Petroleum Generation in the Ukrainian External Carpathians and the Adjacent Foreland

Abstract

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.

Origin and migration of oil from the Ukrainian Outer Carpathians to their Mesozoic basement: the case of the Lopushna traps

“…No ev i dent source rocks have been rec og nized within this suc ces sion in the area. Mid dle Ju ras sic or ganic-rich rocks, oc cur ring in the north western part of the base ment of the Ukrai nian Carpathian Foredeep (Koltun et al, 1998), where the Kokhanivka and Orkhovychi oil fields are lo cated, do not ex ist in its south east ern part, where the Lopushna ac cu mu la tion has been found. Neocomian-Aptian dark grey argillites, which are pen e trated by bore holes in the subthrust se quence in the area of the Lopushna field oc currence, and may spread fur ther un der the Carpathian Overthrust, are of small thick ness (up to 3.5 m) and TOC content (up to 1.6%) and con tain Type III kerogen (Radkovets, 2010(Radkovets, , 2015.…”

“…The char ac ter is tics of these rocks may be assumed to im prove fur ther be neath the Carpathian Overthrust, but no data on this ex ist. Si lu rian rocks within the study area are overmature (Koltun et al, 1998) and sep a rated from the Me sozoic lithostratigraphic as sem blage by a sig nif i cant strati graphic un con formity. Four main struc tural com plexes can be dis tin guished in the plat form base ment within the ter ri tory of the Lopushna field occur rence: Pro tero zoic, lat est Pre cam brian to Early De vo nian, Me so zoic-Paleogene and Neo gene.…”

“…Ear lier in ves ti gations (Lafargue et al, 1994;Roure and Sassi, 1995;Picha, 1996) sug gested that oil mi grated to the Lopushna res er voirs from the Oligocene Menilite Beds of the Outer Carpathians before the main overthrusting event. How ever, the data ob tained by oth ers (Koltun et al, 1998) show that the Menilite Beds reached the mat u ra tion level, cor re spond ing to the "oil win dow", only af ter the Mio cene overthrusting ep i sode and ma ture source rocks oc cur at depths ex ceed ing 4.5 km. Krzywiec et al (2009) con sid ered that Albian shales of the Me so zoic basement might be the source of the Lopushna oil ac cu mu la tions.…”

“…The main as pects of pe tro leum gen er a tion and ac cu mu lation in the area of the Ukrai nian Carpathians have been discussed in ear lier pub li ca tions (Koltun, 1992;Koltun et al, 1998;Kotarba et al, 2005Kotarba et al, , 2007Kotarba and Koltun, 2006;Wiêc³aw et al, 2012). The prin ci pal source rocks have been rec og nized and in ves ti gated, whereas oils and nat u ral gases have been stud ied with re spect to their ge netic char ac ter is tics.…”

“…The prin ci pal source rocks have been rec og nized and in ves ti gated, whereas oils and nat u ral gases have been stud ied with re spect to their ge netic char ac ter is tics. On this basis three main pe tro leum sys tems have been iden ti fied, namely the pe tro leum sys tem of the Cre ta ceous-Paleogene flysch sequence of the Outer Carpathians (Koltun et al, 1998), which includes the main oil fields of the Carpathian Prov ince, the pe troleum sys tem of the Neo gene molasse of the Carpathian Foredeep (Kotarba et al, 2011a), which in cludes the main gas fields, and the pe tro leum sys tem of the base ment of the Outer Carpathians and Carpathian Foredeep (Kotarba et al, 2011b) com pris ing sev eral oil fields in the Me so zoic se quence of the East Eu ro pean Plat form, and oc cur ring on both sides of the Pol ish-Ukrai nian bor der.…”