Depositional Framework and Reservoir Potential of the Silurian Qusaiba Member, Eastern and Central Saudi Arabia

By

 Paul G. Nicholson¹, Stephen Carney¹, Merrell A Miller¹, John Melvin¹

(1) Saudi Aramco, Dhahran, Saudi Arabia

 Although well known for its organic rich "hot shales", the Qusaiba Member also contains sand-prone intervals with exploration potential. In this multi-disciplinary study, new biostratigraphical, sedimentological, petrophysical and seismic analyses have been integrated to develop a framework for the age distribution, depositional setting and reservoir properties of this member in eastern and central Saudi Arabia.

The Qusaiba Member comprises four, third-order depositional sequences of Llandoverian age in the study area. Sedimentation occurred on a marine shelf that progressively shallowed-upwards and became more storm-influenced based on faunal, palynological and lithofacies analyses. Wireline log correlations of these age-constrained sequences, combined with regional seismic profiles, indicate that deposition in the Rhuddanian was significantly condensed over major intra-basinal (basement-cored) highs, whereas large sediment thicknesses accumulated in rapidly subsiding basin lows (typically underlain by Neoproterozoic grabens). Syn-depositional tectonism appears to have been minimal. In the Aeronian, the "Mid Qusaiba Sandstone" was subsequently deposited as a widespread lowstand unit across the entire basin, followed by further regressive (silt and sand-prone) sedimentation through the Telychian. The relatively layer-cake system tract correlations suggest a ramp geometry for this shelf, and that any shelf-slope break (if present) was located far to the northeast in Paleo-Tethys.

The "Mid-Qusaiba Sandstone" has been penetrated in approximately 25 wells in the Ghawar and western Rub' Al Khali areas. These sands typically contain significant gas shows and have flowed up to 3.3 MMscf/d. Although reservoir quality is typically moderate to poor (5-15% porosity and <5mD permeability), natural fractures are thought to significantly enhance deliverability in wells located near major faults. Hence there is both exploration and fracture stimulation potential for this reservoir.