Geological Model and Reservoir Evaluation of the Lower Cretaceous Bab Member in the Zakum Field, Abu Dhabi, U.A.E.

Abstract

ABSTRACT In the Zakum Field, the Shuaiba Formation (Aptian) is the upper most rock unit of the Lower Cretaceous Thamama Group, in ascending order it is divided into three zones, Thamama IA, IB and IC. The last two have been termed the Bab Member, has an average thickness of about 230 ft. and shows a diverse lithology. Minor changes in eustatic sea level at the time of deposition produced a major change in the lithofacies distribution across the field. These range from deep water basinal to shallow shelf sediments. The lower part of the Bab Member (Thamama IB) is divided into three: A progradational carbonate cycle of deep water argillaceous globigirinal limestone at the base (Tar Unit), which could represent a good source rock. Above it are drowned shelf sediments of bioclastic wackestone with a moderately developed reservoir facies (IX unit). These end with a highly dolomitized unit of poorly developed reservoir facies, which forms a comparatively good seal. The upper part of the Bab Member (Thamama IC) consists of calcareous shale interbedded with limestone of good reservoir quality. The limestone is separated into two reservoir facies units (IT & IW) and is well developed in the East, representing a shallow shelf to high energy shoal deposits (oolitic grainstone, calcareous algae, orbitolinid and skeletal debris). These sediments were transported westward and deposited over the slope of the deeper water environment and limiting the reservoir facies to the crest and eastern flank of the field. In order to evaluate the economic potential of the sequence, all of the possible reservoir units were defined using wireline logs and petrographic description. Facies units (IT, IW & IX) were mapped over the field. Units IT & IW represent a stratigraphic trap, with some later influence of structural uplift due to deep seated salt piercement. The oil accumulation in the reservoir units is restricted to the shelf and shelf edge facies. The tilted oil/water contact (OWC) is a pronounced feature of the field. The prolific cyclic carbonate sand facies (IT & IW) as part of abroad carbonate shelf platform extend further East of Zakum Field. Subsequently, development of reservoir quality connot a potential future prospect.