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Abstract: Origin of Natural Gases of the Polish Carpathians: Isotopic and Geological Approach

Maciej Kotarba, Witold Weil, Tadeusz Wilczek

The Carpathian petroleum province includes the two main structural units: the Carpathian Overthrust and the Carpathian Foredeep. The Carpathian Foredeep consists of two structural complexes: Paleozoic-Mesozoic basement and a late Alpine Miocene cover.

Stable isotope ratios and C2+ wet gas index {C2+=(C2+C3+C4+C5)/(C1+C2+C3+C4+C5)100[%]} for gases from the Flysch Carpathian reservoirs vary in the following ranges: ^dgr13C (CH4) from -65.8 to -34.2 ^pmil, ^dgrD (CH4) from -207 to -151 ^pmil, ^dgr13C (C2H6) from -47.2 to -24.0 ^pmil, ^dgr13C (C3H8) from -31.6 to -22.6 ^pmil and C2+ from 0.72 to 33.9. Natural gases, both dissolved with the oils and free accumulated independently in the Carpathian Overthrust flysch reservoirs, were generated mainly during the low-temperature thermogenic processes (within "oil indow") and high-temperature ones as well as sporadically during bacterial processes.

Stable isotope ratios and the C2+ wet index for gases from Paleozoic-Mesozoic deposits vary in the following ranges: ^dgr13C (CH4) from -57.0 to -31.7 ^pmil, ^dgrD (CH4) from -182 to -134 ^pmil, ^dgr13C (C2H6) from -34.1 to -22.5 ^pmil, ^dgr13C (C3H8 from -33.9 to -23.2 ^pmil and C2+ from 2.2 to 13.4. Such extended ranges of values suggest complex origins of bacterial and thermogenic gases involving migration and mixing. The thermogenic generation of hydrocarbons was a multiphase process and traps in all deposits were supplied at various times during the successive hydrocarbon generation phases.

The gases accumulated in Miocene sediments show smaller variations than in Paleozoic--Mesozoic basement: ^dgr13C (CH4) from -69.0 to -61.1 ^pmil, ^dgrD (CH4) from -214 to -178 ^pmil, ^dgr13C(C2H6) from -49.9 to -38.6 ^pmil, ^dgr13C (C3H8) from -31.2 to -29.4 ^pmil and C2+ from 0.17 to 0.56. Methane dominating in the Miocene reservoirs originated during the bacterial reduction of carbon dioxide. Accumulation of bacterial gases within the Miocene strata was facilitated by high sedimentation rate, rhythmic and cyclic deposition of alternating clays and sands and presence of methane hydrate formed in the thermodynamic regime of the Miocene basin.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France