PETROLEUM GEOCHEMISTRY OF THE ZALA BASIN, HUNGARY

The Zala basin is a subbasin within the Pannonian basin of Hungary. Oi l and smaller amounts of gas are produced from Upper Triassic through Miocene reservoirs. Our geochemical study of oils and rocks in the bas in indicate that two, and possibly three, genetic oil types are presen t in the basin. Miocene source rocks, previously believed by explorati onists to be the predominant source rock, have expelled minor amounts of hydrocarbons. The main source rock is the Upper Triassic (Rhaetian) Kossen Marl Formation or its stratigraphic equivalent. Oils derived r om the Triassic source rock are recognizable by their isotopic and bio logical marker composition, and high content of metals. In other areas of Europe, Upper Triassic source rocks have been correlated with larg e oil accumulations (e.g., Molassa and Villafortuna fields, Po basin, and other fields in Italy) or are postulated to be good potential sour ce rocks (e.g., Bristol Channel Trough). Knowledge of the geochemical characteristics of oils derived from these Upper Triassic source rocks and understanding of the source rock distribution and maturation hist ory are important for recognizing Triassic oil-source bed relationship s and for further exploration in other basins in Hungary and other par ts of Europe where Triassic source rocks are present.