A strong relationship between carbonate precipitation and microbial gas generation is evident for the Upper Eocene reservoir
rocks of the North Alpine Foreland Basin. To achieve a better understanding of this relationship, 40 samples of limnic to shallow
marine, gas-, oil- and water-bearing sandstones were studied to determine mineralogy and diagenetic history. The specific mineral parageneses were used to reconstruct changes in the hydrogeochemical conditions over time. Thus, authigenic mineral phases
within reservoir rocks are an important archive for the reconstruction of pore fluid composition changes.
The eogenetic pore space evolution of investigated Eocene sandstones is influenced by their primary mineralogy, which is strongly
controlled by (i) depositional environment, (ii) detrital input and (iii) transport distances. Thus, a low compositional maturity is associated with high feldspar and high clay mineral content. Authigenic clay minerals, formed during several stages of diagenesis,
play an important role for reservoir quality, due to pore space reduction.
During eogenesis, authigenic micritic and sparitic carbonate phases are precipitated, which decreases the pore space. These eogenetic carbonate cements exhibit isotope values of about δ C: -5.9 to +2.2‰ and O: -8.3 to -4.3‰ [VPDB]. Some of these sam18 ples indicate a trend towards lighter δ O values (-17.2‰), which is attributed to meteoric flush.
Within the Eocene sandstones, two types of strongly cemented zones with low permeabilities can be differentiated: (i) extraordi13 nary light δ C (-28.4‰) carbonates, which formed due to degradation of organic matter at the stage of advanced sulfate reduc13 13 tion and (ii) heavy δ C (δ C: +8.7‰), which precipitated at the fermentation zone.
Within the reservoir sandstones telogenesis is characterized by mineral destabilization (e.g. carbonate and feldspar corrosion)
and kaolinite precipitation. The formation of authigenic kaolinite booklets resulted into a decrease in porosity.