Late Carboniferous and Permian (Unayzah/Gharif) Paleogeography of Saudi Arabia and the southern Arabian Plate: Implications for Reservoir Trends

By

Roger J. Price¹, David R. Fell²

(1) Saudi Aramco, Dhahran, Saudi Arabia (2) BP Canada Energy Company, Calgary, Alberta

 A series of 12 paleogeographic maps depict the changing paleogeography throughout Saudi Arabia, Oman and the United Arab Emirates from latest Carboniferous to latest Permian time (approx. 305-245 MYBP). The maps illustrate an overall climatic amelioration from alternating glacial/interglacial periods to arid-tropical and their resultant reservoir facies. A representative well is used from the Haradh/S. Ghawar field (Hrdh 601) to illustrate the stratigraphic positions of the reservoir sands and their nomenclature. The suggested sequence stratigraphy of this Permo-Carboniferous interval is reviewed so as to explain the maps/reservoir trends plus their positions on a local relative coastal onlap curve.

The often contemporaneous Unayzah C/Haradh and Unayzah B/Jawb (Juwayl) are predominantly braided and meandering facies respectively. Desiccation followed, overlain by the marine transgression of the "Haushi Lmst"/Lower Gharif/Saiwan/Unayzah. This represents an initial rift/failed breakup unconformity and connection to paleo-tethys. Contemporaneous and younger aeolian dune deposits are suggested to the west (A2 Unayzah reservoir). Subaerial exposure of the Lower Gharif/Unayzah is overlain by beach/shoreline, then fluvial meandering facies of the Middle Unayzah/Gharif. This inturn is unconformably succeeded by incised valley fill of the Upper Unayzah/Gharif of Murghabian age.

This period is followed by transgression of the lower Basal Khuff Clastics succeeded by sea level drop and incised valley fill of the upper Basal Khuff sands (A1 reservoir), with potential downdip forced regressive shorelines deposited intra-ramp in paleotopographic lows.

Ultimate breakup/drift resulted in transgression of the basal Khuff carbonates westwards, which completely flooded the low relief ramp, effectively shutting off clastic sedimentation.