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

The Drava Depression is located in the southwestern, margin of the Pannonian Basin. The northern zone of the depression is called the deep Drava. Geochemical data imply that in the northwestern part of deep Drava, the mudstones, carbonate-fossiliferous mudstones, compact marls and siltites of Lower Miocene to Badenian age at the depths of more than 3000 m are petroleum source rocks. The organic matter is mostly of terrigeneous origin, with local occurrences of hydrogen-rich terrigeneous components. The source rocks have reached a high level of thermal alteration in the condensate-gas window between 3150 and 3800 m. Hydrocarbons in accumulations are interpreted to be migrated thermogenic, condensate-associated gases and their compositions are consistent with the facies type and maturity of the source rocks. Mineralogical composition of reservoir rocks and specific thermodynamic conditions control the formation of CO2 and H2S. The high CO2 concentrations mainly result from the thermal breakdown of carbonates, while H2S is the product of thermochemical sulphate reduction from anhydrite in the presence of hydrocarbons