109 Since 1961 Paleocene Deposits of the Ukrainian Carpathians

since 1961 BALTICA Volume 33 Number 2 December 2020: 109–127 https://doi.org/10.5200/baltica.2020.2.1 Paleocene deposits of the Ukrainian Carpathians: geological and petrographic characteristics, reservoir properties Halyna Havryshkiv, Natalia Radkovets Havryshkiv, H., Radkovets, N. 2020. Paleocene deposits of the Ukrainian Carpathians: geological and petrographic characteristics, reservoir properties. Baltica, 33 (2), 109–127. Vilnius. ISSN 0067-3064. Manuscript submitted 27 February 2020 / Accepted 10 August 2020 / Published online 03 Novemver 2020 © Baltica 2020 Abstract. The Paleocene Yamna Formation represents one of the main oil-bearing sequences in the Ukrai- nian part of the Carpathian petroleum province. Major oil accumulations occur in the Boryslav-Pokuttya and Skyba Units of the Ukrainian Carpathians. In the great part of the study area, the Yamna Formation is made up of thick turbiditic sandstone layers functioning as reservoir rocks for oil and gas. The reconstructions of depositional environments of the Paleocene flysch deposits performed based on well log data, lithological and petrographic investigations showed that the terrigenous material was supplied into the sedimentary basin from two sources. One of them was located in the northwest of the study area and was characterized by the predominance of coarse-grained sandy sediments. Debris coming from the source located in its central part showed the predominance of clay muds and fine-grained psammitic material. The peculiarities of the terrigenous material distribution in the Paleocene sequence allowed singling out four areas with the maximum development (> 50% of the total section) of sandstones, siltstones and mudstones. The performed petrographic investigations and the estimation of reservoir properties of the Yamna Formation rocks in these four areas allowed establishing priority directions of further exploration works for hydrocarbons in the study territory. Keywords: Carpathians; Skyba and Boryslav-Pokuttya Units; Yamna Formation; reservoir rocks; oil and gas prospective areas Halyna Havryshkiv ([email protected]), Natalia Radkovets ([email protected]), Institute of Geology and Geochemistry of Combustible Minerals of the NAS of Ukraine, Naukova Str. 3a, 79060 Lviv, Ukraine INTRODUCTION the Ukrainian Outer Carpathians (Fedyshyn 1998; Mayevskyi et al. 2014). The Outer Carpathians and their foredeep repre- The Paleocene sequence of the Ukrainian Outer sent one of the oldest and largest petroleum provinces Carpathians and the Carpathian Foredeep are repre- in the World (Fedyshyn 1998; Ulmishek, Klemme sented by turbidite deposits, which in most of the study 1990). The geological history of the Carpathian oil- area, are made up of thick sandstone layers. Most of the and gas-bearing region goes back about 300 years. available studies into the Paleocene sediments of the Manual production of oil in the Carpathians began in Ukrainian Carpathians focus on problems of geologi- the second half of the 18th century in Boryslav. In cal structure, stratigraphy, tectonics, and description of the second half of the 19th century, drilling in the re- the existing petroleum fields, while mineralogical, pet- gion was performed using manual methods, while at rographic and lithological aspects are described in less the end of that century, mechanical methods of drill- detail (e.g. Dolenko 1962; Dolenko et al. 1980; Pylyp- ing were applied (Zuber 1885; Zeh 1889; Krupskyi chuk, Vul 1981; Gabinet 1985; Vialov et al. 1988; Ha- 2001; Vishnyakov et al. 2014). To date, there have vryshkiv 2008; Havryshkiv et al. 2007; Krupskyi et al. been several thousand exploration and production 2014; Vul 2014; Waśkowska et al. 2019). wells drilled in the Ukrainian Carpathians. A total of Paleocene sandstones are among the principal reser- 41 hydrocarbons fields have been discovered within voir rocks in the flysch succession of the Outer Carpathi- 109 ans. The first oil fields, i.e. Strilbychi, Skhidnytsya and tween the Polish and Romanian Carpathians (Fig. 1). Bytkiv-Babche, were discovered in the Paleocene reser- The thrust-and-fold belt of the Outer Carpathians is voirs in the second half of the 19th century. In the second bounded by the Carpathian Foredeep in the northeast half of the 20th century, two more fields of hydrocar- and the Pieniny Clippen Belt in the southwest. It is bons, i.e. Blazhiv and Tanyava, were found (Fedyshyn made up of terrigenous flysch and is thrust over the 1998). The main aspects of petroleum generation and southwestern margin of the East European Platform. accumulation in the area of the Ukrainian Carpathians The evolution of the Outer Carpathians basin star- were discussed in earlier publications (Koltun 1992; ted with the Late Jurassic rifting