The physical and geochemical consistency of the cap rock is primarily important for safe geological storage of
CO2. As a consequence of CO2 injection reactions took place between the minerals of the reservoir, the cap rock
and CO2 saturated pore water. These reactions may change the mineral composition and petrophysical properties
of the storage reservoir as well as the cap rock that provides the only physical barrier that retains carbon dioxide
in the target reservoir formation.
Study of the natural CO2 occurrences delivers information to understand which properties of a cap rock provide
the sustainable closure and retainment. Knowledge of the long term effect of CO2 on the behavior of the cap
rock is an important input in the selection procedure of a potential CO2 injection site. Yet, very few data exist
on geochemical properties and reactivity of the cap rocks. During normal commercial operations the reservoir is
typically cored, but not the cap rock.
This study may enhance our knowledge about possible mineralogical reactions, which can occur in clayey-aleuritic
cap rocks. The Mihályi-Répcelak natural CO2 occurrence is believed to be leakage safe. There is no known
seepage on the surface. It is suggested that the aleuritic clay rich cap rock occurring at the natural reservoir can
stop CO2 migration into other reservoirs or to the surface. The most important characteristics of cap rocks that
they have low permeability (<0.1 mD) and porosity (eff.por. = 4%) and high clayeyness (approx. 80%). However,
we demonstrate that in addition to these parameters the geochemical properties of cap rock is also important.
In order to characterize the natural CO2 occurrence, we applied the following analysis, like XRD, FTIR, SEM.
The petrophysical properties are determined from the interpretation of geophysical well-logs and grain size
distribution.
The most important result of this study that adequate petrophysical properties do not completely define the
suitability of a cap rock. The effective porosity (~4 %), permeability (0.026 mD) and clayeyness (~80%) data
imply that the studied aleurolites are good cap rocks. The mineral composition of cap rock is similar to that
of reservoir rock, however, the ratio of components is different. The mineralogical analysis and petrography
yield to the reaction between CO2 and the cap rocks. The most visible effect of CO2 presence is the dawsonite
precipitation after albite dissolution within the cap rocks. Therefore, the CO2 may migrate through the cap rocks
in geological time scale, however the total system could be leakage safe
