Abstract: Source Rocks of Petroleum Systems in the South Atlantic Margins

Wiles, R. L. F. - BP Exploration London; A.S. Pepper - BP Exploration Houston; S.J. Cawley - BP Exploration Aberdeen

During the early Cretaceous to recent rift-drift history of the South Atlantic Basin, numerous episodes of organic rich mudstone (ORM) deposition occurred as a result of interactions between tectonically controlled basin morphology and climatic / ocean current conditions. These ORMs can be recognized in both industry wells on the present shelf-slope and academic research wells on the ocean floor. Geochemical oil-to-rock correlation allows these ORMs to be identified as being related to oil families encountered along both the South American and Southern African margins, establishing numerous petroleum systems.

We carried out paleo-reconstruction of the South Atlantic by restoring the ocean basin, along transform faults, on a free air gravity map. This allows us to broadly understand the structural and geomorphic settings under which ORM deposition occurred and interrogate the remarkable similarities and subtle differences seen in oil family groups on the conjugate South African and South American margins. The ability to understand source rock distribution in this way is a pre-requisite to predicting new petroleum systems in currently untested areas along the margins.

Our poster shows four paleo-reconstructions representing key ORM depositional episodes and the resulting source rocks and petroleum systems on the African and South American margins. The ORM depositional episodes are:

1. Lower Cretaceous freshwater lacustrine ORMs deposited in early syn-rift topographic depressions, through less tectonically influenced and more widely distributed saline lacustrine ORMs;

2. Aptian ORMs deposited in hypersaline environments associated with the rift-drift transition in the central South Atlantic;

3. variably carbonate-rich Albian ORMs developed most strongly along the restricted West African margin north of the Walvis Ridge in the early drift phase;

4. deposited during the later drift phase: sapropelic Upper Cretaceous ORMs resulting from widespread Late Albian-Cenomanian-Turonian (and locally younger) "oceanic anoxic events" and/or local upwelling; Upper Cretaceous through Paleogene marine ORMs influenced by local terrestrial run-off.

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