The successful exploration and appraisal of a field requires deep insight into the petroleum system of the given basin. Fluid inclusion petrography and microthermometry combined with the geochemical analysis of source rock extracts, crude oil, and the petroleum trapped within fluid inclusions by Molecular Composition of Inclusions (MCI) procedures offer additional information about the petroleum system. The case study from the Pannonian Basin represents the use of fluid inclusions as the witnesses of geological and geochemical processes in a fractured reservoir and the adjacent sedimentary basin. In the study area, fractures in volcanic carrier beds and reservoir rocks were cemented by different minerals, in particular by zeolites. Based on fracture-filling mineral sequences as well as their textural relationships, four distinct vein types can be distinguished, but only the youngest one preserves the migrated hydrocarbon-bearing fluid trapped in zeolites (analcime, heulandite) and allows oil to accumulate in a reservoir scale. Petrographically, two different petroleum inclusion assemblages could be distinguished. The very infrequent, single primary HC1 inclusions with weak yellow fluorescence colour were trapped earlier than the abundant primary HC2 inclusions with intense blue fluorescence colour. The UV-fluorescence micro-spectroscopy strengthened the distinction of the two different types and established the similarity of crude oil produced in the area and late (HC2) fluid inclusion oil. Based on the homogenization temperatures of the primary aqueous inclusions co-genetic with HC2 petroleum-bearing inclusions, the migration of the HC2 fluid in the fracture system took place in the temperature range of 130-160 °C. Molecular composition of the fluid inclusion oil was correlated with rock extracts of representative source rock samples (MS1, MS2) and crude oil produced in the area. The biomarker signatures of the fluid inclusion oil and the crude oil are very similar to each other. The crude oil looks unbiodegraded, akin to MS2 rock extract. Both oils appear to represent a marine (carbonate) influenced source facies but deposited in a slightly more anoxic environment than the source rock samples derived, probably from a deeper part of the basin. The results confirm that fluid inclusion measurements could be used as effective exploration tools for comprehension of the petroleum system in the area.
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