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 Exte rnal Carpathians, and in the adjacent autochthonous foreland. The accu mulations occur in three different structural settings: in the Carpath ian flysch belt; in the pre-Neogene foreland autochthon; and in Miocen e foredeep molasse. The petroleum potential of organic-rich rocks belo nging to a number of different sedimentary cycles and tectonic units w as studied by Rock-Eval pyrolysis. Two significant organic-rich units, both with a basin-scale distribution, occur within the External Carpa thians: (i) a unit comprising the Lower Cretaceous Shypot and Spas For mations; and (ii) the Oligocene-Lower Miocene Menilite Formation. The Menilite Formation includes the principal source rocks in this area, a lthough Lower Cretaceous source rocks may also have generated hydrocar bons. Thermal maturation of the flysch and consequent petroleum genera tion 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 cease s at around 6 km. In more internal tectonic units, a ''frozen oil kitc hen'' occurs at shallower depths, indicating an increasing amount of t ectonic uplift and erosion. The autochthonous foreland is part of the SW margin of the East European Platform, and is partially overthrust b y the Carpathian orogen. Rocks belonging to several different sediment ary cycles are present here. Pre-Mesozoic rocks in the foreland are ov ermature, at least within the central and SE parts of the study area. Petroleum accumulations are therefore most probably confined to the Me sozoic-Cenozoic sedimentary cover, within which the Middle Jurassic Ko khanivka Formation has the highest generation potential.