The Sequence Stratigraphy of the Maastrichtian (Upper Cretaceous) Reservoir at Wafra field, Partitioned Neutral Zone, Saudi Arabia and Kuwait: Key to Reservoir Modeling and Assessment

Dennis W. Dull, Raymond A. Garber, and W. Scott Meddaugh
Chevron Energy Technology Company, Houston, TX

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 made it a much less attractive resource than other productive intervals at Wafra field. Less than 1 percent of the OOIP in the Maastrichtian has been produced. 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 Maastrichtian reservoir was deposited during an initial transgressive phase (TST) that evolved into a high stand (HST). Within this sequence stratigraphic framework, ten high frequency sequences (HFS) could be correlated, albeit with some difficulty, across the entire field.

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 within 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 barrel oil Maastricthian resource at Wafra field.