Print this pageThe Toca Carbonate Play in the Congo Basin, Equatorial West Africa: Controls on Facies and Reservoir Quality
HARRIS, NICHOLAS B.
The Toca Carbonate is a lacustrine carbonate in the Congo basin, offshore Congo and Cabinda (Angola). Though it produces oil at high rates in a few wells (at Takula field in Cabinda and the Viodo Marine 1 well, Congo), it is unpredictable with rapid facies and reservoir quality changes over short distances. The Toca occurs within the upper part of the pre-rift sequence, the non-marine suite of rocks deposited in the evolving rift basin between Africa and South America in the early Cretaceous.
Examination of cores from 18 wells indicates that the Toca occurs at 3 or more distinct stratigraphic levels: two within the lacustrine calcareous shales known as the Marnes Noires, the major hydrocarbon source rock in the basin; and one within the overlying shale, the Argilles Vertes. The lowermost Toca interval consists almost entirely of gastropods and coated grains, whereas the uppermost Toca consists largely of bivalve shells. The middle Toca is a complex mixture of components found in both the upper and lower units.
The evolving lithology of the Toca affected both its distribution within the basin and its reservoir quality. Carbonate material generally formed on structural highs, but the gastropods and coated grains comprising the lower Toca never lithified in shallow water; instead they avalanched into deeper water adjacent to the highs. The bivalve shells lithified on shallow water platforms, thus carbonate in the middle and upper Toca intervals is preserved on the structural highs. Examples of core and seismic data are shown.