The main plays in the southern part of Orenburg Region are the Devonian upper Eifelian carbonate and Givetian-lower Frasnian clastic-carbonate sediments, where the Zaykin, Rostashin, Lebyazhin, Garshin-Yefimov, Dolin, and Donets gas condensate and oil fields have been discovered.
The Rubezhin-Ural uncompensated paleo-downwarp is now recognized in the upper Eifelian carbonates on a basis of lithology and sedimentary thickness. It extends for several hundred kilometers across the south of Orenburg Region and is 75-80 km wide. Its central part is bounded on north and south by border zones. The regional Kamelik-Chagan flexure in Devonian sediments extends parallel to this downwarp some 10-20 km to its south. Detailed seismic surveys have been made within the downwarp and to the south of the Kamelik-Chagan flexure.
Within the downwarp as a whole the thickness of the upper Eifelian carbonates is 137-224 m. Along the north border this thickness is 180-224 m. Biohermal rocks are present there. The latter are gray limestone. Organic buildups 60-95 m high occur in this north border zone. The south border is similar to the north border and may also consist of biohermal deposits.
In the central uncompensated part of the downwarp the sedimentary thickness is 137-158 m. These deposits are dark gray, almost black pelitic radiolarian limestones containing beds of argillite and combustible shale.
The Rubezhin-Ural downwarp is expressed also on the top of the Tournaisian Stage. Profiles of deep well extend across both the north and south flanks. Amplitude of this downwarp for the upper Eifelian rocks is more than 1 km on the north flank and more than 400 m on the south flank. These parameters are 850 and 67 m, respectively, for the Tournaisian.
The most favorable area for exploration is the north border zone of the Rubezhin-Ural downwarp, where bioherms are present in the upper Eifelian and thick sandstones with good reservoir properties in the carbonate-clastic section of the Givetian and lower Frasnian.
Taken from Borisova and Fomina, 1987; digested in
Petroleum Geology, vol. 23, no. 11/12, one map and two cross
sections.
Copyright 2002 James Clarke. You are encouraged to print
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