--> Abstract: Angolan Atlantic-Margin Petroleum Systems: Pre- and Post-Salt Source-Rock Control, by R. Burwood; #90933 (1998).

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Abstract: Angolan Atlantic-Margin Petroleum Systems: Pre- and Post-Salt Source-Rock Control

Burwood, Ralph - Fina Exploration Ltd.

Recent deepwater license awards and frontier drilling have re-vitalized interest in Angolan margin prospectivity. Existing hydrocarbon habitat and play concept considerations are described In "petroleum system" terms thereby providing a critical benchmark for the ongoing exploration (Fig. 1).

Basins in the contiguous Lower Congo Coastal andKwanza provinces currently account for cumulative reserves exceeding 6 GBOR, largely reservoired in Pinda carbonates of the former area. Elsewhere productive aggradational wedge, carbonate-platform and Pre-Salt traps host oils of disparate character supporting a diversity in contributory, source rock(s) and petroleum systems. These two basins constitute discrete source rock-driven hydrocarbon habitats deriving from facets of the Early Cretaceous-initiated opening of the southern Atlantic. Prominent here was the widespread evaporitic deposition of Aptian salt (Loeme Fm.) which separates two distinct sedimentary-tectonic episodes of the Pre-Salt, lacustrine, rift phase (Synrift I, II) from the marine, drift phase, Post-Salt sequences.

The core of the Lower Congo habitat Is dominated by the Pre-Salt Bucomazi Fm.-sourced "poly" petroleum system. Often super-rich and thickly developed, these sediments reveal marked organofacies and kerogen kinetic variations between their Synrift I (basin fill-ORZ) and II (sheet drape) sequences as observed in the type section well CABGOC 86-1. These constraints mediate the realization of hydrocarbon potential and control the compositional diversity of the resultant progenic oils. In this way the composite provenance of the super-giant Malongo trend oils (ORZ dominated) can be differentiated from the more modest Late Bucomazi-derived coastal Zaire and N.W. Angola plays (Tschiende, N'Zombo fields etc).

Southwards, and in deeper water occidental areas, less extensive Pre-Salt source potential is thinly developed (e.g., Guelengue-1) but still remains prospective as witnessed by charge to the Kiame and Kiabo fields. Additionally, more prolific Late Cretaceous-Paleogene labe Fm. marine clastic-sourced petroleum system(s) are productive yielding oils of unique (e.g., Bufalo, Cavela and Impala, etc.), and/or mixed Pre-Salt hybrid (e.g., Sulele, Palanca, Pacassa), provenance. 18(a) Oleanane abundance, being prominent in the labe (Tert.) derived oils, helps differentiate between the relative Late Cretaceous versus Paleogene source contributions.

South of the Ambriz Spur onshore Pre-Salt (Infra-Cuvo, Cuvo Fms) potential (e.g., Maculungo-1) becomes more tenuous. The offshore wells Lukunga-1 and Falcao-1 again show intermittently developed source horizons of variable organo-facies signature. Shows ascribable to such sourcing (e.g., Mubafo, Cegonha through Ametista to Mucua), in addition to marginal onshore production (Luanda, Cacuaco fields), have been observed. Elsewhere, Post-Salt sourced petroleum systems (Middle Binga and Margas Negras Fms), yielding both individual (e.g., Tobias and Quenguela North fields, respectively) and hybrid oils, set the scene. Organic-rich Albian micrites and Late Cretaceous-Paleogene black shales provide additional, as yet unproven, offshore potential.

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