Origin of natural gases in the Paleozoic-Mesozoic basement of the Polish Carpathian Foredeep

Abstract

Origin of natural gases in the Paleozoic-Mesozoic basement of the Polish Carpathian ForedeepHydrocarbon gases from Upper Devonian and Lower Carboniferous reservoirs in the Paleozoic basement of the Polish Carpathian Foredeep were generated mainly during low-temperature thermogenic processes ("oil window"). They contain only insignificant amounts of microbial methane and ethane. These gaseous hydrocarbons were generated from Lower Carboniferous and/or Middle Jurassic mixed Type III/II kerogen and from Ordovician-Silurian Type II kerogen, respectively. Methane, ethane and carbon dioxide of natural gas from the Middle Devonian reservoir contain a significant microbial component whereas their small thermogenic component is most probably genetically related to Ordovician-Silurian Type II kerogen. The gaseous hydrocarbons from the Upper Jurassic and the Upper Cretaceous reservoirs of the Mesozoic basement were generated both by microbial carbon dioxide reduction and thermogenic processes. The presence of microbial methane generated by carbon dioxide reduction suggests that in some deposits the traps had already been formed and sealed during the migration of microbial methane, presumably in the immature source rock environment. The traps were successively supplied with thermogenic methane and higher hydrocarbons generated at successively higher maturation stages of kerogen. The higher hydrocarbons of the majority of deposits were generated from mixed Type III/II kerogen deposited in the Middle Jurassic, Lower Carboniferous and/or Devonian strata. Type II or mixed Type II/III kerogen could be the source for hydrocarbons in both the Tarnów and Brzezówka deposits. In the Cenomanian sandstone reservoir of the Brzezowiec deposit and one Upper Jurassic carbonate block of the Lubaczów deposit microbial methane prevails. It migrated from the autochthonous Miocene strata.