Electrical resistivity tomography of the Douala-Massoumbou Paleocene- Eocene aquifer (Cameroon Atlantic Margin)

Abstract

The Paleocene aquifer of the Douala-Massoumbou sub basin is a rhythmic sequence of sand and shale deposits. Resistivity pseudosections and profiles of half-distance between current electrodes of 350m were acquired at 20 different sites of the Douala-Massoumbou subbasin. These data coupled with mud and gamma-ray logging of deep groundwater boreholes led to the identification of two major sand sequences through the  Paleaocene-Eocene stratigraphic section. The upper sequence associated with Ypresian 30-50 m thick unconformity made up with semi-conductive records, is punctuated by lenses of substantially water bearing sand (WBS). While the lower sequence locally associated with H2S and iron pollution plumes, displays high conductive records. In such sequences, the  distribution of sand and clay deposits settled by channel incision appears fundamental in predicting reservoir geometry and the hydrological potential of the Paleocene WBS. Keywords : Sag basin, electrical tomography, Paleocene formation, water-bearing sand. L’aquifere du Paleocene du sous-bassin de Douala-Massoumbou est une sequence rythmique de depots de sable et d’argile. Des sondages geoelectriques tripole (OA=350m) et des tomographies de resistivite ont ete acquis sur 20 sites differents de la zone de faible enfouissement de cet aquifere. Ces donnees, couplees aux resultats des diagraphies gamma-ray et des deblais des forages hydrogeologiques profonds, ont permis d’identifier deux sequences de sable hydrostratigraphique au sein de l’aquifere paleoceneeocene. La sequence superieure semi-conductrice correspond a depots de sable aquifere sain, epaisse de 30 a 50 m et associes a la discordance ypresienne. La sequence inferieure tres conductrice, correspond a des chenaux de sable aquifere pollue par le sulfure d’hydrogene (H2S) et le fer. La mise en correlation de l’ensemble des resultats obtenus a permis de constater que la prise en compte du developpement spatial des chemins hydrologiques fossiles est un critere fondamental pour la predire la distribution et l’exploitation des niveaux de sable aquifere sain du Paleocene du sous-bassin de Douala-Massoumbou. Mots cles : Bassin de fosse d’effondrement, tomographie electrique, Formation paleocene, Sag basin, electrical tomography, Paleocene formation, niveaux de sable aquifere sain.