Fluid-inclusion evidence of petroleum migration through a buried metamorphic dome in the Pannonian Basin, Hungary

Some of the petroleum currently produced in the Pannonian Basin in Hungary stems from buried metamorphic basement domes that form structural highs between Neogene sedimentary sub-basins. In the Békés sedimentary sub-basin, the source rocks of the petroleum are known to be of Middle to Late Miocene age, whereas at least part of the petroleum in the adjacent Szeghalom basement dome appears to be of Lower Miocene age or even older. Virtually nothing is known about the compositions, distribution, sources and migration history of this older petroleum, although such variables are crucial to future exploration strategies. This study addresses the first two of these gaps in knowledge by providing a basic characterization of the older petroleum trapped as fluid inclusions in fracture-hosted quartz crystals within the Szeghalom Dome (SzD). Fluid-inclusion samples from 9 wells at depths of 1900 to 2200 m were analysed by visible- and CIE-calibrated UV microscopy, microthermometry, Laser Raman- and 1H NMR spectroscopy. Two major types of Early Miocene oils were found, distributed over 5 distinct temporal generations: an earlier, immature brown oil succeeded by a light gas condensate. The early petroleum was undersaturated with respect to methane gas and it migrated through the fracture network at temperatures between about 160 to 130 °C, entraining blebs of bitumen and flushed by weakly-saline water. The later gas-condensate migration consisted of a heterogeneous mixture of volatile oil + methane-rich gas + weakly-saline water, at slightly lower temperatures between 135 and 120 °C. The temperature evolution was caused by cooling of the metamorphic wall rocks during basement uplift, culminating in Middle-Miocene exhumation. The various hydrocarbons are unevenly distributed across the Szeghalom Dome, showing that pre-Middle-Miocene fluid migration through the fracture network was compartmentalized at the km-scale.