Sequence Stratigraphic and Tectonic Setting of Aptian (Cretaceous) Shuaiba Reservoirs, Bab Basin, Middle East

Kerans, Charles, Bureau of Economic Geology, Austin, TX

Aptian (Cretaceous) Shuaiba reservoirs represent one of the most important producing intervals in the Middle East. The Shuaiba Formation is a 100-110 m shallow-shelf carbonate blanket whose constant thickness belies what is actually one of the most diverse suites of reservoirs in the region. Examination of core, wireline log, and lesser seismic data on 14 Shuaiba fields forms the basis for a sequence stratigraphic framework characterizing strati­graphic, structural, and diagenetic heterogeneities of these reservoirs.

Abrupt deepening atop the Thammama/Kharib platform in the Early Aptian generated space for an aerially extensive shelf and Bab intrashelf basin covering parts of Oman, the U.A.E., Qatar, Saudi Arabia, and the adjoining Arabian Gulf. The first two of 5 Shuaiba sequences are Early Aptian and form the transgressive sequence set, consisting of Lithocoduium-mollusc packstone/grainstone (LSh 1), followed by Lithocodium-coral-mol-lusc and Orbitolina skeletal wackestones and lesser Lithocodium banks (LSh2). LSh1-2 reservoirs include the Lekhwair, Qarn Alam, and Musalim. Development of an inner-to-outer ramp transition in LSh2, where this sequence displays a local doubling in thickness, focus­es nucleation of latest Early Aptian rudist banks of LSh3 (HSS), the main reservoir facies for the giant Al Huwaisa, Shaybah and Bu Hasa fields. Following subaerial exposure at the end of the Early Aptian (top LSh3), a significant decrease in accommodation, minor influx in sili­ciclastic sediments, and reactivation of regional structural elements, creates the setting for a series of strongly diachronous mixed clastic-carbonate sequences USh1-2, FSSS) that hosts the Safah, Al Barakah, Al Rayan, Lekhwair East, Dhulaima and related fields.