Abstract: The Tchibouela Field, Lower Congo Basin: A Seismic Approach to the Understanding of its Trapping Mechanism

Brandon C. Nuttall, Terence Hamilton-Smith, Michel Nutten, Daniel Pelerin, Patrick Sorriaux, Rene Vernet

The Tchibouela oil field, located offshore in the lower Congo basin has been discovered in 1984 and has been developed in 1987. It is presently the major producing field in Congo with an output of roughly 50,000 bbl/d and initial recoverable reserves exceeding 200 million barrels.

This paper is intended to report how the interpretation of seismic data prior to discovery has been enhanced by a careful analysis of amplitude anomalies and associated pull down effects.

The first Tchibouela well (TBM.1) was drilled in 1975 to pre-salt objectives (Toca limestones). Albeit dry at the objective level, this well found minor oil shows in post-salt series (Albian and Cenomanian) where no structural closure had been previously reported.

Eight years later, thanks to improvement in seismic acquisition and processing, seismic anomalies were detected to the South East of that well and subsequently interpreted as gas occurrences at stratigraphic levels assumed to be of Senonian and intra-Cenomanian age. The upper anomaly was thought to be the origin of significant pull down effects.

A corrected depth conversion evidenced a large structural closure previously overlooked at these levels. A well (TBM.2) drilled in 1984 to these objectives eventually confirmed this interpretation and became the discovery well of the Tchibouela field after testing small quantities of gas in the Senonian and oil in the Turonian and Cenomanian series.

On the other hand, this well showed also that the lower intra-Cenomanian amplitude anomaly was related to lithologic variations rather than gas occurrence, namely a local dolomitization of sands deposited within a coastal environment. The thickness of this dolomitic heterogeneity varies from a few feet on the edges of the field to more than three hundred feet at the top. This possible permeability barrier proved not to be watertight and its impact in relation with the water drive mechanism of the field will be discussed.

AAAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994