Potential deep water petroleum system, Ivory Coast, West Africa

Bird, Scott, Kirk Geno, Peter Van Slyke, Fred Strawson, Gonz Enciso, Ocean Energy, Ocean International Ltd., Houston, TX

Abstract

Recent data acquisition and interpretation has refined industry knowledge of the structural framework and source potential of the offshore Ivory Coast. A major offshore positive structural feature, termed the Grand Lahou high, is located south of and separated from the well-documented Lion-Quebec high by an additional syncline visible on seismic. Like the Lion-Quebec high, the Grand Lahou high is bounded to the south by south-dipping normal faults; sediments below the 98 Ma (Albian) unconformity dip northward. These sediments consist of Aptian/Albian clastics that grade up section from lacustrine to marginal marine. Overlying sediments dip regionally southward and consist of fluvial to shallow marine sediments deposited from 98 Ma to 90 Ma. Outer shelf and slope sediments were deposited from 90 Ma (late Turonian) to Miocene time. Major slope canyon systems developed in the Campanian-Maastrichtian and to a lesser extent during the Tertiary. Upper Albian and Cenomanian source rocks penetrated elsewhere in the basin have been confirmed along the Grand Lahou high, although at greater richnesses (up to 15% TOC) than previously encountered. Oil geochemistry from wells along the western end of the Lion-Quebec high also suggests the presence of a previously unrecognized Albian-Aptian lacustrine source rock.

All of the petroleum accumulations to-date in Cote d’Ivoire have been associated with faulting along the Lion-Quebec high. Hydrocarbons below the 98 Ma unconformity are found in fault traps or in sands just beneath the unconformity surface on the up-thrown sides of faults. Late Albian sands were deposited in half graben topographic lows on the 98 Ma unconformity surface. Stratigraphic traps are formed where these sands pinch out onto adjacent highs. Later Albian and Cenomanian sands have been deposited over the 98 Ma highs. Partial erosion of these shallow marine sediments by subsequent unconformities (96.5 Ma, 90 Ma, 88 Ma) results in additional reservoirs where the sands are not completely eroded from the pre-existing highs. Reactivation of the normal fault system has folded the post 98 Ma section, augmenting closure. On some features, drape over or later movement of the fault systems folded slope canyon reservoirs as high in the section as Maastrichtian, resulting in structural trapping of hydrocarbons. Gas prone middle Albian source rocks (Morrison et al., 2000) as well as oil prone Albian-Aptian and upper Albian/Cenomanian source rocks are believed to be mature in major synclines adjacent to the Lion-Quebec and Grand Lahou highs. Migration towards the major highs is assumed to be along carrier beds and up normal faults bounding these features.

Future deepwater discoveries may be found along the Grand Lahou high or in deeper basinal areas of the Grand Bassam sub-basin. Trap types and reservoirs along the Grand Lahou high would be similar to those found in shallower water along the Lion-Quebec high as Albian and Cenomanian fluvial and shallow marine sandstones are in fault and stratigraphic traps on Albian highs. Other potential accumulations may be found in stratigraphic traps associated with Campanian through Maastrichtian slope canyon systems, which have been identified in shallower water. These same canyon systems feed into the Grand Bassam sub-basin, and may compose slope and basin floor fan sand reservoirs.