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The Sarah Formation: A Glaciogenic Reservoir Analogue in Saudi Arabia


The Sarah Formation is a glaciogenic sedimentary unit of the latest Ordovician (Ashgillian) ice age deposited along the Gondwana Margin. It forms part of an extensive, but discontinuous belt of outcrop deposits that extends from the Arabian Peninsula to westernmost North Africa. The short-lived ice age resulted in initial incision and large-scale palaeovalley (PV) generation. Subsequent filling of the PVs juxtaposed the Sarah reservoirs against different older Ordovician marine source rocks and sealed them with a major Silurian source rock. This unique setting makes these Sarah glacial deposits one of the main reservoir targets in the subsurface of Arabia. Our study currently focusses on PV fills exposed in the Tayma area, northwestern Saudi Arabia. The exposures provide excellent reservoir analogs for the glaciogenic PV fills in the subsurface and for other time-equivalent units on the Arabian Peninsula and in North Africa. Outcrop-based reservoir characterization provides a unique insight to the 3D architecture and heterogeneity of PV reservoirs and allows the pre-drill prediction of reservoir quality distribution. Based on our work in the Rahal Daba'a PV, which includes sedimentological sections, general depositional environment mapping and architectural analysis (photomosaics), we present a preliminary 3D static reservoir analogue model. We deduce the evolution of the complex network of the Sarah PVs to the shoreline as well as depositional controls on facies change and reservoir architecture. This initial model captures the spatial and temporal lithofacies’ heterogeneity within the Rahl Daba'a and the 3D connectivity of reservoir geobodies. Reservoir quality data and mechanical properties derived from samples tied to measured sections are assigned to sandstone geo-bodies and lithofacies. They provide the crucial link between the Sarah outcrops in northwestern Saudi Arabia and equivalent subsurface reservoirs. The static 3D model incorporates porosity/permeability and density/velocity cubes for forwards seismic modeling, which is an important approach to reservoir architectural prediction in the subsurface. The approaches taken so far are part of a larger workflow, which will incorporate terrestrial LiDAR, near-surface seismics, calibration wells behind the outcrop and dynamic modeling approaches.