Crude oil samples from uncommonly hot (>170°C) reservoirs of the SE-Pannonian Basin were studied for stable carbon and hydrogen isotope compositions of their different fractions. From two fields, 15 samples of different depths and temperatures were chosen for this study. The aim was to study the impact of extreme reservoir conditions on the isotope ratios of the different fractions and to find the ratios that show correlation with increasing depth and temperature.
We have shown that the behaviour of isotopes in these very hot oils differs from those from lower temperatures. The combined application of carbon and hydrogen isotope techniques is useful and may provide approximate information on reservoir conditions.
Authors:István Vető, János Csizmeg and Csanád Sajgó
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.
Authors:József Fekete, Csanád Sajgó, István Horváth, Zoltán Kárpáti, István Vetõ and Magdolna Hetényi
The geochemical facies of Hungarian thermal waters were the object of this study. Samples were separated into groups by relative ages (δ18O values). Mature and immature subgroups were formed on the basis of dissolved (semi)volatile organic compounds. The oldest (connate) waters form one group with a small number of samples. The subgroups containing different small molecular-sized soluble aromatics differ sharply in their chemical features (sodium, hydrogen carbonate, iodine, ammonium etc. content). The origin of the organic matter may differ in the subgroups as inferred by their different δ18O values and ten times greater halogen contents.
Our results show that the decomposition of organic matter produces small molecular-sized aromatic compounds and also influences the amounts of inorganic components in thermal waters, through the increase of feldspar hydrolysis and carbonate dissolution.