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Origin of natural gases in the autochthonous Miocene strata of the Polish Carpathian Foredeep

Kotarba M. J.

Annales Societatis Geologorum Poloniae

2011

Vol. 81, No 3

409-424

Methane concentrations in natural gases accumulated in the autochthonous Miocene strata of the Polish Carpathian Foredeep (between Kraków and Przemyśl) usually exceeded 90 vol%. Methane and part of the ethane were generated during microbial reduction of carbon dioxide in the marine environment, mainly during the sedimentation of Miocene clays and muds. It is possible that this microbial process has continued even recently. Higher light hydrocarbons (mainly propane, butanes and pentanes) were generated during the diagenesis and the initial stage of the low-temperature thermogenic process. Very small changes in the values of geochemical hydrocarbon indices and stable isotope ratios of methane, ethane and propane with depth are evidence for similar gas generation conditions within the whole Badenian and Lower Sarmatian successions. Only in a few natural gas accumulations within the Upper Badenian and Lower Sarmatian reservoirs are thermogenic gases or thermogenic components present, both generated from mixed, type III/II kerogen. These thermogenic gases, now accumulated mainly in the bottom part of Miocene strata, probably resulted from thermogenic processes in the Palaeozoic– Mesozoic basement and then migrated to the Miocene strata along the fault zones. The presence of low hydrogen concentrations (from 0.00 to 0.26 vol%) within the Miocene strata is related to recent microbial processes. Carbon dioxide and nitrogen, which are common minor constituents, were generated in both microbial and low-temperature thermogenic processes. However, CO2 has also undergone secondary processes, mainly dissolution in water during migration. Hydrogen sulphide, which occurs in natural gases of Lower Badenian strata, was most probably generated during microbial sulphate reduction of the Lower Badenian gypsum and anhydrites.