The sandstones of the Permo-Carboniferous Shajara Formation form the main part of the Unayzah Reservoir in the Greater Arabian Basin. It is divided into three reservoirs, namely from base to top Lower, Middle, and Upper Shajara reservoirs. Mercury intrusion technique was carried out on representative sandstone samples collected from the type section and the three reservoirs are generally characterized as heterogeneous megaporous reservoirs. The best reservoir quality is assigned to the lower sand unit of the Lower Shajara followed by the Middle Shajara Reservoir.