In spite of its considerable size and the presence of mature oil source rocks, the Neogene Danube Basin is characterized by the absence of commercial oil accumulations. However, important CO2 fields have been discovered in the basin, which have been possibly explained recently by displacement of oil by later migration of CO2 into the reservoirs. Previous studies performed independently in the southern, Hungarian part of the basin (known as the Little Hungarian Plain) have attempted to model petroleum generation/migration history, to identify sources of the CO2, and to understand the tectonic evolution of the basin and its deep-water flow regime. We were attempting to combine the results of these studies to interpret the fluid-migration history and reveal the influencing processes. This work supports the hypothesis that CO2 could have played a key role in preventing the formation of oil accumulations. During the latest Miocene, early mature oil and saline water, the latter formed by dissolution of up to now unidentified halites, moved together toward the Mihályi High, a regional uplifted structure in the central part of the basin. Modest amounts of oil could have been trapped there while saline water mixed with the low-salinity water of the reservoirs, with the CO2 arriving later and displacing the oil.

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