oa Prediction of Unayzah Reservoir Quality Ahead-of-the-Bit

Recent research at Saudi Aramco have examined the feasibility of predicting areas of good reservoir<br>quality - “sweet spots” - in the Permo-Carboniferous Unayzah reservoir away from areas of well control<br>using stratigraphic forward modeling. The Unayzah Group reservoir interval was deposited above the<br>Hercynian Unconformity over a 56 Ma. time period during the upper Carboniferous to lower Permian in<br>central and eastern Saudi Arabia, and consists of a succession of sandstones and siltstones that reflect<br>changing climatic conditions, from glacial and peri-glacial conditions in the lower Unayzah (Unayzah-C<br>and lower -B), to fluvial, lacustrine and ultimately eolian conditions in the upper Unayzah (upper<br>Unayzah-B and -A). Stratigraphic Forward Modeling has been applied to predict the distribution of<br>facies, grain size, porosity, and reservoir architecture in this diverse suite of rocks. Initial conditions<br>and paleotopography were established above the Hercynian Unconformity, with sediment erosion,<br>transport, and deposition modeled along this surface; lower Unayzah (especially the Unayzah-C)<br>sediments were modeled as the products of fluvial depositional systems intercalated with repeated<br>glacial advances and retreats. Modern analogues for this interval include glacial outwash plains in front<br>of retreating glaciers in Iceland and Argentina. Middle Unayzah sediments were modeled as<br>predominantly fluvial and lacustrine systems that arose during glacial retreat and collapse, while upper<br>Unayzah sediments were modeled as the result of primarily eolian deposition. Modern analogues for<br>upper Unayzah eolian sedimentation occur in modern-day Saudi Arabia. Model results correspond to<br>observed sedimentary facies and initial reservoir quality in cores and logs, and with stratal geometries<br>defined to the limit of seismic resolution. Results from this research will be used in conjunction with<br>diagenetic modeling in Saudi Aramco’s on-going reservoir quality prediction effort to develop better pre<br>-drill risk estimates for exploration efforts in this interval.