Carbonate-Evaporite Sequence of the Upper Jurassic Arab Formation: Ancient Analog to the Recent Coastal Sediments of Abu Dhabi, United Arab Emirates

A. S. Alsharahan, Gregory L. Whittle

The Upper Jurassic Arab Formation is a major hydrocarbon reservoir in the Arabian Gulf region. The Arab Formation comprises four members, Arab A to D (or I to IV), which were cyclically deposited as transgressive and regressive carbonate-evaporite units. It can be divided into five lithofacies: 1) oolitic/peloidal grainstone, 2) dolomitic grainstone, 3) dolomitic mudstone, 4) dolomite and 5) massive anhydrite. The best reservoirs occur in association with interparticle porosity in grain-supported lithologies and intercrystalline porosity in dolomites and dolomitic limestones.

The formation represents four depositional settings: 1) supratidal sabkha, 2) intertidal mud flats, 3) shallow subtidal lagoonal and 4) open marine shelf. These depositional settings were a response to the fluctuating sea level conditions during the Upper Jurassic. The primary diagenetic processes include cementation, leaching and dolomitization.

The Recent sediments of the Abu Dhabi coastal flats provide one of the best Holocene analogs to ancient carbonate/evaporite sequences. These Holocene coastal flat sediments occur as a thin veneer covering pre-Holocene carbonate and mixed clastic-carbonate sediments. The carbonates of the Arabian Gulf accumulate on a broad shelf rimmed by organic reefs and coral-algal sands. The inner shelf is capped by supratidal islands that protect coastal lagoons and enable the development of wide tidal flats. The carbonate barrier islands, tidal channels, tidal deltas and shallow lagoons are the site of accumulation of oolitic sands and finer grained mud-size carbonates. The lagoons are rimmed by algal flats, which in turn have been partly covered by prograding supratidal sabkhas.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995