There have been many studies carried out in the past decades attempting to develop strategies for a safe injection of CO2 into storage sites without leakage and contamination of valuable resources. Leakage from these storage sites through capillary seals is one of the long-standing issues, which require a comprehensive assessment prior to CO2 injection. However, surface wettability, which is one of the major parameters controlling the capillary pressure of seals, is a poorly understood parameter, changing as a function of temperature, pressure, surface roughness, etc. The aim of this paper is to provide a deeper insight into the surface wettability of minerals in rocks, especially sandstone and shales, under subsurface conditions for a better assessment of the structural integrity in CO2 storage sites. The results obtained from a series of contact angle measurements under different pressure and temperature conditions indicated that quartz- and feldspar-dominated sandstones are strongly water wet, while kaolinite-dominated shales have a weakly water to intermediate wet system. It is also apparent that the interfacial tension is a function of the pressure and temperature conditions. It appears that the size of pore throats in rocks is the major contributor to capillary threshold pressure and it must be determined very accurately. It is recommended that surface wettability and interfacial tension under reservoir conditions are determined a priori to ensure that the seal integrity analysis is as concise as possible.