PETROLEUM GENERATION IN THE UKRAINIAN EXTERNAL CARPATHIANS AND THE ADJACENT FORELAND

Major hydrocarbon accumulations are located in western Ukraine within the Mesozoic-Cenozoic flysch sequence in the frontal parts of the External Carpathians, and in the adjacent autochthonous foreland. The accumulations occur in three different structural settings: in the Carpathian flysch belt; in the pre-Neogene foreland autochthon; and in Miocene foredeep molasse. The petroleum potential of organic-rich rocks belonging to a number of different sedimentary cycles and tectonic units was studied by Rock-Eval pyrolysis.
Two significant organic-rich units, both with a basin-scale distribution, occur within the External Carpathians: (i) a unit comprising the Lower Cretaceous Shypot and Spas Formations; and (ii) the Oligocene-Lower Miocene Menilite Formation. The Menilite Formation includes the principal source rocks in this area, although Lower Cretaceous source rocks may also have generated hydrocarbons. Thermal maturation of the flysch and consequent petroleum generation resulted mainly from tectonic burial during Miocene overthrusting. In the external parts of the Carpathian Flysch Belt, oil generation in the Menilite Formation begins at a depth of around 4.2 km and ceases at around 6 km. In more internal tectonic units, a "frozen oil kitchen" occurs at shallower depths, indicating an increasing amount of tectonic uplift and erosion.
The autochthonous foreland is part of the SW margin of the East European Platform, and is partially overthrust by the Carpathian orogen. Rocks belonging to several different sedimentary cycles are present here. Pre-Mesozoic rocks in the foreland are overmature, at least within the central and SE parts of the study area. Petroleum accumulations are therefore most probably confined to the Mesozoic-Cenozoic sedimentary cover, within which the Middle Jurassic Kokhanivka Formation has the highest generation potential.