Late and deep fermentative methanogenesis as reflected in the H-C-O-S isotopy of the methane-water system in deep aquifers of the Pannonian Basin (SE Hungary)

H and O isotope ratios and H and C isotope ratios of methane as well as the abundances of some major and minor dissolved components were measured in 26 subsurface waters from SE Hungary, produced from late Neogene aquifers within the upper two kilometres in depth. Mixing with meteoric water of glacial age is of importance. The hydrocarbon gas (HC gas) content of the great majority of the waters is dominated by isotopically light bacterial methane. The depth of the onset of thermal gas generation varies from 0.7 to 1.2 km but the amount of thermogenic methane leaving the kerogen was not sufficient to overprint the light isotopic signature of the methane in the waters. About two thirds of the waters (Group I samples) are characterised by low sulfate content (<5 mg/l), high chloride/sulfate ratio (>20) and methane δD values less negative than −260‰. They also show a direct relationship between the H isotope ratio of the methane and that of the water. These waters experienced exhaustive bacterial sulfate reduction followed by a relatively shallow bacterial methanogenesis. In a third of the waters (Group II samples) the methane δD values are more negative than −278‰. The Group II samples, which are not affected by an admixture of glacial age meteoric waters contain a relatively high amount of sulfate (35–45 mg/l) and do not show a direct relationship between the H isotope ratio of the methane and that of the water. In Group II waters bacterial sulfate reduction was not exhaustive and the residual sulfate inhibited shallow bacterial methanogenesis. As a result of continuing burial, the aquifers were heated up to 40–60 °C, thereby allowing a relatively intensive bacterial acetogenesis, followed by acetate fermentation to methane. Our results show that deep acetate fermentation and early thermal gas generation can occur closely in space and time, even partly overlapping with each other.