--> Abstract: Petroleum Geochemistry of the Zala Basin, Hungary, by J. L. Clayton and I. Koncz; #90987 (1993).

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CLAYTON, JERRY L., U.S. Geological Survey, Denver, CO; and I. KONCZ, Hungarian Hydrocarbon Institute, Nagykanisza, Hungary

ABSTRACT: Petroleum Geochemistry of the Zala Basin, Hungary

The Zala Basin is a small, deep sub-basin located within the Pannonian Basin system. Production in the Zala Basin began in 1943 with the discovery of the Nagylengyel field which has the largest initial reserves of any field found to date in Hungary. The Zala Basin contains as much as about 7,000 m of sedimentary rocks of Mesozoic and Tertiary age. The main producing intervals are Triassic and Upper Cretaceous carbonate rocks, with secondary production occurring in rocks of Miocene age sandstones. Organic geochemical analyses indicate that two main genetic oil types are present in the Zala Basin--"Nagylengyel" and "Szen" types. Oils of the Nagylengyel type have low gravity (API 13-30 degrees), high concentrations of V, Ni, and S, isotopically light hydrocarbons, low pristane/phytane (P /Ph) ratio (<1.0), and no oleanane. Oils of the Szen type have higher gravity (31-39 degrees ) and Pr/Ph ratios (>1.0), low metal and sulfur content, high amounts of oleanane, and isotopically heavy hydrocarbons. Geochemical features of the Szen group are variable and may indicate more than one sub-type of oil within this larger group.

Explorationists previously hypothesized that Miocene shales were the major source rocks for Zala Basin oils. Geochemical composition of the Nagylengyel-type oils, however, strongly suggest a carbonate source rock, which led us to hypothesize a Mesozoic carbonate source rock. Geochemical analyses confirmed that organic-rich (as much as 10% organic carbon) marls of the Triassic Kossen Marl are the source of the Nagylengyel oil-type. Secondary amounts of oil have been expelled by Miocene clastic source rocks, which are the likely source of the Szen oil-type. The Triassic source rocks expelled oil at fairly low thermal maturities during Miocene time. These Triassic source rocks are equivalent to the Triassic source rocks of northern Italy that formed the giant Molassa field in the Po Basi . An understanding of the distribution and thermal maturity of the Triassic source rocks is important for further exploration in the Pannonian Basin system and other parts of eastern Europe.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.