Geochemistry of formation waters in the Pannonian Basin (southeast Hungary)

Twenty five deep-water samples collected in the southern part (Szeged area) of the Pannonian Basin were analysed for their chemical and isotopic compounds. Hydrogen and oxygen isotopes indicate that most of the waters have a palaeometeoric origin. They infiltrated during a cold period. Waters in the deepest lithostratigraphical positions are mixtures of palaeometeoric water and oil field water squeezed from the thick sequence of fine-grained sediments underlying the thermal water aquifer. Most of the waters are NaHCO3 type with 1200–4400 mg/l of TDS. Na and HCO3 increase with depth and/or temperature. Based on chemical and isotopic compositions, waters are divided into three groups and these reflect differences in the lithostratigraphy of the basin. The wide range of carbon stable isotope variation (−14.5<δ13C<0) suggests the existence of different carbon sources. In the shallowest samples soil CO2 and dissolved carbonate minerals could account for the dissolved inorganic carbon. In deeper layers additional carbon from the transformation of the sedimentary organic matter is required due to the increase of alkalinity with depth and δ13C values. The evolution of the sediment organic matter starts with kerogen formation accompanied by a CO2 production and is followed by the transformation of kerogen yielding CO2 from its oxygen-containing functional groups. Such evolution and methane formation can explain the wide range of 13C contents measured in the total dissolved inorganic carbon.