Abstract:
A 3D seismic data set was used to analyze the structural framework and depositional systems of a Cretaceous extensional basin adjacent to a Cretaceous strike-slip margin active during the opening of the Equatorial Atlantic Ocean. 1800 km2 of data was collected by Petroleum Geo-Services in 1986 and covers blocks CI-104 and 105 offshore of the Ivory Coast. The complex structure in the area of interest is a prolongation of strike-slip faulting along the St. Paul fracture zone to the southwest and is similar to rotated fault blocks described by previous workers on the eastern side of the Ivory Coast basin. Previous studies and the Ocean Drilling Project (ODP) cores near the Ivory Coast-Ghana Ridge east of my study area show the slope and base-of-slope sedimentary rocks to be primarily siliciclastic. Four tectono-sedimentary phases were identified and related to Cretaceous extensional tectonics: 1) pre-rift Aptian lacustrine phase, 2) Aptian-Albian active rift phase, 3) late Cretaceous thermal uplift phase, and 4) late Cretaceous and Tertiary post-rift passive margin phase. The Aptian pre-rift and Aptian-Albian syn-rift phase exhibit tilting and strike-slip of the fault blocks during the rifting. The thickness of the pre-rift sedimentary rocks total 0.4 seconds two-way-time (TWT), while the post-rift rocks average 5 seconds TWT in the central basin. The geometric patterns of the structural features were analyzed and show similarities with observed and modeled strike-slip tectonics in other areas. The late Cretaceous and Tertiary post-rift deposits were transported by gravity into the base-of-slope environment within a narrow graben formed between tilted rift blocks. Seven, laterally-extensive unconformities were interpreted from seismic data and are tied to the 3492-m-deep K1-2X well. The age of the unconformities corresponds to the age of regressions identified on a eustatic sea level curve for the Gulf of Guinea. These unconformities separate the Jurassic pre-rift sequence, Aptian-Albian sequence, late Cretaceous sequence, Paleocene-Eocene sequence, Oligocene sequence, early Miocene sequence, and Upper Miocene-Pleistocene sequence. Post-rift depositional facies (e.g. slumps, channel complexes, drapes) and erosional features (e.g. canyons, bypass unconformities) were imaged in the seismic survey. Chaotic, convergent, and drape seismic facies differentiate among sedimentary facies and were identified and mapped using seismic facies analysis. The post-rift paleogeographic maps show a submarine channel complex, interpreted to provide the bulk of the sandy reservoir facies, flowed southwesterly during the late Cretaceous and early Tertiary and changed to a more southerly orientation in the Upper Miocene-Pleistocene sequence.