Abstract: Integrated Deterministic Approach in Reservoir Simulation: Example of the Albian Nkossa Reservoir - Offshore Congo

BIONDI, PATRICK, FREDERIC WALGENWITZ, CATHERINE BAGLINIERE and HUGHES GREDER

The 400m-thick Nkossa reservoir is a typical example of mixed carbonate-siliciclastic platform deposits. The high degree of reservoir heterogeneity is due to a large range of lithologic compositions, to high frequency elementary depositional sequences and to a high frequency diagenetic overprint (stratigraphic-controlled dolomitization).

The sedimentological model corresponds to a carbonate ramp during the periods of high accomodation space, and to a mixed carbonate-siliciclastic ramp during the periods of low accomodation space. The best reservoir layers are located in the inner to mid ramp deposits, whereas the seal is made of the outer ramp deposits. The diagenetic model, set up from isotopic data and log interpretation, corresponds to a brine reflux dolomitization. The lateral shift of dolomitization front related to base level changes during the Albian transgressive megasequence, both accounts for the vertical and lateral distribution of dolomite, and for the mean rate of dolomitization.

The close link between depositional settings and dolomitization allows a deterministic approach to be used to assess the high degree of reservoir heterogeneities. This approach is based upon core-log calibration and stratigraphic correlations of 18 wells. Fifty-three elementary sequences (5 to 15m thick) were defined and led to an equal number of facies, diagenesis, petrophysics and barrier maps. The 3D integration of layer characteristics with structural data has also been done prior to reservoir simulation.

The purpose of this contribution is to illustrate that a deterministic approach proved to be an efficient way for reservoir modeling.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria