Geological Model of the Bu Hasa Shuaiba Reservoir, Abu Dhabi

Abstract

Abstract The Aptian Shuaiba formation is the major oil reservoir of the Bu Hasa field. The formation consists of rudistid, carbonate buildups with associated back-reef and fore-reef limestones. Algal limestone buildups and normal marine limestones also occur. Extensive coring combined with excellent log correlations, particularly in the northern portion of the field, have resulted in a revised geological model. The new geological model has had a major impact on the selective injection and production programme as the reserve distribution is different programme as the reserve distribution is different within the various pseudo reservoirs. Introduction Bu Hasa is the westernmost ADCO producing field (Fig. 1) and is the only one with a lower Cretaceous Shuaiba carbonate buildup and oil accumulation (Fig. 2). The Bu Hasa Shuaiba reservoir has been the object of a full-field development plan since late 1980 with up to 5 drilling and 2 completion rigs active in doubling the number of wells drilled in the last 4 years, as were drilled in the first 18 years since discovery in 1962. Development plan objectives are to maximize ultimate oil recovery and to build a 20 year sustainable production capacity. Work programmes include improvement of sweep efficiency by increasing well-completion density and the provision of full-field pressure support to replace withdrawals. The bulk of the oil production since 1965 has been from cased or open hole completions in the central and southern sectors of the field and, in particular, from the prolific producers in the particular, from the prolific producers in the Unit II rudist packstones. The northern sector lead been sporadically appraised and little tested or produced because of the presence of numerous dense produced because of the presence of numerous dense intervals within a finer-grained assemblage of rocks. It was quickly established, during late 1981 and early 1982, that the existing northern geological model was inaccurate. To re-examine the previous northern model, detailed porosity log cross-sections, 1/200 scale, were constructed with datum as base Shuaiba. All northern wells were tied together in a grid (Fig.3) similar to that of seismic survey and these sections were expanded rapidly as new logs came in from tie field. Strong northwest-southeast sedimentary trends were recognized both in log correlations and on isopach maps. Log correlation was much more difficult at right angles to these trends until infill drilling bridged some of the 3-4 kms gaps. THE GEOLOGICAL MODEL The Aptian Shuaiba formation is the major oil reservoir of the Bu Hasa field. The Base of the Shuaiba is a dense limestone separating the Shuaiba from the underlying oil bearing Thamama Zone B reservoir. Across the field the Shuaiba starts with algal wackestones and packstones of lower Unit A deposited in a shallow restricted marine environment (fig. 4). Then a rise in sea level occurred indicated by the presence of the overlying chalky limemudstones and wackestones of upper Unit A and lower Unit B containing Orbitolina and small planktonic foraminifera. A return to algal colonization is observed from the wackestones and boundstones of upper Unit B. This type of sedimentation is thin in the north but thick and continues into lower Unit C (Subunit C3) in the south. in the north a transgression took place expressed by the presence of the fore-reef chalky limemudstones and wackestones of lower Unit D. These limestones contain planktonic foraminifera which suggest open marine circulation. To the south the coral development of Subunit C2 occurred followed by algal colonization of Subunit C1 in an open to restricted marine bay to shelf lagoon complex. In the north major periods of aggrading or upbuilding can be seen within the fore-reef, plus sediment progradation across the older deposits represented by the sedimentary cycles of upper Unit D, Units F, C and to. These units consists of varying facies from packstones/wackestones containing planktonic foraminifera, Orbitolina and Miliolids planktonic foraminifera, Orbitolina and Miliolids (Upper Unit D, Units F and Ho); to limemudstones/wackestones containing Cynophyceae and sponge spicules (Unit F and C). Overall upper Unit D, Units F, G and Ho are wedge shaped in cross-section.