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2 Annales Societatis Geologorum Poloniae

2 2011

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Microbial gas system and prospectives of hydrocarbon exploration in Miocene strata of the Polish and Ukrainian Carpathian Foredeep

Kotarba M. J., Peryt T. M., Koltun Y. V.

Annales Societatis Geologorum Poloniae

2011

Vol. 81, No 3

523-548

Molecular composition of natural gases accumulated in autochthonous Miocene strata of the Polish and Ukrainian Carpathian Foredeep is dominated by methane, which usually constitutes over 98 vol%. Methane was generated by the carbon dioxide reduction pathway of microbial processes. Ethane was generated both during microbial and thermogenic processes ("oil window") and propane at the initial stage of the low-temperature thermogenic processes, and also by the microbial processes. The rhythmic and cyclic deposition of Miocene clays and sands as well as the vigorous generation of microbial methane caused that the gas produced in claystone beds was accumulated in the overlaying sandstones, and capped, in turn, by the succeeding claystones. Such generation and accumulation system of microbial gases gave rise to the formation of multi-horizontal gas fields. Analysis of the distribution of immature humic dispersed organic matter in the Upper Badenian and Lower Sarmatian sequences indicates that it is practically homogeneous. A migration range of microbial gases was insignificant and locations of their accumulations would depend only on the existence of proper type of traps (compactional anticlines situated above basement uplifts, sealed by the Carpathian Overthrust and/or by faults; stratigraphic pinching out and stratigraphic traps related to unconformities). Another situation is encountered in the south, beneath the Carpathian Overthrust. The thickness of the autochthonous Miocene strata in this area is more than 1,500 metres. Geochemical studies reveal that from a depth of 2,500 metres starts the process of low-temperature thermogenic hydrocarbon generation (“oil window”). At greater depths, more than 7,500 metres, within the autochthonous Lower Miocene basin only the high-temperature methane ("gas window") could be produced and accumulated.

Evaluation of hydrocarbon potential of the autochthonous Miocene strata in the NW part of the Ukrainian Carpathian Foredeep

Kotarba M. J., Więcław D., Kosakowski P., Koltun Y. V., Kowalski A.

Annales Societatis Geologorum Poloniae

2011

Vol. 81, No 3

395-407

The quantity, genetic type, maturity and hydrocarbon potential of dispersed organic matter were determined for the complete sequence of the autochthonous Miocene ranging from the Lower Badenian Sandy-Calcareous Series to the Lower Sarmatian Upper Dashava Formation of the Bilche-Volytsia Unit. Geochemical analyses were conducted on 78 core samples collected from 11 wells in the Ukrainian Carpathian Foredeep between the Ukrainian-Polish state border and the Stryi River. The most favourable source-rock parameters characterize the Upper Badenian Kosiv Formation where the highest TOC contents, from 0.44 to 2.01 wt% (median 0.76 wt%), were found. Only slightly lower values were obtained for the Lower and the Upper Dashava formations – from 0.01 to 1.45 wt% (median 0.72 wt%) and from 0.62 to 0.77 wt% (median 0.71 wt%), respectively. In the Lower Badenian Sandy-Calcareous Series, the Lower Badenian Baraniv beds, and the Upper Badenian Tyras Formation, the TOC content is lower and varies from 0.00 to 0.77 wt%. Immature type III (terrestrial) kerogen dominates the analysed sections of the Kosiv and Dashava formations. Marine organic matter was detected sporadically, and only in the Upper Badenian Kosiv Formation in the vicinity of Kokhanivka, and in the Upper Badenian Kosiv and Tyras formations.