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Sequence Stratigraphic Controls on Carbonate Reservoir Heterogeneity, Mishrif Formation, West Qurna I Field, Iraq


The Mishrif reservoir comprises the main discovered developed reservoir at West Qurna I field in southern Iraq and has been on production intermittently since 1999. Core, log and dynamic reservoir data are being integrated to characterize and model the impact of reservoir heterogeneity on reservoir performance and development plans, including waterflood response. Reservoir heterogeneity is driven mainly by stratigraphy and depositional facies, and an integrated sequence-stratigraphic model has been developed to guide reservoir characterization and modeling. Skeletal grainstone comprises a relatively small volume of the reservoir, but forms the dominant flow units in the reservoir. Permeabilities range from 10's to 1000's of millidarcies based on integration of core and well test measurements. Permeabilities in grainstones are enhanced by dissolution associated with subaerial exposure at sequence boundaries. Outer-shelf packstone and wackestone facies comprise the bulk of the reservoir rock volume and are dominated by microporosity. These facies exhibit high porosity (> 20 %), but low permeability (< 10 millidarcies) due to small pore throat sizes. The permeability contrast in the Mishrif presents significant challenges to reservoir development and management. One challenge is optimizing waterflood effectiveness. Production logging confirms that water is moving preferentially through the higher-permeability grainstone, potentially by-passing a large rock volume with lower permeability. The geometry and connectivity of the grainstone bodies vary significantly by sequence-stratigraphic interval, ranging from field-wide thief zones, to more discontinuous grainstones of limited lateral extent (1-2 km). Waterflood design and completion strategies need to be developed and managed on a zonal basis to account for these geologic variations. Productivity of low permeability microporous rock presents another challenge and alternative development and completion options are being evaluated.