--> Abstract: Application of Organic Geochemistry to Petroleum Exploration in Bekes Basin, Hungary, by J. L. Clayton and I. Konca; #91004 (1991)

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Application of Organic Geochemistry to Petroleum Exploration in Bekes Basin, Hungary

CLAYTON, JERRY L., U.S. Geological Survey, Denver, CO, and I. KONCA, Hungarian Hydrocarbon Institute, Szazhalombatta, Hungary

The Bekes basin is a subbasin within the Pannonian basin, containing about 7000 m of post-Cretaceous sedimentary rocks. Oil and gas are produced from reservoirs (depths of about 1000-3300 m) of Precambrian through Tertiary age located on structural highs around the margins of the basin. We conducted organic geochemical analyses of oil, gas, and potential source rocks basinwide. The purpose of this study was to provide an understanding of hydrocarbon generation and migration that can be integrated with other exploration methods to predict the occurrence of undiscovered oil and gas in the basin, particularly in possible stratigraphic traps located basinward of currently producing structures.

The geochemical results indicate that three major genetic oil types are present in the basin. Rocks of Miocene age containing as much as 5 weight % organic carbon are the major source rocks, although Pannonian (s.l.) age rocks containing as much as 2 weight % organic carbon have also expelled both oil and gas. Hydrocarbons were expelled from the source rocks over a wide range of thermal maturities as the source rocks were rapidly buried during Tertiary subsidence (extension) of the basin. Oils migrated updip through adjacent fractured basement rocks (including Proterozoic metamorphic rocks and upper Paleozoic metamorphic and sedimentary units) and porous Tertiary strata to basin-margin structural traps. As a result, oils of relatively low thermal maturity (API gravities < 30 degree ) occur in the same fields or producing structures as more thermally mature oils including condensates and gases, and all three genetic oil types occur together in some of the producing structures.

Using source rock maturation data and carbon isotope ratios of methane, we estimated depth of generation and vertical migration distances for gas. These calculations indicate that as much as 3500 m of vertical migration has occurred in some cases. Isotopically heavy (> -7%) CO is the predominant species present in some shallow reservoirs and has probably been derived via long-distance vertical and lateral migration from thermal decomposition of carbonate minerals in Mesozoic and older rocks in the deepest parts of the basin. Some microbial gas is present in shallow reservoirs. In general, vertical communication is limited between stacked reservoirs in individual fields or producing structures. Therefore, the composition of gas (and oil) frequently varies considerably between indivi ual producing horizons over a short vertical distance within a single field. These findings provide a guide that, when integrated with the geology of specific plays, can help predict the occurrence of undiscovered hydrocarbon accumulations.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)