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 CretaceousThamama Group, in ascending order it is divided intothree 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 and 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 and IX) were mapped over field. Units IT and IW represent a stratigraphic trap, with some later influence of structural uplift due to deepseated salt piercement. The oil accumulation in the reservoir units is restricted to the shelf and shelfedge facies. The tilted oil/water contact (OWC) is a pronounces feature of the field. pronounces feature of the field. The prolific cyclic carbonate sand facies (IT and IW)as part of abroad carbonate shelf platform extend furtherEast of Zakum Field. Subsequently, development of reservoir quality connot a potential future prospect. Introduction Zakum Field is one of the giant off-shore oil fields in the Arabian Gulf. The field extends 50 Km. long along East-west direction and 30 Km. wide alongNorth-south direction. The field was discovered in 1963 by seismic survey and the discovery was confirmed by drilling ZK-1.Here the water depth of the Gulf range from 20–80 ft. The field is located 80 Km. North-west of Abu Dhabicity (Fig.1). Production from Zakum started in October, 1963 at about 50,000 b/d from the Lower Cretaceous carbonates. The Zakum structure is a distinctive East-west feature contrasting to the general N-S and NE-SW striking lines of other fields in Rub-al-Khali basin. The structural anomaly of the Zakum field is probably related to the deep seated salt diapirism, which began to move during the Cretaceous. The field is abroad, gentle, low amplitude asymmetrical anticline with gentle dip of less than one degree on the west and steeper dip of less than three degrees on the East and North-east. The vertical closure of the structure is estimated around 1200–1500 ft. for Thamama IA above the OWC. P. 797