Carbon dioxide capture and storage (CCS) is considered as the most promising technological method to decrease atmospheric Carbon dioxide (CO2) emissions. However, due to density difference between the injected CO2 and the formation brine, CO2 tends to move vertically upwards in the reservoir, which results in leakage risk. This vertical migration of CO2 can be prevented by four trapping mechanisms, i.e. structural/residual trapping, capillary trapping, solubility trapping, and mineral trapping. The capacities of structural and residual trapping are highly affected by rock wettability. Clay wettability is one of the crucial parameters in evaluation CO2 geo-sequestration. However, the literature data show that there are many uncertainties associated with clay experimental measurements. One of these uncertainties is the influence of the effect of gas density on the clay mineral wettability. Thus, here, we compared the wettability of a clay mineral (i.e. montmorillonite) with three different gas densities scenarios (i.e. low (Helium), moderate (Nitrogen) and high (CO2) gas densities). To do so, we measured the advancing and receding contact angle (i.e. wettability) of montmorillonite for CO2/water, nitrogen/water, and Helium/water systems at a constant (333 K) under four different pressures (5, 10, 15, and 20 MPa). The brine composition used was 4 wt% NaCl, 4 wt% CaCl2, 1 wt% MgCl2 and 1 wt% KCl, for all gas density scenarios. The results indicate that gas density has a significant effect on the clay mineral wettability and that both advancing and receding contact angles increase with an increase in gas density. The results show that the higher gas density gas has higher contact angle with montmorillonite, measured at the same temperature and pressure. For example, the advancing contact angle of montmorillonite at 333 K and 15 MPa was only 50ͦ (water-wet) for the Helium/water system, while it was 101ͦ (intermediate-wet) for the CO2/water system. Thus, we conclude that the gas density affects the clay wettability and that higher gas density leads to increase the CO2-wettability of the clay. This phenomenon reduces the estimated CO2 geo-sequestration capacity and containment security. This implication can be utilised to improve the wettability predictions, and thus, the assessments of reservoir CO2 geo-sequestration capacity.