Austria committed to reduce greenhouse gas emissions by 13 % based on the level of 1990. One possibility to avoid emissions is the geological storage of these gases. CO2-injection into oil reservoirs is a common EOR method and might result in substantial incremental recovery. Injecting CO2 into gas reservoirs also leads to limited incremental gas recovery and additional income accordingly. Further, the basins from which hydrocarbons are produced are not too far from industry sites. Hence, geological storage of CO2 could be economically attractive and might contribute to achieving the Kyoto target of Austria. To evaluate the potential of CO2 geological storage in Austria, the eleven largest oil and thirteen largest gas fields were investigated. The storage capacity of these fields was determined as 465 million tons. The fields were ranked according to their geological suitability, and the results indicated that the best candidate reservoirs are naturally fractured. When injecting gas into such a reservoir, various mechanisms are interacting. One of them is molecular diffusion, known to be relatively slow and thus often considered to be of minor importance and negligible for conventional simulation studies. For sequestration purposes however, where the relevant time periods are significantly longer, diffusion could become a key effect for incremental oil production. This influence of diffusion during gas injection in fractured reservoirs was studied in this thesis.