From Bergs to Ergs: Stratigraphic Development and Architecture of Unayzah Reservoirs in East-Central Saudi Arabia

John Melvin, Christian J. Heine, and Ronald A. Sprague
Saudi Aramco, Dhahran 31311, Saudi Arabia

The prolific Permo-Carboniferous Unayzah reservoirs of Saudi Arabia display considerable thickness variation, and are subdivided into three stratigraphic units. The lowermost, Unayzah-C, comprises tightly-cemented and fractured sandstones. Initially deposited on a fluvioglacial outwash braidplain, they were subsequently overridden and deformed by a major glacial readvance. They are overlain unconformably by the Unayzah-B glacial retreat facies, a mixed unit comprising discontinuous, poorly organized (and locally glacially tectonized) massive diamictites, glaciolacustrine turbidites and ice-distal stratified diamictites in its lower part. Its upper part comprises more laterally continuous, non-glacial facies dominated by floodbasin siltstones, but including eolian and fluvial sandstones and terminating in a significant paleosol.The contact with the overlying Unayzah-A is disconformable, and represents a significant surface of desiccation. The Unayzah-A comprises various eolian facies (including erg-center and erg-margin), as well as playa lake and ephemeral stream deposits. A robust layering scheme is identified both within and between these diverse facies, which is based on paleo-water table fluctuations in Unayzah-A times, and thus is considered to have sequence stratigraphic significance. An overall upward-wetting trend is thought to be related to post-glacial transgressive effects. The Unayzah-A is truncated by the Pre-Khuff Unconformity, and overlain by Basal Khuff Clastics comprising paleosols, and estuarine to shallow marine sandstones that herald the eventual establishment of the marine carbonates of the Khuff Formation.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005