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Abstract: Untitled

Rene Vernet

The N'Kossa field, located offshore in deep water (170 - 300 meters water depth), is the most recent major oil field discovered in the Republic of Congo.

Although the structure was conspicuous enough to be identified by an early seismic survey carried out in the mid 1970's, the structure was not drilled until 1983. The results obtained by this first well, albeit encouraging, led to the discovery being classified as subcommercial when the water-depth was taken into account. Subsequently, appraisal works, including 2D and 3D seismic surveys and three delineation wells, confirmed the presence of major oil reserves and led to the present field development.

The sandy and carbonate reservoirs of the Sendji formation, Albian in age, are presently proven to trap a total oil and gas column up to 400 meters with producible reserves estimated at about 55 million tons (400 million barrels).

Using 3D seismic data and available development wells, the actual geometry of the structure may be described as a complex turtle-shaped feature extending over 30 km2. Regionally, the N'Kossa structure is surrounded by large Upper Albian to Cenomanian depositional troughs and minor Aptian residual salt ridges. The absence of Albian platform deposits in the area indicates an early formation of the structure and explains why it is draped by sealing shales. Local salt tectonics and possible regional raft tectonics are the proposed mechanism for structuring.

N'Kossa reservoirs are complex in nature: vertical and lateral variations are related to mixed siliciclastic and carbonate depositional environments. Such a complexity is better understood using high resolution sequence stratigraphy which has been used to model reservoir layering distribution and variations in reservoir properties. Observation of genetic units a few meters thick and definition of composite sequences using core descriptions and log analysis, have led to improve reservoir correlations and field evolution.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France