Mantle-related CO2, metasedimentary HC-N2gas and oil traces in the Répcelak and Mihályi accumulations, W-Hungary — mixing of three fluids of very different origin

Abstract

Numerous accumulations of CO2 and nitrogen-rich natural gas are known in the hot Pannonian Basin System (PBS), where even the mixture of these two fluids is a common phenomenon. The Danube Basin, part of the PBS, is characterized by the predominance of CO2 and nitrogen-rich natural gas over “normal” natural gas. The multistacked Répcelak and Mihályi gas accumulations (southern, Hungarian part of the Danube Basin) display an upward increase of nitrogen-rich natural gas at the expense of CO2. This study, using the abundant public data, the published results and the new biomarker data obtained from oil traces, attempts to explain the formation of these multistacked accumulations. A synoptic view of the vertical changes in gas composition, the maturation history of the basin and its basement, the chronology of the Neogene basaltic volcanism and the biomarker pattern of the oil traces resulted in the recognition of the metasedimentary origin of the nitrogen-rich natural gas and in a relative chronology of the mixing of the two gases and the oil.