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.
Abstract:The Jurassic/Cretaceous stratigraphic complex forming a part of the sedimentary cover of both the eastern Małopolska Block and the adjacent Łysogóry-Radom Block in the Polish part as well as the Rava Rus'ka and the Kokhanivka Zones in the Ukrainian part of the basement of the Carpathian Foredeep were studied with geochemical methods in order to evaluate the possibility of hydrocarbon generation. In the Polish part of the study area, the Mesozoic strata were characterized on the basis of the analytical results of 121 core samples derived from 11 wells. The samples originated mostly from the Middle Jurassic and partly from the Lower/Upper Cretaceous strata. In the Ukrainian part of the study area the Mesozoic sequence was characterized by 348 core samples collected from 26 wells. The obtained geochemical results indicate that in both the south-eastern part of Poland and the western part of Ukraine the studied Jurassic/Cretaceous sedimentary complex reveals generally low hydrocarbon source-rock potential. The most favourable geochemical parameters: TOC up to 26 wt. % and genetic potential up to 39 mg/g of rock, were found in the Middle Jurassic strata. However, these high values are contradicted by the low hydrocarbon index (HI), usually below 100 mg HC/g TOC. Organic matter from the Middle Jurassic strata is of mixed type, dominated by gas-prone, Type III kerogen. In the Polish part of the study area, organic matter dispersed in these strata is generally immature (T max below 435 °C) whereas in the Ukrainian part maturity is sufficient for hydrocarbon generation.
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