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UKRAINE: SEDIMENTARY BASINS:
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The Dnieper-Donets Basin
Globalshift records that the Dnieper-Donets basin has yielded most of Ukraine’s oil and gas. It is a long and narrow Late Devonian rift located in the northeast of the country with a small part in the southeast in Russia.
Sediments are very thick, with a column up to 15 kms in the centre. Precambrian basement of the Ukrainian shield borders the basin on the southwest whilst to the northeast is the Voronezh regional basement high which is covered by a thin layer of Palaeozoic sediments.
In the southeast it merges with the Donbas folded belt which is a structurally inverted and deformed extension of the rift system. In the northwest it extends into the Pripyat Devonian rift basin of Belarus.
The basin has four sequences. The pre-rift platform includes Middle Devonian clastics, deposited in an intracratonic basin. The Upper Devonian syn-rift sequence is up to 5 kms thick comprising marine carbonate, clastic, and volcanic rocks and two deformed salt formations of Frasnian and Famennian age. The post-rift sag sequence comprises Carboniferous and Lower Permian clastic marine and alluvial deltaic rocks up to 11 kms thick in the southeast.
The Lower Permian includes a salt that acts as a regional seal. In Early Permian time the southeastern basin areas were uplifted and eroded and the Donbas foldbelt was formed. Finally a post-rift platform comprises Triassic through Tertiary rocks deposited in a shallow platform depression which extends beyond the basin boundaries.
Most oil and gas is reservoired in Lower Permian rocks below the salt with the remainder in the Lower Carboniferous. Traps are primarily salt-cored anticlines or drapes over Devonian horst blocks. These are sourced by Devonian syn-rift and Carboniferous post-rift shales. Although these beds had matured by the end of the Palaeozoic, inversion probably led to remigration of hydrocarbons. Basin models are thus very complex. Since most of the traps are salt-related new modern seismic acquisition and processing techniques could perhaps reveal further small oil and gas accumulations.
The Carpathian Basin
The Ukrainian part of the Carpathian folded belt and its foreland basin is in the west of the country, spreading into to Poland, Slovakia, Hungary and Romania. It is a complex plate boundary with most oil and gas production coming from the folded belt within thrust and imbricate units. Many of the old fields were discovered by surface geological study without seismic data. A few fields also lie within the platform sequence on the foreland.
Additional small fields are likely to be found in both areas but the small complex structures require detailed and modern seismic acquisition and interpretation to image the deep traps and reservoirs.
The Black Sea Basin
The Ukrainian part of the Black Sea shelf has two parts; the Gulf of Odessa and the Sea of Azov. However the productive part of Sea of Azov is now part of Russia (disputed) as is most of the offshore folded belt (called the Sudak folded belt) south of Crimea and the Kerch peninsula.
Exploration in the Gulf of Odessa has targeted inverted structural traps formed by compressional tectonic events during the Late Eocene and Middle Miocene. The main productive horizons are in Upper Cretaceous, Paleocene, Eocene, Oligocene and Lower Miocene sequences. Oligocene to Lower Miocene Maykop shales and Aptian-Albian mudstones are thought to be the source rocks in the region. There are many undrilled structures on the shelf as well as potential stratigraphic traps within Paleocene and Eocene units. In addition deeper Mesozoic sediments are barely explored.
The shelf extends into deep waters bounding the Sorokin foredeep basin from the north. Prominent syn-rift structures such as the Tetyaev High provide the potential for structural traps.
Globalshift believes that Ukraine may have significant unconventional resource potential.
The Dnieper-Donets basin contains deep tight gas reservoirs of Carboniferous age which are yet to be exploited. Elsewhere the country has a Silurian to Lower Devonian shale succession similar to Poland. The main reservoirs are thought to be located near the Polish border in the foreland of the Carpathians and between Moldova and the Black Sea coast.
Ukraine has three productive regions: the Dnieper-Donets rift in the northeast, the Carpathian folded belt and its foreland in the west, and the offshore Black Sea shelf and associated deep waters in the south. The rest of the country is underlain by the Pre-Cambrian Ukrainian Shield.