ABSTRACT: RABI Field, Gabon, West Africa--Impact of Horizontal Well Technology on Appraisal and Development of a Giant Oil Field

LAK, BARBARA, Shell Gabon, Gabon, West Africa

The RABI field was discovered by Shell Gabon in August 1985 and put on stream in January 1989. The field is located near the equator in an area covered by dense rain forest. The estimated STOIIP is 222 million st cubic meters (1.4 billion STB), contained in an oil rim 46 m (150 ft) thick.

The reservoir is composed of unconsolidated, highly permeable sands belonging to two formations of Early Cretaceous age. The lower Dentale Formation, deposited in a continental rift system, consists of a fluviolacustrine sequence of alternating sands and shales, the latter acting as baffles and barriers to flow. It is unconformably overlain by the thin transgressive sequence of the Gamba Formation, through which pressure communication between the individual subcropping Dentale reservoir units is achieved.

The field forms an anticlinal structure, which is severely faulted along the crest. It is sealed by the overlying evaporitic Ezanga Formation, which forms salt domes over the crest of the structure. Seismic definition of most of the field is poor owing to the thick Ezanga evaporites and associated younger clastic infill zones occurring in the dissolution sinks above the salt domes. The definition of the field is therefore mainly based on well results, with seismic data contributing only on the flanks where the salt sequence is thinner.

Appraisal drilling and the first phase of development drilling were performed with conventional vertical wells. Since 1990, horizontal drain holes with completed sections of up to 1000 m have become the standard for development wells. As a result of their long horizontal reach, some of these wells also fulfilled a distinct appraisal function, resulting in better flank definition. Application of horizontal well technology has made it possible to cluster well locations, improving the economics of the field while at the same time minimizing the impact on the environment.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)