Abstract: Regional Evaluation of the Pre-Salt System of Cabinda, Angola

Orsolini, P. - Elf; J. R. Scheevel - Chevron; P. Jeronimo - Sonangol; V. Barletta - Agip

The Pre-Salt sedimentary section of Eastern Cabinda (Area A), is located in the Lower Congo Basin. The Stratigraphic column consists of Neocomian to Upper Barremian lacustrine siliciclastic and carbonated rocks deposited on top of the metamorphic Mayombe Basement during the Pre-Atlantic continental stretching phases. The end of crustal extension is marked in the late Barremian/Early Aptian by the development of a major regional erosional unconformity overlaid by the thin and laterally extensive sandy Chela formation and the Upper Aptian Loeme Salt formation.

Over 500 MMBO have been produced from Pre-Salt formations in the last 25 years from sandy and carbonated reservoirs. Additional exploration objectives are still being recognized with new improved seismic imaging.

A reevaluation of Cabinda Pre-salt Petroleum System based on a comprehensive well dataset, an improved time equivalent framework provided by Palyno/Ostracods zonations and recent high quality seismic surveys has been performed by a four geoscientist team provided by the J.V. partners in Cabinda: 1. Sonangol, 2. Chevron, 3. Elf Aquitaine Production, 4. Agip. The group worked in San Ramon (California) for a 15-month period. Each Company provided specific technical assistance based upon their "in house" expertise or innovative tools.

Cabinda petroleum system can be briefly described as follows:

- Reservoirs. They correspond to fluvial or lacustrine shoreface sandstone and nearshore lacustrine diagenetically enhanced carbonates. The Sandy Lucula formation is the largest producer followed by the Toca carbonates. Two type of Lucula sandstone have been differentiated based on their age and depositional /structural setting.

- Seals. They are provided by the Loeme Salt and the silty dolomitic shales of the Bucomazi formation.

- Traps. The closures are formed primarily through a combination of rotated block highs and stratigraphic or erosional pinch outs.

- Source rocks: organic lacustrine shales, siltstone and carbonates with TOC averaging 7% (up to 30%) and type I kerogen.

- Timing of oil migration. The geochemical modeling establishes that the onset of oil generation and migration started in the Upper Cretaceous. This process has been enhanced by Tertiary sediment downloading and is still active today. The Middle Bucomazi Section which is the richest potential source rock in the area is still immature or marginally mature in Eastern Cabinda. The Bulk of the oil in place in the Pre-Salt is therefore assumed to derive from the lower part of the Bucomazi section. Oil fingerprints establish that all Post Salt oil accumulations in Eastern Cabinda have a Pre-Salt origin and is therefore migrated updip through openings in the Loeme Salt regional seal.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil