Sequence Stratigraphy of the Maastrichtian Wafra Carbonate Reservoir in the Partitioned Neutral Zone (PNZ), Kuwait and Saudi Arabia

Niall Toomey*, W. Scott Meddaugh, Kera Gautreau, and Stewart Griest
Chevron, USA
*[email protected]

The Maastrichtian (Upper Cretaceous) reservoir at Wafra field was discovered in 1959 and has produced about 20 million barrels of oil; less than 1.0% of the estimated original-oil-in-place. The Maastrichtian sediments were deposited during arid conditions in shallow water on a gently dipping ramp in normal marine and restricted lagoonal settings. They are dominated by subtidal dolomite and limestone inter-bedded with thin organic-rich shales. Facies types are dominantly wackestone, rudist rudstone, peloidal packstone, and localized grainstone, with minor mudstone and rudist floatstone. Moldic, inter- and intra-crystalline porosity are common. The average porosity is about 15% and reaches 30–45% in productive zones. Permeability averages about 30 mD with readings as high as 4.0 Darcy. Previous stratigraphic studies of the Maastrichtian interval at Wafra field, largely based on five cores along the crest of the structure, resulted in a field-wide and regional stratigraphic framework. This study, utilizing additional data from cored flank wells, examined the relationship between reservoir characteristics and depositional/stratigraphic controls. Core and thin sections were described with particular attention to depositional texture, grain types, pore types, diagenetic features and stratigraphic surfaces. Depositional facies maps constructed for each stratigraphic surface within the reservoir interval indicated that facies belts are generally oriented in a northwest to southeast trend, parallel to the paleo-shoreline. These depositional environment maps revealed that facies bodies have geometries ranging from arcuate to elongate. The refined stratigraphic framework and depositional model has been incorporated in an updated reservoir model to validate reservoir development strategies for this important heavy oil resource.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain