The depositional regime on the abyssal plain of the Congo Fan in Angola

Evans, D. G., WesternGeco, London

Abstract

The Miocene channel play of offshore Angola contains the recent giant discoveries of Girassol, Dahlia, etc. From recent 3D surveys that extend across the salt front onto the abyssal plain in the outer Congo Fan, it can be seen that this play can now be continued out into the ultra-deep water into a new highly prospective frontier area. This presentation covers this area’s prospectivity and the use of the models generated here in the salt province.

The Congo Fan of Angola shows evidence of extensive sand deposition from the Oligocene onwards. This sand deposition extends beyond the current exploration targets in the salt diapir province onto the abyssal plain, an area that has been covered by recent 3D seismic data sets.

By using these data, the morphology of differing styles of sand deposition can be shown in a variety of ways including amplitude extraction, timeslices, opacity, and 3D visualization. All these methods work well in this area because the strong amplitude/acoustic impedance contrast between sand and shale is combined with the approximately flat-lying nature of the abyssal plain. These factors allow the depositional morphologies to be shown without the influence or bias of interpretation.

In this province of offshore Angola it can be demonstrated also that the Cretaceous and early Tertiary on the abyssal plain shows little or no seismic evidence of sand deposition. However, this section is a prime candidate to contain mature source rocks, the evidence for which can be demonstrated by the clear ‘DHIs’ seen in the overlying sands. Micro-faults that act as hydrocarbon migration conduits can be seen on the data.

The abyssal plain is at first sight the ultimate location for the ‘subtle’ (i.e., stratigraphic) trap, however, closures can also be shown to exist as drapes over basement features and allochthonous salt bodies.

The models generated on the abyssal plain can be applied in the far more heavily structured salt diapir province, to provide an understanding of the channel and fan systems in this area. The multiple DHIs here, combined with numerous sands and four-way dip closures suggest that this is a highly prospective area.