Composition and orlgln of gaseous hydrocarbons in the Miocene strata of the Polish part of the Carpathian Foredeep

Abstract

The results of molecular and stable carbon isotope analyses of methane, ethane and propane, and stable hydrogen isotope analyses of methane to explain the origin of gaseous hydrocarbons accumulated in the whole Badenian and Early Sarmatian sequence of the Carpathian Foredeep, at the depths from 161 to 2,670 meters. Composition of tested gases is dominated by methane which concentration usually exceeds 98 vol. %. Methane was generated during carbon dioxide reduction pathway of microbial processes. Higher gaseous hydrocarbons (mainly ethane and propane) which are usually minor constituents (concentrations less than 0.2 vol. %), were generated during diagenetic processes and at the initial stage of the low-temperature thermogenic processes. Only the gas encountered in the Tarnów-45 well, in the bottom part of Upper Badenian sequence atypical, high-temperature, thermogenic gas generated from the oil-prone, marine organie matter (type II kerogen) which remains at transformation level corresponding to 1.1-1.6% in vitrinite reflectance scale. This gas has migrated from the Mesozoic basement where such thermogenic gases were encountered. Natural gas accumulated in the Upper Jurassie carbonate trap of the Lubaczów field is typically microbial and has migrated from the autochthonous Miocene strata along the fault zone.