Geochemical characterization of natural gas manifestations in Greece

The Greek region is characterized by intense geodynamic activity with widespread volcanic, geothermal and seismic activity. Its complex geology is reflected in the large variety of chemical and isotopic composition of its gas manifestations. Basing on their chemical composition the gases can be subdivided in three groups, respectively CO2, CH4 or N2-dominated. On oxygen-free basis these three gases make up more than 97% of the total composition. The only exceptions are fumarolic gases of Nisyros that contain substantial amounts of H2S (up to more than 20%) and one sample of Milos that contains 15% of H2. CO2-dominated gases with clear mantle contribution in their He isotopic composition (R/Ra corrected for air contamination ranging from 0.5 to 5.7) are found along the subduction-related south Aegean active volcanic arc and on the Greek mainland close to recent (upper Miocene to Pleistocene) volcanic centers. These areas are generally characterized by active or recent extensive tectonic activity and high geothermal gradients. On the contrary, gases sampled in the more external nappes of the Hellenide orogen have generally a CH4- or N2-rich compositions and helium isotope composition with a dominant crustal contribution (R/Ra corr < 0.2). The chemical and isotopic characteristics of the emitted gas display therefore a clear relationship with the different geodynamic sectors of the region. Gas geochemistry of the area contributes to a better definition of the crust-mantle setting of the Hellenic region.