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

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.