Depositional Environments and Reservoir Potential Assessment of the Paleocene Ewekoro Formation, Dahomey Basin

The Paleocene carbonates of the Ewekoro Formation exposed in Ewekoro and Shagamu areas have been mapped and logged. The total thickness of the formation varies form one area to the other. In thickness of ca. 12m and in the Shagamu quarry, it measured a thickness of ca. 23m. The limestones consist of grey to dark grey nodular to massive bodies. In general, it is highly fossiliferous with increasing sandy components toward the base of the formation. Petrographic study of the thin section and the polished slabs reveal at least five carbonate microfacies including bioclastic packstone, sandy bioclastic packstone, bioclastic packstone grainstone, algal bioclastic grainstone and oncolitic grainstone. These rock types consist of many bioclastic grains and few non-bioclastic grains. Corals, algae, echinoderms, gastropods, pelecypods and bryozoans represent the main bioclastic components while quartz, lithoclasts and few peloids represent the non-bioclastic components. Benthic foraminifera, few ostracods, carbonaceous organic matter and detrital material are present in the associated shale beds. The effects of diagenesis had led to reduced porosity and permeability in the carbonate rocks. This has given the rock a poor reservoir potential. Integration of sedimentologic, petrologic and paleontologic data from the study areas suggest that the carbonates are deposited in shallow marine shelf lagoonal environment during the Maastrichtian Paleocene marine transgression in to the basin with many short lived changes