and extension (Kosa- Koltun et al. 1998; Kotarba, Koltun 2006; Ślączka et al. kowski et al. 2018; Golonka et al. 2006) with a con- 2006; Więcław et al. 2012; Kosakowski 2013; Radko- sequent post-rift thermal subsidence and deepening vets et al. 2016; Karabyn et al. 2019). of the sedimentary environment, which resulted in Presently, all the above mentioned 5 fields in the the deposition of a thick flysch succession. The gene- Yamna Formation reservoirs are operating. However, rally well-oxygenated depositional conditions, which there are significant preconditions for forecasting and supposedly prevailed during the Turonian to Eocene, discovering new petroleum accumulations in this tur- conditioned high rates of sedimentation (Kosakowski bidite, essentially sandy succession. The thrust-and-fold et al. 2018; Bieda et al. 1963; Ślączka et al. 1999). structure of the Outer Carpathians, which is intensely Periods of intense turbidite deposition, caused by ma- developed in the Skyba and Boryslav-Pokuttya nappes jor sea-level drops, resulted in the formation of thick of the Ukrainian Carpathians, suggests the existence sandstone sequences in the Paleogene succession, the of multiple anticline structures. Hence, the information Paleocene Yamna Formation (the object of our study) on the distribution of rocks with good reservoir proper- being one of them. The compressional deformation ties throughout the study territory allows determining of the Outer Carpathians, which took place during the directions for further petroleum exploration works. Oligocene to the Early Miocene, led to the formation Since the Paleocene sequence in the study territory of the Carpathian thrust-and-fold belt and the Carpat- has been proved to bear petroleum, the establishment hian Foredeep (Kosakowski et al. 2018; Ślączka et of additional criteria for new hydrocarbons accumula- al. 1999). tions prospecting is the task of priority importance. Ob- The Carpathian Foredeep is subdivided into jectives of this study are as follows: to reconstruct the three separate units, i.e. Bilche-Volytsya, Sambir propagation of Paleocene sediments in the Carpathian and Boryslav-Pokuttya. A number of tectonic units basin using palinspastic restoration techniques; to es- have been identified within the Ukrainian Outer Car- tablish variations in the Yamna Formation thickness pathians (Fig. 1): Skyba, Krosno, Dukla, Porkulets, within the Skyba and the Boryslav-Pokuttya nappes of Chornogora, Rakhiv and Magura (Shakin et al. 1976; the Ukrainian Carpathians; to estimate proportions of Kruglov et al. 1985; Hnylko 2011). The study area different rock types in the total section; to single out covers the Skyba and the Boryslav-Pokuttya Units, areas with the maximum occurrence of sandstones, where the Yamna Formation represents one of the siltstones, mudstones; to perform detailed studies of main reservoir rock sequences for hydrocarbons ac- petrographic features and reservoir properties of the cumulations in the Carpathian petroleum province. Yamna Formation. To accomplish these tasks, we have The Boryslav-Pokuttya Unit represents a com- investigated a considerable number of well-logs dis- plex of superimposed nappes, each of which usually tributed throughout the study territory and carried out comprises a flysch succession covered by molasse. lithological, mineralogical, petrographic, palaeoceano- Northeastward, it is thrust over the Sambir unit and is graphic investigations, the combined results of which partially covered by the Skyba nappe in the southwest allowed delineating the potential areas for further oil (Figs 1 and 2). and gas exploration within the study territory as well The Skyba Unit extends along the frontal part of as those where petroleum occurrence is less probable. the Carpathians (Glushko1968; Vialov et al. 1988; The identification of prospective oil and gas accumula- Krupsky 2001). It is the largest tectonic unit of the tion areas will help planning further exploration works Outer Carpathians and is made up of a flysch succes- more efficiently and determine first priority explora- sion. The Skyba Unit consists of a number of thrusts, tion targets within the Skyba and the Boryslav-Pokut- stretching for hundreds of kilometers from northwest tya nappes of the Ukrainian Carpathians. to southeast and overlying one another with varying amplitudes. In the Ukrainian Carpathians, the Skyba GEOLOGICAL OVERVIEW OF THE STUDY Unit is thrust over the south-western part of the Bo- AREA ryslav-Pikuttya Unit of the Carpathian Foredeep and covers a large part of it. The amplitude here is at least The Ukrainian Carpathians, constituting the north- 15–25 km (Figs 1 and 2). According to the deep drill- eastern segment of the Carpathian Arc, are situated be- 110 Fig. 1 Sketch

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