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 isot opic compounds. Hydrogen and oxygen isotopes indicate that most of the waters have a palaeometeoric origin. They infiltrated during a cold p eriod. Waters in the deepest lithostratigraphical positions are mixtur es of palaeometeoric water and oil field water squeezed from the thick sequence of fine-grained sediments underlying the thermal water aquif er. 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 r ange of carbon stable isotope variation (-14.5<delta(13)C<0) suggests the existence of different carbon sources. In the shallowest samples s oil CO2 and dissolved carbonate minerals could account for the dissolv ed inorganic carbon. In deeper layers additional carbon from the trans formation of the sedimentary organic matter is required due to the inc rease of alkalinity with depth and delta(13)C 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 evolut ion and methane formation can explain the wide range of C-13 contents measured in the total dissolved inorganic carbon. (C) 1997 Elsevier Sc ience B.V.