Facies, Stratigraphy and Lateral Stratigraphic Continuity in Lower Jurassic Reservoir Analogue Outcrops at Wadi Naqab, United Arab Emirates

Abstract

The Lower Jurassic succession at Wadi Naqab (United Arab Emirates) was analysed in detail. The objectives were (1) to update the depositional age through biostratigraphy, (2) identify the main facies types and interpret the depositional environment, and (3) quantify the lateral continuity and bed-thickness changes and pinch outs of carbonate beds. The properties of the exposed rocks are well known in the subsurface, however little is known on the lateral facies changes. The latter potentially has important impact on subsurface correlation and estimating sweep efficiency. Detailed biostratigraphy analysis shows the presence of dasycladalean algae and foraminifera that indicate a Hettangian-Upper Pliensbachian age. The Upper Sinemurian-Lower Pliensbachian limit was identified with confidence. In terms of depositional environment, there is a distinct transition from inter-bedded inner ramp tidal flats and ramp barrier shoal type deposits towards shallower, restricted lagoons in the upper part of the succession. Based on our data, the Hettangian-Upper Sinemurian succession from Wadi Naqab was recognized/suggested as a new facies analogue for e.g. the Bathonian Uwainat Member (Araej Formation) in Qatar. This part of the Wadi Naqab succession consists of interbedded wackestones and grainstone which were deposited in ramp barrier shoals and inner ramp tidal flats. Similar wackestone/grainstone alternating layers can be found in the Uwainat Member, for whcich the environment of deposition is represented by ramp barriers and inner ramp areas with some outer ramp depositional features. By contrast, at Wadi Naqab outer ramp deposits occur only at higher stratigraphic levels. A detailed 3D digital outcrop model is used to quantify lateral changes by measuring bed thickness variation at cm-resolution. At first sight it appears all beds are completely continuous with constant thickness. Detailed measurements indicate the lateral continuity of the upper part of the succession showing two different thickness trends in the SW-NE direction. Some of these beds are thinning towards NE (by as much as 2.00 m per 100 m) while the other category includes beds which are thicker towards NE and thinner in the SW. Pinch-outs are common for several SW-NE thinning beds with the pinch-out occurring towards the NE. Such changes and pinch outs often are not identified in the subsurface and their impact on well correlation and flow behaviour are overlooked.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016