Petroleum geochemistry of the deep part of the Drava depression, Croatia

The Drava Depression is located in the southwestern, margin of the Pannonia n Basin. The northern zone of the depression is called the deep Drava. Geoc hemical data imply that in the northwestern part of deep Drava, the mudston es, carbonate-fossiliferous mudstones, compact marls and siltites of Lower Miocene to Badenian age at the depths of more than 3000 m are petroleum sou rce rocks. The organic matter is mostly of terrigeneous origin, with local occurrences of hydrogen-rich terrigeneous components. The source rocks have reached a high lever of thermal alteration in the condensate-gas window be tween 3150 and 3800 m. Hydrocarbons in accumulations are interpreted to be migrated thermogenic, condensate-associated gases and their compositions ar e consistent with the facies type and maturity of the source rocks. Mineral ogical composition of reservoir rocks and specific thermodynamic conditions control the formation of CO2 and HPS. The high CO2 concentrations mainly r esult from the thermal breakdown of carbonates, while H2S is the product of thermochemical sulphate reduction from anhydrite in the presence of hydroc arbons. (C) 1998 Elsevier Science Ltd. All rights reserved.