Abstract: Exploration for Deep-Water Reservoirs, Offshore Angola

Raposo, António J. M. - Sonangol; Mark A. Sykes* - Exxon Exploration Co.

Seismic survey data from the deep-water Lower Congo Basin, offshore northern Angola, show that the area potentially contains significant sandstone reservoirs. The basin extends from offshore Congo Republic to the Baía do Bengo, Angola. Up to 12,000 meters of sediment has been deposited in the basin since its inception in the Cretaceous. The sedimentary succession reflects continued subsidence throughout this period and comprises Portlandian-Barremian fluvial and lacustrine deposits overlain by a thick Aptian-Recent marine section. The onset of temporal incursions of the proto-Atlantic Ocean into the subsiding basin is denoted by the occurrence of salt. Subsequently, continued subsidence permanently submerged the basin and fully marine strata were deposited, comprising early paralic and nearshore deposits overlain by offshore mudstones, siltstones and deep-water sandstones, were laid down.

Since the middle Oligocene, three agents have promoted deep-water sandstone deposition in the Lower Congo Basin. Firstly, uplift of the West African coast has steepened the continental margin hereabouts and exposed crystalline basement rocks at the basin edge and in adjacent fluvial catchments. Secondly, northward drift of Africa has caused the basin hinterland to pass from a climatic zone to a more humid one, promoting erosion and delivery of sediment to the basin. Thirdly, the development of the Antarctic ice cap, and ensuing periodic glaciations, have significantly lowered and fluctuated sea level respectively, facilitating clastic transport into the deepest parts of the basin.

A significant amount of seismic survey data has been obtained over the Lower Congo Basin in recent years. The presence of bright, high-amplitude reflections in the late Oligocene and Miocene succession in these data suggest the presence of deep-water reservoirs. Three types of deep-water reservoirs are observed. Firstly, confined channel complexes, which appear as linear, frequently erosive features exhibiting internally discontinuous high-amplitude seismic character. Secondly, overbank deposits, which exhibit more uniform, concordant, and internally continuous seismic character. These overbank strata were deposited as sediment gravity flows that escaped the banks of their confining channel and spread sand over the surrounding sea floor. Thirdly, leveed channels, which also exhibit conformable seismic reflections but which have a constructive geometry that results from the build-up of the levee crest above local topography. Modern examples of sandy sediment gravity flows in Angola have been documented from the Congo Canyon, where they have been responsible for the breakage of Luanda-São Tomé telephone cables.

The deep-water reservoir sandstones in the Lower Congo Basin appear similar to examples found in the North Sea and the Gulf of Mexico subsurface. Because these basins have been densely drilled and extensively studied, they provide useful insights into the likely nature and performance of Lower Congo Basin deep-water reservoirs. Additional analog information is provided by modern deep-water channels from the Mississippi, Amazon, and Indus submarine fans.

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