PETROGRAPHY, SEDIMENTOLOGY, DIAGENESIS AND RESERVOIR CHARACTERISTICS OF THE SHUAIBA AND KHARAIB FORMATIONS (BARREMIAN-MID APTIAN) CARBONATE SEDIMENTS OF ABU DHABI, UNITED ARAB EMIRATES

The Lower Cretaceous Thamama Group is a thick sequence of carbonate platform deposited over the Gulf area in a time span of as much as 30 million years. It is composed of four formations in ascending order: Habshan (Berriasian-Valanganian), Lekhwair (Hauterivian-lower Barremian), Kharaib (lower Barremian-early Aptian) and Shuaiba (Early to mid Aptian). In this study, the Kharaib and Shuaiba Formations of the Thamama Group are described and analyzed from cores and thin sections from four major oilfields (Zakum, Bab, Asab and Bu Hasa).; In the Bu Hasa oilfield the Shuaiba Formation accumulated on a shelf margin as a shoaling upward cycle and is composed mainly of rudistid and algal sediments. While in the Zakum oilfield the Formation accumulated in an intrashelf basin, and is dominated by lime mudstone and wackestone textures typically of a deep to open marine shelf setting. Grainstone and packstone textures typically associated with a shoal environment are only present in thin intervals. The Kharaib Formation in the fields studied accumulated as a shelf sediment consisting of depositional cycles (transgressive-regressive) in which repetitive sequences of rock types are spread over vast areas. This transgressive-regressive sequence is made up of a repetition of restricted marine deposits superposed by shoal sediments overlain by restricted marine sediment which in turn is overlain by a sequence of open marine shelf deposits.; Petrographic evidence reveals that diagenesis of these sediments was complex and included: the marine phreatic, mixed and meteoric phreatic and burial diagenesis. Thin sections and core analysis of samples indicates that their pore system of mainly primary interparticle porosity was occluded early by rim and pore filling cements, while intraparticle porosity was produced as a result of meteoric leaching and is occasionally occupied by equant cement. A secondary porosity mainly of vuggy and moldic pore types was produced by leaching of skeletal organisms, a result of subaerial exposure. Compaction produced fracture porosity which is sometimes occluded by blocky cement, and rare intercrystalline porosity.; The reservoir quality of the Lower Cretaceous carbonates depends upon two parameters (a) texture and depositional environment; and (b) porosity modification by diagenetic processes. The original sediment composition and texture strongly influenced the effect of diagenesis resulting in an interplay of porosity and permeability creating or destroying processes, which either upgraded or degraded original reservoir quality.