STRATIGRAPHY, SEDIMENTARY ENVIRONMENT AND DEPOSITIONAL EVOLUTION OF THE KHUFF FORMATION IN SOUTH-CENTRAL SAUDI-ARABIA

The Upper Permian Khuff Formation, a major gas reservoir unit in Saudi Arabia and other Arabian Gulf countries, has been studied in outcrops south of lat.23 degrees N in south-central Saudi Arabia. Fifteen stra tigraphic sections were studied and measured; the maximum thickness of the Khuff Formation in this area is about 148m. The formation non-con formably overlies the Proterozoic Pocks of the Arabian Shield here, an d the underlying Unayzah Formation is absent due to depositional pinch -out The overlying Sudair Shale (Lower Triassic) is found only in smal l, isolated outliers. The Khuff Formation is composed of alternating s equences of carbonates and siliciclastic racks, and the proportion of siliciclastics increases southwards. The transgressive Khuff carbonate s mark a significant change in the pattern of sedimentation from domin antly siliciclastic to dominantly carbonate. This change is attributed to a combination of factors, all of which acted in favour of carbonat e rather than siliciclastic deposition. These factors include a change in the palaeolatitudinal position of the Arabian Plate towards lower latitudes within the subtropical zone; the consequent change in palaeo climate towards warmer temperatures; tectonic subsidence associated wi th the opening of the Nea-Tethys and closure of the Palaeo-Tethys Ocea ns; and the late Early Permian melting of continental glaciers in the area. The major Late Permian marine incursion onto the Arabian Plate r esulted in the deposition of extensive carbonates over the Arabian She lf in a shallow-marine tidal-flat system. The shelf was barred and bec ame differentiated into an inner shelf iii the west (where restricted circulation prevailed), and an open-marine, outer shelf in the east; m ost sediments were deposited within the photic zone below fairweather wave-base. The depositional environment changed laterally to the east fr-om a shelf-setting to a slope-and-basin (with turbidites) in Iran. The sedimentation of the Khuff Formation involved four third-order dep ositional cycles, which reflect relative changes in sea-level. Each cy cle began with a transgressive phase, and ended, with a regressive pha se during which sedimentation occurred in the supratidal zone. The dur ation of these cycles ranged between 1.33 and 4.08 million yr. Differe ntial tectonic subsidence is invoked to explain these third-order cycl es. The Hail-Jawf-Rutbah-Mosul Arch ill the north, and the Hadramaut A rch in the south acted as barriers to the advancing Permian Tethys Sea . The eastern tip of the Arabian Shield was submerged by this transgre ssion.