Tectono-Sedimentary Controls on a Mixed Clastic-Carbonate Shallow Marine Reservoir: Lower Sendji Carbonate (Albian), N’Kossa Field, Offshore Congo

Wonham, Jonathan, Untung Ashari, Lucile Schuwer, Marie-Corinne Devilliers, Thang Nguyen, Total, Paris La Defense Cedex, France

The N’kossa field reservoir is more than 400 metres thick and produces from a number of different lithologies of the Lower Sendji Carbonate (Albian) including sandstone, dolomite and grainstone intervals.

The reservoir overlies mobile Aptian salt. Post-reservoir development, the rafted block which defines the field underwent a lateral translation of 25 km, gliding on salt. Today, the reservoir is subdivided into a number of panels which mainly reflect fault movement during the early phase of rafting.

With more than forty wells, the lateral reservoir variability is now well understood and proves to closely reflect relationships between syn-sedimentary tectonics and deposition/early diagenesis. Three structural unconformities subdivide the reservoir into discrete phases of deposition marked by differences in depositional character and thickness trends.

Dolomite is a major reservoir type. It was developed in relation with the development of anhydrite over sabkha tidal flats bordering a lagoon. Dolomite was preferentially developed over structural highs and passes laterally into compact limestones in areas where accom­modation space creation was more rapid.

To be correctly understood, these structural-sedimentary relationships need to be placed in the context of an evolving structural milieu which was transformed initially by salt move­ment and later by accelerating extension on listric faults.