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 stratigraphic sections were studied and measured; the maximum thickness of the Khuff Formation in this area is about 148m. The formation non-conformably overlies the Proterozoic Pocks of the Arabian Shield here, and the underlying Unayzah Formation is absent due to depositional pinch-out The overlying Sudair Shale (Lower Triassic) is found only in small, isolated outliers. The Khuff Formation is composed of alternating sequences of carbonates and siliciclastic racks, and the proportion of siliciclastics increases southwards. The transgressive Khuff carbonates mark a significant change in the pattern of sedimentation from dominantly siliciclastic to dominantly carbonate. This change is attributed to a combination of factors, all of which acted in favour of carbonate 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 palaeoclimate towards warmer temperatures; tectonic subsidence associated with the opening of the Nea-Tethys and closure of the Palaeo-Tethys Oceans; and the late Early Permian melting of continental glaciers in the area. The major Late Permian marine incursion onto the Arabian Plate resulted in the deposition of extensive carbonates over the Arabian Shelf in a shallow-marine tidal-flat system. The shelf was barred and became differentiated into an inner shelf iii the west (where restricted circulation prevailed), and an open-marine, outer shelf in the east; most 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 depositional cycles, which reflect relative changes in sea-level. Each cycle began with a transgressive phase, and ended, with a regressive phase during which sedimentation occurred in the supratidal zone. The duration of these cycles ranged between 1.33 and 4.08 million yr. Differential tectonic subsidence is invoked to explain these third-order cycles. The Hail-Jawf-Rutbah-Mosul Arch ill the north, and the Hadramaut Arch in the south acted as barriers to the advancing Permian Tethys Sea. The eastern tip of the Arabian Shield was submerged by this transgression.