One of the most secure storage sites for CO2 injection is in depleted gas reservoirs. To ensure that the CO2 is trapped securely and will not escape to the surface, storage in such formations must be study carefully prior to injection in such formations. After the injection, the injected CO2 will undergo several trapping mechanisms; namely: hydrodynamic, solubility and mineral trapping. The extend of geochemical reactions involved depend on the composition of the injected fluid introduced in the aquifer, the composition of the initial minerals assemblage and the aquifer brine. In this paper, the importance of biological/microbial mechanisms towards the impact on the storage capacity was studied using reactive transport modelling. The results obtained shows that the presence of microbial compound such as organic matter contributes to the enhancement of mineral precipitation, resulting in secure long-term storage.