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Abstract: Coastal Angola and Zaire: A Geochemical Contrast of the Lower Congo and Kwanza Basin Hydrocarbon Habitats

Ralph Burwood, Bernd Mycke

The Lower Congo Coastal and Kwanza provinces cumulatively account for reserves of ca 6 GBOR. These are dominantly reservoired in Pinda carbonate traps of the former basin. However, with production from a range of accretionary wedge, carbonate platform and Pre-Salt reservoirs, a diversity in oil character presupposes complex hydrocarbon habitats charged by multiple sourcing.

Each of these two major Atlantic margin salt basins constitutes a different, source rock driven, hydrocarbon habitat. As classic passive margin pull-apart basins, Early Cretaceous initiated rift events (Pre-rift, Synrift I, II, etc.) evolved into the Drift phase opening of the southern Atlantic. A striking feature of this progression was widespread evaporite deposition of the Aptian Loeme Salt. This separates two distinct sedimentary and tectonic domains of the Pre- and Post-Salt.

The core Lower Congo habitat is dominated by the Pre-Salt Bucomazi (!) petroleum system. These lacustrine, often super-rich, sediments reveal considerable organofacies variations between their basin fill (Synrift I) and sheet drape (Synrift II) development, accounting for the compositional diversity in their progenic petroleums. Of crucial impact is a cognate diversity in their kerogen kinetic behaviour. This controls the conditions and timing of generation and realization of charge potential.

With the Lower Congo habitat extending southwards to the Ambriz Spur, the Bucomazi facies proper appears restricted to the northern and deeper proto-lake trend. Over the more weakly subsident margins such troughs host inferior sheet drape potential. Elsewhere, the Upper Cretaceous-Paleogene marine clastic labe (!) petroleum system is hydrocarbon productive, yielding petroleums of unique, and/or mixed Pre-Salt, source provenance.

A lesser attractive Pre-Salt source system is reflected in the Kwanza habitat. Consequential effects of salt movement have proved important for halokinetic-induced structuration, Cretaceous block rafting and pseudograben (Tertiary Trough) source deposition. Marginal reserves of Cuvo(!) derived petroleums are known. With limitations in charge potential at this level, the modest Kwanza reserves (0.1 GBOR) have a Post-Salt provenance deriving primarily from the Eocene Margas Negras (!) and subordinate Aptian Middle Binga (!) petroleum systems. Cases of multisourced petroleums abound. In addition to organic-rich Albian micrites, Upper Cretaceous black shales provide unproven (?) offshore potential.

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