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Abstract: Stratigraphy and Sedimentation of the Unayzah Reservoir, Central Saudi Arabia

Muhittin Senalp, Abdulaziz Al-Duaiji

Significant reserves of Arabian super light oil, condensate, and associated gas occur in the various genetically different sandstone bodies of the upper Permian Unayzah and Khuff Formations in Central Saudi Arabia.

The Unayzah Formation which rests unconformably on the older formations is composed of red colored, poorly sorted conglomerate, sandstone, siltstone, mudstone, caliche and nodular anhydrite. Facies changes occur due to the presence of various subenvironments and possible faulting and structural growth in the basin during deposition. However, the entire Unayzah Formation shows an overall fining and thinning-upward sequence. It was deposited as coalescing alluvial fans dominated by braided streams which graded into meandering stream and playa lakes under arid to semi-arid conditions. Eolian processes were also inferred.

A marked unconformity which is indicated by the occurrence of thick caliche and soil horizons separates the Unayzah and the overlying Khuff Formation.

The Khuff Formation consists primarily of marine shale, marl, and fine- to very coarse-grained sandstones in the lower parts; shale, limestone, dolomite, and anhydrite in the upper parts. The sandstones were deposited as incised channel fills and their associated low stand deltaic sediments as a result of fluctuating sea level during the deposition of the Khuff Formation. The base of the incised channels represent a sequence boundary. Red colored and rooted paleosols were formed on the underlying marine sediments. During relative sea level rise, good quality reservoir sands were deposited by aggradation within the incised channels. Sand deposition within the channels terminated at the same time, and the area was covered by shallow marine limestones, shales and marls during maximum sea level highstand.

Although the Unayzah reservoir occurs in both the Unayzah and the Khuff Formations because of their different geometry, continuity, and reservoir quality, they have been, studied separately.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France