Abstract: Unsnarling of Petroleum Systems Through Geochemical Methods, Offshore Cabinda, Angola

Schoellkopf, N. B.; B. A. Patterson - Chevron Overseas Petroleum Inc. and J. M. Gaulier - IFP

Offshore Cabinda (Block 0) oils arise from three prolific source rocks: the lacustrine Lower Cretaceous Bucomazi Gp., the marine Upper Cretaceous/Lower Tertiary labe and Landana Fms., and the marine Tertiary Malembo Fm. Oil-to-source relationships provide working hypotheses of hydrocarbon generation, migration and entrapment that can be tested using basin modeling.

Bucomazi-sourced oils have unique isotopic signatures, high gammacerane/ hopane and C26/C25 tricyclic terpane ratios, and lack C30-steranes. Rich Type I Bucomazi kerogens are major contributors to pre-rift, syn-rift, and post-rift reservoirs throughout Block 0. Modeling reveals that large generative depressions in the Takula, Area C/A and Malongo sub-basins have charged the fields of Area A.

Iabe-Landana and Malembo oils are easily differentiated from lacustrine oils, but difficult to separate from each other. Marine oils differ isotopically from lacustrine oils, contain C30-steranes, variable amounts of oleanane and have low gammacerane/hopane and C26/C25 tricyclic terpane ratios. Iabe-Landana source rocks contain excellent Type II kerogen; in Block 0, only the lower labe is mature.

Malembo oils typically contain more oleanane and have slightly lower gammacerane/hopane and C26/C25 tricyclic terpane ratios. The lower Malembo comprises Type II to II-III kerogen with good-to-moderate source potential in westernmost Block 0. This section is mature in deep Tertiary troughs and is a major source of oil into adjacent Pinda structures.

Maturation models confirm that many Area B and C oils are of mixed origin. Oil and gas from the Bucomazi mix with labe and Malembo-sourced oils to fill post-salt reservoirs.

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