Interaction of Tertiary Deep-Water Depositional Systems with Actively Evolving Structures — Paleogeography of the Lower Congo Basin, Offshore Angola

Arlene Anderson¹, Donald K. Sickafoose¹, Tim R. Fahrer¹, Richard R. Gottschalk², Frank J. Goulding², Michael Porter², and Ian A. Watson^1
1ExxonMobil Exploration Company, Houston, TX
²ExxonMobil Development Company, Houston, TX

The Angola continental margin has undergone a complex history of gravity-driven deformation. The interaction of actively evolving structures and the depositional system is a primary control on Tertiary reservoir distribution and architecture. Local changes in gradient can lead to rapid lateral changes in reservoir distribution and geometry. Understanding this complexity and the related lateral variability as the Lower Congo Basin (LCB) evolved through time is critically important to focusing exploration efforts in the basin.

Along the Angola margin, up-dip extension and translation are accommodated by down slope contraction and extrusion of salt. There is also considerable lateral variability in structural style from north to south along the margin related to differences in 1) volume of salt, 2) magnitude of Neogene extension - contraction, 3) rapid lateral gradients in timing and magnitude of strain, 4) relief on the detachment surface, 5) focus and volume of sediment deposition, and 6) pre-existing mechanical variability within the deformed Cretaceous strata. Because Tertiary strata are syntectonic deposits, lateral variations in structural style strongly influenced the present-day distribution of reservoir sands on the LCB slope.

Deep-water clastic reservoirs in the LCB range from Rupelian to Messinian in age and were deposited on the LCB slope; they are generally organized into confined and weakly confined slope channel complexes. As the Angola margin evolved, sediment gravity flows funneled through a maze of active structures, forming multi-cycle stacked channel complex systems. In the distal reaches of the paleo-slope where the paleo-bathymetry was more subdued, the sands form single cycle, digitate anastomosing to distributary bodies. Sheet geometries are absent here simply due to the continued presence of a very low slope gradient.

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