Challenges in Reservoir Characterization and Oil Recovery in a Complex Carbonate Reservoir, the Kambala Toca, Offshore Angola

Ricardo Digregorio¹, Steve Jenkins¹, Ben Wang¹, Joey Legaspi¹, Andry Nabasir¹, Lucas Pedro¹, Matthew Hoehn¹, and Wayne Narr^2
1Chevron Africa/Latin America Exploration & Production, Chevron, Houston, TX
²Chevron Energy Technology Company, Chevron, San Ramon, CA

The Toca Formation is a lacustrine carbonate reservoir with a complex depositional and diagenetic history and OOIP of about 1,950 MMBO. It shows extremely variable thickness and quality, with a spatially varying occurrence of fractures and vugs. Some wells have produced more than 10 MMBO but geologic and operational complexity resulted in wells of varying rates. Despite these challenges, the Kambala Toca Field was successfully developed. It was discovered in 1971 at a water depth of 55 ft and reservoir depth of 10,200 ft TVDSS. The field is a tilted fault block oriented NW-SE. The producing interval is a dolomite-prone facies which is controlled by depositional setting and the action of later diagenesis. It is better developed in the west flank, where shallow lacustrine facies prevails. Deep lacustrine deposits, mostly marls and shales, dominate the east flank. Lithology, secondary porosity and fractures play primary roles in well deliverability. Reservoir distribution was mapped using seismic attribute methods with acceptable results, but the presence of this facies is a necessary but insufficient condition to warrant good well performance. For that reason a better understanding of the distribution and relative importance of fractures and vugs contributing to matrix permeability enhancement is required. A dual-porosity, single-permeability, finite difference simulation model was used to match production history. Strong water influx and anisotropic fracture permeability are key parameters adjusted during production history match. This paper summarizes efforts to systematically describe key reservoir properties and the integration of seismic, core, image logs and production log data. It also proposes alternatives to incorporate these elements into the simulation model. Drilling strategies, including horizontal well drilling, and completion options are also presented.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery