The Maastrichtian (Upper Cretaceous) reservoir is one of five prolific oil reservoirs in the giant Wafra oil field. Although discovered and first produced in 1959, the reservoir is currently in early development because of its low but variable oil gravity, high sulfur content, relatively high water cut, and apparent compartmentalization. This made it a much less attractive resource than other productive intervals at Wafra field. Less than 1% of the original oil in place in the Maastrichtian has been produced.

The Maastrichtian oil production is largely from subtidal dolomites at an average depth of 760 m (2500 ft). Carbonate deposition occurred on a very gently dipping, shallow, arid, and restricted ramp setting that transitioned between normal-marine conditions to restricted lagoonal environments. The average porosity of the reservoir interval is about 15%, although productive zones have porosity values as much as 30–45%. The average permeability of the reservoir interval is about 30 md; individual core plugs have measured permeability as much as 1200 md. This study was undertaken to (1) determine reservoir volumetrics, (2) understand the areal and stratigraphic distribution of intervals likely to yield higher volumes of better quality oil, and (3) provide a reservoir property model for use in fluid-flow simulation.

The key to modeling the reservoir was the construction of an appropriately detailed sequence-stratigraphic framework for use in building a geostatistical reservoir model with high-quality descriptions from five cored wells in the reservoir.

The geostatistical model of the Maastrichtian reservoir demonstrates the layered and compartmentalized nature of the reservoir and clearly shows that the location of the reservoir facies in the Maastrichtian is controlled by the original depositional fabric and subsequent dolomitization, both of which have been influenced by the paleotopography. Such understanding is critical to efficiently develop the 1.5 billion bbl of Maastrichtian oil at Wafra field.