Rabi-Kounga Field, Southern Gabon

M. G. A. Boeuf

The Rabi-Kounga field recently discovered by Shell onshore Gabon is located at 2°S lat. in an area of central Africa covered by dense, uninhabited, primary rain forest, 140 km south of Port Gentil, the country's economic capital.

The field lies mostly within the Sette Cama Block where Shell Gabon is operator on behalf of an association in which the partners are Shell Gabon, Elf Gabon, and SNEA. As presently defined, the Rabi-Kounga field is 12 km long and 3-4 km wide, extending northward into the Dianongo Block operated by Elf Gabon for the same association. With an expectation of 1,300 million bbl (STOIP), it appears to be the largest oil accumulation discovered in Gabon.

The hydrocarbons of the Rabi-Kounga accumulation are contained within the reservoir sands of the Gamba and Dentale Formations, which form part of a rift basin of Early Cretaceous age, the Dianongo basin. The cap rocks are late Aptian evaporitic deposits (Ezanga Formation). The salt deposits and their thin underlying clastics (Gamba Formation), which unconformably overlie the continental Dentale sequence, bear evidence of the first marine influence and reflect the phase of continental separation between Africa and South America. Post-salt sediments are fully marine, corresponding to a phase of sedimentary progradation. They are represented by carbonates and clastics that were deposited on a subsiding continental margin.

Trapping in the Rabi-Kounga field is provided by structural closure at the base of the Ezanga salt, where Gamba and Dentale reservoirs are in fluid communication. Such closures at the base of salt are difficult to define in the Dianongo basin, as a complex seismic velocity distribution in the overburden related to halokinesis, salt dissolution, and clastic infill distorts the seismic time expression below the salt. The complexity of the overburden combined with the limited amount of well velocity data make accurate time-to-depth conversion difficult, and structural mapping at the base of salt is therefore extremely vulnerable to errors in depth conversion. The field was discovered in 1985 on the basis of reinterpretation of seismic data acquired between 1970 and 1977 and the integrati n of 1981-1982 data that showed a marked improvement in quality and resolution.

More than 20 years have elapsed between the two most significant discoveries ever made onshore Gabon: Gamba-Ivinga in 1963-1967 and Rabi-Kounga in 1984-1985. During this time, efforts were made to improve structural control over an area that remains difficult from both a seismic and a logistic point of view. Many disappointments were experienced, but success has finally been achieved as a result of perseverance combined with the use of state-of-the-art seismic technology, all favored by a long concession life.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.