In western Georgia the Rion River valley opens broadly into the eastern end of the Black Sea. This swampy lowland is the site of the Kolkhid depression, a post-Hercynian Mesozoic-Cenozoic downwarp that measures some 50 km east to west and 30 km north to south. It is the western part of the terrane south of the Caucasus; this terrane may be a micro-continent. "Kolkhid" is the ancient Colchis, the mythical home of Medea. It was the goal of Jason and the Argonauts, who retrieved the golden fleece (fleece used to recover placer gold - as I have read.)
The sedimentary fill of Kolkhid depression consists of six structural complexes, at the boundaries of which are marker reflecting horizons.
Complex I consists of Modern friable sand-clay deposits 5-20 m thick. Longitudinal wave velocities are in the 450-1600 m/sec range.
Complex II combines the lower part of the Holocene and the Pliocene. It comprises conglomerates and coarse sands up to 300 m thick with wave velocity of 1500-1600 m/sec.
Complex III is upper Pliocene sands and weakly cemented sandstones, siltstones, and silty clays 1150-1250 m thick with velocities up to 2100-2300 m/sec.
Complex IV includes sediments of the lower Pliocene, Miocene, and Paleogene. The upper part is conglomerate and coarse sandstone of the Pontian, passing downward into finer sandstone and siltstone and then into clays and marl. Limy clays of the upper Eocene grade into marls and clayey limestone of the middle and lower Eocene. Thickness of this complex is 400-1800 m, and wave velocity is 3500 m/sec.
Complex V is Cretaceous, largely limestone with marl in the upper part and volcanics in the lower. Thickness is 1400-1800 m, and average wave velocity is 4200 m/sec.
Complex VI combines rocks of the Jurassic and possibly also the Paleozoic. Its upper part is a pile of Middle Jurassic effusives more than 3000 m thick. These overlie a Lias clastic-carbonate unit. Thickness of this complex may reach 6000 m. Wave velocity is 4800-5100 m/sec.
Structure in the Kolkhid depression has an east west trend. From north to south the principal structures are the Khobi syncline, Central Kolkhid high, Paleotom depression, and Guriy foredeep, which is thrust northward onto the Paleotom depression.
Both reservoir rock and seals are recognized in the study area. Reservoir rocks are present in Lower Jurassic and upper Bajocian- Bathian clastics, Aptian-Neocomian and Turonian-Paleocene carbonates, and Pliocene-Quaternary clastics. Between these are seals in the lower Bajocian, Upper Jurassic, Albian-Cenomanian, and lower-middle Sarmatian complexes. In the Pliocene-Quaternary complex Meotis clays are a seal for an underlying sand.
Gas, gas-condensate, and oil pools are expected in Lower Jurassic sand- stones, and gas-condensate and gas-oil pools in the lagoonal-continental sediments of the Bajocian-Bathian complex. The Aptian-Neocomian carbonates have been thoroughly flushed. The Turonian-Paleocene complex has been eroded in many places and pools destroyed. The Neogene-Quaternary complex is oil-gas-bearing only in the Guriy foredeep, where pools occur in Sarmatian sandstones. (From Likholatnikov (1993); digested in Petroleum Geology, vol. 31, no. 1, p. 10-11, four structure maps and one cross section.)
Copyright 2000 James Clarke. You are encouraged to print out this News
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