Bimodal pore size behavior of the Shajara Formation Reservoirs of the Permo-Carboniferous Unayzah Group, Saudi Arabia

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. One sample collected from the upper part of the Lower Shajara was described as low quality due to its fine grain characteristic and its proximity to the unconformity surface. Reservoir quality is controlled to a large extent by the depositional facies and specifically by rock texture illustrated by petrophysical description. The quality of the three reservoirs of the Shajara Formation, increases with the increase of grain size and grain sorting.