Online Journal for E&P Geoscientists

Droste, Henk ^*1
(1) Shell International E&P, The Hague, Netherlands.

The Upper Jurassic along the eastern edge of the Arabian Plate is a stratigraphic puzzle with many missing pieces. In the United Arab Emirates and Saudi Arabia this interval contains the prolific Arab reservoirs. These were the main target of the first exploration well in northern Oman but unfortunately, the well found no Arab and was abandoned as a hugely disappointing dry hole. It became apparent that in Oman Middle Jurassic shallow water carbonates are overlain by deep water deposits of poorly defined Late Jurassic-Early Cretaceous age.

The nature of the lateral change has been a point of discussion for many years. In the subsurface stratigraphic thinning of the Upper Jurassic has been observed, while in the Oman Mountains there is evidence of erosion and truncation of the Middle Jurassic. Interpretations were hampered by differences in and inconsistent use of stratigraphic nomenclature across political boundaries and, most importantly, very poor biostratigraphic age control. Several models have been proposed: 1) lateral facies change to condensed deep water facies 2) latest Jurassic uplift and subaerial erosion followed by drowning 3) submarine erosion associated with block faulting. Recent biostratigraphic data combined with seismic observations allowed the construction of a new stratigraphic model which will help to define the stratigraphic trapping potential within this interval.

The truncation of the Middle Jurassic carbonate platform is related to uplift and subaerial exposure of the eastern plate margin at the end of the Middle Jurassic. Uplift was followed by major collapse of the eastern margin of the Arabian plate and the development of drowning unconformity overlain by deep water condensed sediments. The collapse, as well as the preceding uplift may be related to the initiation of the transform fault margin located in the Arabian Sea just east of Oman.

Following the early Late Jurassic flooding event, a prograding carbonate shelf complex was established in Saudi Arabia and the UAE consisting of the Arab and Hith formations, while in northern Oman deposition took place in a basinal setting. Only by Early Cretaceous time this shelf reached north Oman in the form of the Salil/Habshan formations and continued to a position of approximately the present-day northern Oman coastline in the middle Cretaceous. Due to climatic changes the facies association of this shelf changed from an arid evaporitic carbonates to a more humid association with shales.