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Abstract: Enhancement of a Hybrid Petroleum System by Raft Tectonics: Example of Onshore Angola

Carlos Cramez, Ghislain Choppin De Janvry, Bernard C. Duval, J. L. Pittion

Northern Angola shelf area is part of the western Lower Congo basin.

The Pinda formation, containing most of the area oil and gas reserves, belongs to a generally transgressive carbonate sequence with a marked backstripping configuration. Three main sources were identified, the lacustrine Pre-salt Bucomazi formation, largely present in the gas window, the rich marine Upper labe formation (Senonian and Paleocene), and the Tertiary Malembo, of lower quality. Integrated geochemical studies and basin modeling showed an early generation from Pre-salt rocks in the deepest grabens, while generation from the labe and Malembo, and the final mixed charging of traps, is very recent.

This hybrid petroleum system is strongly enhanced by salt tectonics.

Firstly, a combination of large open windows in the Aptian salt and intense faulting in the updip ends of fault blocks facilitated early vertical migration from the Bucomazi source rock.

Secondly, hydrocarbons are trapped in Pinda structures related to early halokinesis due to Aptian salt. These salt movements caused the segmentation of the Pinda platform and the development of allochthonous blocks or rafts.

Thirdly, the main reservoir developments of the Pinda formation are related to high energy carbonates accumulated on structural positive features.

Fourthly, the respective roles of vertical and lateral migration are controlled by the degree of separation of the rafts, with more lateral migration taking place in the preraft more "connected" easternmost domain.

Finally, hydrocarbons of Tertiary origin are linked to depocenters, where lateral space was created to accommodate the extension.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela