ORIGIN OF OIL AND GAS OCCURRENCES IN THE PLIOCENE SEDIMENTS OF THE PANNONIAN BASIN, HUNGARY

Considerable volumes of oil and gas in the Pannonian basin have been d iscovered in Pliocene sediments having good reservoir characteristics. The Pliocene sediments, belonging to delta facies, contain quantitati vely very good, but immature source rocks at depths of 0.5-2 km. The o rigin of the oil and gas accumulations in the Pliocene reservoirs is v ery different, due to diverse conditions of hydrocarbon generation and secondary migration. The aim of this paper is to establish the origin of oil and gas trapped in Pliocene reservoirs, in order to have a pic ture of the migration and accumulation systems. Different cases examin ed, by means of the geochemical characteristics of oils and gases, ind icate that the Pliocene reservoirs contain indigenous dry gases genera ted by bacterial and early catagenetic processes, and vertically migra ted hydrocarbon-rich wet gases associated with oil, and carbon dioxide -rich gases derived from high temperature zones of the earth crust or from volcanic bodies.