The Upper Jurassic Hadriya and Hanifa carbonate grainstone reservoirs in Berri Field, Saudi Arabia, contain large reserves of Saudi Extra Light crude oil. New geologic models of each reservoir have been built using sequence-stratigraphic principles. These new models have a chronostratigraphic architecture which accommodates the facies transitions observed in cores sampled along the ramp-to-basin profile. Each reservoir contains, from oldest to youngest:a highstand systems tract represented by aggrading and prograding grainstones, capped by a type-2 (subaqueous) sequence boundary;a shelf margin wedge systems tract characterized by grainstones showing maximum basinward progradation; anda transgressive systems tract typified by a series of backstepping grainstones capped by a surface of relative drowning. These new models clarify the elements controlling fluid movement within the reservoirs. Introduction Berri field is located about 100 km north of Dhahran in the eastern provence of Saudi Arabia. The anticlinal trap lies partly on and partly offshore, just northeast of Jubail. Of the seven Upper Jurassic reservoirs in this field, the Hanifa and Hadriya are currently under full development because they contain Arabian Extra Light crude (Fig. 1). Both reservoirs have been undergoing peripheral water injection since 1973. In 1988 ARAMCO initiated a joint study with a team from Mobil Research and Development Corporation to construct new geologic models of the Hanifa and Hadriya reservoirs that would eventually be used for reservoir simulation. Sequence stratigraphic principles were used in formulating the new models. Since no high-resolution seismic data was available over Berri Field, sequence stratigraphic interpretations were based solely on core and log data. This included 33 cores and 92 logs in the Hadriya, and 32 cores and 142 logs in the Hanifa. The application of sequence stratigraphic principles led to the recognition of previously undefined reservoir geometries. Subsequent integration of production and engineering data confirmed the importance of these geometries in controlling fluid distribution and movement. The new correlations had an especially profound affect on the three dimensional architecture of the Hanifa reservoir. PALEOGEOGRAPHIC SETTING The Middle Jurassic Arabian Platform was characterized by minimal platform-interior bathymetric relief and by widespread shallow-marine, carbonate sedimentation. Beginning in the Upper Jurassic, however, a series of intrashelf basins and intervening arches were formed (Fig 2). The intrashelf Arabian Basin was separated from the Southern Gulf Basin of Qatar and the United Arab Emirates by the Qatar Arch. P. 489^

The paper presents a case study of a major carbonate reservoir under peripheral waterflood.It illustrates the importance of good surveillance data not only for monitoring performance but also for characterizing reservoirs.It shows that fluid flow in a lithologically complex and highly stratified reservoir can be much simpler than would be projected from geology alone.The paper also discusses a production strategy that has improved areal and vertical sweep.In this study, the available surveillance data including well performance, geochemical analysis, cased and openhole logs were utilized to determine water encroachment patterns, areal and vertical sweep, and identify the reasons for lack of sweep in certain areas of the reservoir.The observed flood performance has been incorporated into an updated geological model of the reservoir that will form the basis for a forthcoming 3-D simUlation model.