KOSTIC, ALEKSANDAR, University of Belgrade, Faculty of Mining and Geology, Dept. of Petroleum and Coal Geology, Belgrade, Yugoslavia

ABSTRACT: The Generative Petroleum Potential of the Tertiary Sediments in the Banat Depression (Pannonian Basin)

The Banat depression (Serbia) contains various local Tertiary depressions up to 2500-5000 m deep, with markedly varied burial histories that had a great impact on the source rock maturity and petroleum generation in this area. An average temperature gradient in deeper parts of the Tertiary sequence is generally about 4, 6 degrees C/100 m. Oil window usually begins at present-day temperatures of about 120 degrees C that generally corresponds to the depth interval of 2000-2500m.

The principal source rocks for oil are Lower Pannonian and Sarmatian clayey limestone and marl with kerogen types I and II, at maturity levels that mostly correspond to vitrinite reflectance of 0.50-1,00 % Rr. Locally, there are also some thin richer layers of Badenian and Ottnangian-Carpathian shale that contain a more mature kerogen type II or I. The Upper Pannonian sediments (Lower Pontian in Yugoslav terminology) usually contain only gas-prone kerogen type III, but a great volume of these sediments is in the katagenetic stage in some depressions (gas generation in "oil window"). The younger (Pontian and Pliocene) sediments are not source rocks of any thermogenetic hydrocarbons, since they are immature.

The principal source rocks are generally of uniform thickness and initial quality, so that the key factor for quantity of generated oil is their maturity. The oil reserves are greatest in the areas where they have been exposed to higher temperatures. Since there are some local depressions where the Pannonian and Sarmatian source rocks are in early maturation stage, the critical investigations in such depressions was related to transformation ratio and maturation history.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid