Sequence Stratigraphy and Sedimentology of the Upper Jurassic Arab and Hith Formations with Emphasis on Anhydrite Deposits, Abu Dhabi, United Arab Emirates

Abdulla Al-Mansoori*, Christian J. Strohmenger, Abdelfatah F. El-Agrab, Ahmed A. Khouri, and Abdullah Al-Aiderous
ADCO
*[email protected]

The Kimmeridgian-Tithonian Arab and Hith formations are part of the highstand sequence set of a second-order supersequence, built by five third-order composite sequences (J70, J80, J90, J100, and J105) and bounded on top by sequence boundary J110_SB. The overall depositional environment envisaged for the Arab Formation is that of a barrier-shoal complex with open-marine, offshore sedimentation to the east and a protected, evaporitic, intra-shelf basin to the west. A barrier-shoal complex was developed along the platform margin and deposition was dominated by oolitic grainstones. Concomitant deposition of sabkha, tidal flat, salina and lagoonal sediments occurred westwards, and open-marine mudstones and wackestones were deposited eastwards of the barrier-shoal complex. The reservoir quality is strongly controlled by the depositional environment and the lithofacies types. The best reservoir is present within grain-dominated lithofacies types of the barrier shoal complex. Relatively poor reservoir quality is characteristic of mud-dominated lithofacies types that occur in open-marine environments. In the intra-shelf basin the dominantly dolomitized lithofacies types show quite good reservoir qualities within thin intercalated packstone to grainstone layers, interpreted as tidal channels or washovers. During Hith time, the restriction of the intra-shelf basin increased and predominantly salina-type anhydrite after gypsum was deposited. The focus of this study is on the depositional environment of the anhydrite sediments, as it is important to distinguish between salina-type (saltern) and sabkha-type evaporites. In contrast to sabkha-type deposits, where the evaporites are forming within the host rock (sediment-dominated: late highstand and lowstand systems tracts), salina-type deposits represent subaqueous evaporite precipitations (evaporite-dominated: late lowstand and transgressive systems tracts deposits). Distinguishing between the different anhydrite depositional environments is crucial for the correct sequence stratigraphic interpretation of the Arab and Hith carbonate-evaporite successions.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain