-carbonate-sulfate on the north, sand-clay varieties on the southwest, and volcanics on the northeast.
Salt comprises most of the complex. Other rocks account for 10-15 percent and in some places up to 40 percent of the section. On a basis of these non-salt types nine fields are recognized.
Field I is situated in the extreme northwest of the downwarp. The main rocks there are dolomitized limestone and more rarely limestone, dolomite, anhydrite, and carbonate-sulfate rock.
The non-salt rocks of Field II in the north part of the downwarp consist of 28-50 percent anhydrite, 20-35 percent carbonate- sulfate rocks, and 20 percent limestone. Stromatopoids are widely distributed in the salt somplex. The stromatopoid-bearing members are composed largely of limestone, anhydrite, and carbonate-anhydrite rock. In the Predrechin synclinal zone the carbonate and sulfate rocks pass into salt, and in the peripheral parts of the downwarp into clastics.
The stromatolites differ from the surrounding rock by being composed almost entirely of cakcite, although in other regions they are dolomite. These stromatolites are shallow-water features associated with shallows on the sea floor. They formed by algal growth and precipitation of calcite. It can be supposed that in late Famennian time the stromatolites formed in a relatively wide range of salinity permitting precipitation of both calcium carbonate and sulfate.
Field III has up to 37 percent clastics; the rest is anhydrite (30 percent), carbonate-sulfate rock (14 percent), limestone (10 percent), and marl (10 percent). The clastics are largely well-sorted quartz siltstone. The association of these siltstones with clays, marls, and salt suggests deposition in a lagoonal environment.
Fields IV, V,and VI are in the central part of the downwarp.
Field IV is largely limestone (36 percent) and marl (22-34 percent). These are gray to dark gray, some almost black. Stromatolitic and biomorphic limestones are present as well as anhydrite.
Field V is characterized by a higher clay content, 22-24 percent marl, and 22-33 percent clay.
In Field VI there is a greater amount of pure and clayey limestone as well as sulfate rock. Up to 5 percent volcanics are also present.
Field VII has 31-32 percent clastics, 20-30 percent marl, 13-15 percent anhydrite, and 10 percent clays and limestone.
Fields VIII and IX in the southern part of the downwarp have largely clastic facies due to their proximity to the Ukrainian Shield. Sandstone and siltstone comprise 43 percent of Field VIII and 74-79 percent of Field IX.
There is great interest in Field IX because a flow of oil has been recovered from clastic rocks of the lower part of this section. These are sandy beds, fluvial or deltaic, which are 80-148 m thick and extend in a northeast direction as a belt 1.5-2.5 km wide. Beyond this belt this member is only 20-40 m thick. The most favorable for lithologically sealed pools for oil are Fields II( (carbonate-sulfate rocks), VIII (clay-clastic rocks), and IX (clastics).
Taken from Obrovets, Vysotskiy, and Kislik, 1986; digested
in Petroleum Geology, Vol. 23, No. 7/8; one lithofacies map,
one lithologic column, one cross section.
Copyright 2003 James Clarke. You are encouraged to print
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