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Natural Fractures and Their Impact on the Tight Sand Plays of the Ordovician Sarah Formation, Northern Arabia


The Sarah Formation was deposited as paleovalleys fill during a short-lived Ashgillian ice age, Ordovician. It is characterized by heterogeneous facies both laterally and vertically resulting in diverse hydrocarbon reservoirs including high porosity/high permeability to tight sandstones. The tight sandstones rely on natural fractures and/or hydrofracture stimulation in their gas delivery. The occurrence of natural fractures in the tight sandstone bodies in the Sarah, and their influence on the reservoir performance are addressed in this study. Cores, image logs, full wave sonic, open hole logs, well tests, and seismic data have been used in the exploration and formation evaluation. The natural fractures in the Sarah are syndepositional, early diagenetic, and tectonic in origin. Syndepositional/early diagenetic deformation of semiconsolidated sediments is manifested as faults, injectites, and slump structures which may play a role in trapping/sealing mechanisms due to their potential sealing properties. The tectonic fractures include faults and joints, which occur as dispersed features or clusters in zones of subseismic resolution. Tectonic fractures' impact on reservoir performance varies widely. Partly mineralized fractures are characterized by channel type apertures that facilitate fluid flow. Barren fractures are predominantly closed under reservoir conditions and have limited impact on fluid flow. Fully mineralized fractures do not enhance permeability, and in some cases they might have acted as conduits to mineralizing fluids, which occluded the porosity of the matrix in the immediate vicinity of the fractures, thus impairing reservoir properties. The vast majority of tectonic fractures are steeply dipping with NNW-SSE to NNE-SSW strikes, nearly parallel to the current day maximum in-situ horizontal stress. This regional pattern is related to remote Arabian Plate stresses rather than local stresses, and facilitates hydrofracture stimulation and borehole planning and completion design.