Saline aquifers are considered as one of the most suitable options for geological storage of CO2. After being injected, due to its lower buoyancy and viscosity in comparison to the resident brine, the majority of the CO2 plume tends to move updip and form a thin layer underneath the caprock at the top of the reservoir. In this study, we have questioned the widespread use of the simplified geological model and have shown the impact of the structure of the sealing unit on the long-term CO2 migration direction and trapping mechanisms through a systematic approach. To achieve this goal, a comprehensive sensitivity analysis is performed using 375 different scenarios to study the impact of well-location, small scale topography variations of caprock (rugosity) and aquifer slope on CO2 migration and trapping mechanisms. The results show an increase in the amount of dissolved and trapped CO2 by increasing the tilt angles.