A Petroliferous Transform-Margin Basin, Cote d'Ivoire, West Africa

John C. Harms, J. M. Bruso Jr., R. L. Wallace, J. A. Canales, and N. Koffi

Break-up transform margins, formed by large dominantly strike-slip faults as continents separate, are distinct in structural style and stratigraphic sequence from subduction or purely extensional margins. A continental margin defined by such a transform zone is sharp, precipitous, and places an essentially complete continental crust abruptly against oceanic or highly attenuated continental crust. Structure fields dominated by horizontal translation with an overprint of uplift and subsidence related to thermal effects of a laterally migrating asthenosphere plume. Stratigraphic sequences begin with relatively deep-water lacustrine deposits and are followed by marine conditions as ocean connections develop. Because bathymetry tends to be steep across the transform zone, mari e deposits along this zone represent slope environments with many erosional canyons and canyon fills, and these facies are vertically stacked through time.

The offshore Cote d'Ivoire Basin is an excellent example of a transform margin documented by more than 110 wells, an extensive 2-D seismic grid, a growing number of 3-D surveys, and several productive fields. The sedimentary section exceeds 5000 m, beginning with Aptian(?)-Albiar deep lacustrine facies. Marine incursion occurred in the Albian, followed by deformation, uplift, and erosion in later Albian. A series of major uplifts developed offshore along strands of the St. Paul fracture zone. The uplifts contain many SE-trending normal splay faults. The uplifts and NE-tilted fault blocks are the major petroleum targets within the Albian section.

Marine inundation occurred in the Cenomanian and deposition of narrow shelf, slope, and basin-floor facies followed. Subsequent loading and thermal subsidence caused only gentle structuring and rare listric faulting. Upper Cretaceous petroleum traps are mainly related to stratigraphic variations caused by submarine canyon cutting and filling.

The Cote d'Ivoire Basin provides a valuable model of transform margin processes and petroleum occurrences.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California