Abstract

In 1989 oil was discovered south of Riyadh in siliciclastic sandstones of the Permian Unayzah Formation, as well as in the basal Permian Khuff Formation. This oil is a high gravity (43-53° API) premium crude with low sulphur content (generally less than .07%) that is classified as Arabian Super Light. In central Saudi Arabia Paleozoic oil and gas accumulations are trapped primarily in structural closures controlled by faults or by drape of the reservoirs over underlying fault blocks. This high gravity Paleozoic oil differs from the lower API crudes (27° to 38° in the main producing fields, with higher sulphur content, 0.8 to 2.9%) that are produced from structurally trapped Jurassic carbonates in the onshore portion of eastern Saudi Arabia.

Gas with associated condensate has also been discovered in the Unayzah and basal Khuff formations. The source of these hydrocarbons is a high gamma ray shale at the base of the Qusaiba Shale Member of the Qalibah Formation. The Silurian Qusaiba Shale Member subcrops beneath the Unayzah Formation and is a regionally significant source rock in Saudi Arabia. Equivalent Silurian shales are important source rocks elsewhere in the Middle East and North Africa.

The Unayzah Formation was deposited on a major unconformity and represents the first regionally extensive sequence following a period of uplift and substantial erosion (greater than 1,500 meters in some areas). This uplift was initiated in Devonian time, with major structural development occurring by the Late Carboniferous, in the same time frame as the European Hercynian orogeny (late Paleozoic orogenic era). The Unayzah Formation thickens to the west, south and southeast of a central Arabian upland that covered much of central and eastern Arabia. Alluvial and fluvial depositional environments are interpreted for the Unayzah Formation in existing wells drilled south of Riyadh. Overlying the Unayzah Formation, basal Khuff sandstones were deposited as channel and valley fill sediments on an eroded Unayzah surface. The contact between the Unayzah and Khuff formations is unconformable and generally marked by a caliche zone at the top of the Unayzah Formation.