The Dnieper-Donets and Pripyat basins are underlain by a Devonian rift system that extends southeastward into the deformed Donbas fold belt. The two basins are probably among the geologically oldest oil- and gas-productive rift basins of the world. Gas was first discovered in the Dnieper-Donets basin in 1950; in the Pripyat basin the first oil discovery was made only in 1964. At present, both basins are in a declining stage of production; however, the exploration potential has not been exhausted.

The basement dips southeastward along the rift strike from a maximum of about 5 km (3.1 mi) in the Pripyat basin to possibly as much as 16-18 km (9.9-11.1 mi) in the southeast part of the Dnieper-Donets basin. The basement and rift fill are cut by a large number of faults into many tilted fault blocks. Salt tectonics further complicates the structure. The basins are separated by a synrift uplift composed largely of Devonian volcanics.

The sedimentary cover consists of prerift platform (Middle Devonian-lower Frasnian), synrift (upper Frasnian-Famennian), postrift sag (Carboniferous-Lower Permian), and postrift platform (Triassic-Cenozoic) sequences separated by major unconformities. The sequences are composed of a variety of clastic and carbonate rocks, local Devonian volcanics, and three thick salt formations--two in the synrift Devonian sequence and one in

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the Lower Permian. A Riphean-Vendian (Proterozoic) rift fill occurs at base of the sedimentary cover in the western Pripyat basin. Another Riphean rift filled with unknown rocks possibly underlies the Devonian in the southeast part of the Dnieper-Donets basin.

The Late Devonian rift was formed due to clockwise rotation of the Ukrainian shield relative to the rest of the Russian craton. Hercynian collision along the southern edge of the craton resulted in strong deformation in the Donbas fold belt. Northwestward, the intensity of deformation decreases rapidly, and no signs of compression are observed in the Pripyat basin.

The Pripyat basin produces oil from carbonate rocks of the prerift platform and synrift sequences. Most of reserves are in faulted structural traps along crests of tilted fault blocks. Source rocks have not been geochemically identified, but they are probably present in both productive sequences. The source rocks are mature in the north zone of the basin where all fields are concentrated. In the rest of the basin, source rocks are largely immature except possibly in the deepest depressions. Time of maturation is uncertain, but may be late Paleozoic or late Mesozoic-Cenozoic.

Almost all hydrocarbon reserves of the Dnieper-Donets basin have been found in the postrift sag sequence, largely in clastic reservoirs. Salt-assisted structural traps dominate, although structural traps related to basement tectonics and stratigraphic traps are present. The Lower Permian salt seal controls the major part of the reserves; however, new discoveries come chiefly from the deeper, Lower Carboniferous section. Source rocks have not been geochemically identified; probably the most important source rocks occur in the lower part of the postrift sequence in the central and southeastern areas of the basin. The synrift Devonian sequence also contains as yet unidentified source rocks. Both Devonian and lowermost Carboniferous source rocks are overmature in much of the basin, result ng in the dominance of gas in hydrocarbon reserves. In addition, gas could have been generated by coal and coaly shales in the Carboniferous section. In most areas, the maximum maturation had been achieved by pre-Late Permian time. Both lateral and long-range vertical migration were involved in formation of fields.