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Sendji Formation reservoir delineation based on 2-D and 3-D inversion, Yombo Field, offshore Congo

CMS Energy, Houston, Texas, U.S.

Corresponding author: J. Van Horn, jvanhorn@cmsenergy.com

The first 20% of the full text of this article appears below.

Seven fields with prolific Sendji Formation production have been discovered offshore Congo. These fields have ultimate recoveries of 100 million b/o or greater. Only the shallow waters (less than 200 m) of the play have been adequately explored. The inability to predict the amount of economically recoverable oil after initial discovery and delineation is a major production challenge. The Sendji Reservoir is a complex sequence of limestones, dolomites, and sandstones deposited as several distinct facies. These facies vary both laterally and vertically, and have vastly different reservoir characteristics. Predicting the configuration of these facies is the key to predicting reservoir performance and calculating the amount of oil that is economically recoverable. Predicting intra-Sendji facies distribution is a significant challenge to optimizing development drilling.

The largest oil field in the Congo is one example of this production challenge. N'Kossa Field, discovered by Elf in 1984, was estimated to have 900 million barrels of oil and condensate in place. Initial delineation drilling and testing supported a recovery factor assumption of 47%, meaning more than 400 million barrels could be economically produced. Yet N'Kossa has significantly underperformed and will probably produce only 300 million barrels, or 33% recovery. This shortfall is due to reservoir heterogeneity revealed only after producing several development wells.

CMS Oil and Gas operates Yombo Field within the Marine I Exploitation block, which is in the Lower Congo basin offshore the Republic of Congo (Figure 1). Sendji reservoir heterogeneity is experienced in Yombo Field. Amoco discovered Yombo in 1988. Five initial wells were drilled to delineate the field. Commercial pay was discovered in both the Cretaceous Cenomanian-age Tchala Formation and the Albian-age Sendji Formation. Yombo currently produces about 16 000 b/d. Production is almost evenly split between the two formations.

View larger version (108K): [in this window] [in a new window]   Figure 1. Yombo Field lies within the Marine I Exploitation . . . [Full Text of this Article]