The Rio Muni Basin underlies the continental shelf of the west African Republic of Equatorial Guinea, located between Gabon and Cameroon. The basin is located above a zone dominated by northeast-southwest trending oceanic fracture zones and their continental extensions. These constitute the boundary between the Equatorial Atlantic margin and the west African salt basin. The tectonic setting has resulted in the Kio Muni Basin exhibiting a complex tectonostratigraphy with characteristics of both the passive margin style of Gabon and Angola and the more transform-dominant style of the northern Gulf of Guinea.

Exploration history

Despite its location between the prolific petroleum provinces of the Niger delta to the north and the Gabon coastal basin to the south, the Rio Muni Basin has been overlooked by the industry for much of the last decade. Six wells were drilled between 1968 and 1991 in the shelf and onshore areas of the basin. Focus was on Albian and Aptian objectives. Although they proved a viable source rock, they did not result in the discovery of any accumulations. In 1997, following a review of the west African margin, Triton Energy licensed Blocks F 81 G, which comprise an area of 5270 km², covering both the shelf and deepwater areas in the central part of the basin. Following acquisition and interpretation of new 2D seismic data, the Ceiba 1 exploration well was drilled in 1999 and proved a working hydrocarbon system in the postrift sequence. This establishes a new high-potential oil play in this part of the Gulf of Guinea.

Figure 1. Blocks F&G cross section.

End_Page 15---------------

Tectonostratigraphy

The tectonostratigraphic evolution of the basin can be divided into a number of separate phases. To date, no wells have penetrated strata older than middle to late Aptian and clear definition of the nature and age of an early rifting or transtensional phase is lacking. Despite this, there is limited seismic evidence to support differential subsidence of possible early to mid Aptian age. It is overlain by a thick, upper Aptian continental/lacustrine succession followed by Albian-Cenomanian marine section. The upper part of the succession appears to be regional in extent and is likely the result of postrift subsidence. Following the onset of postrift subsidence, the Albian-Cenomanian sequence was locally deformed into a series of rafts that detach on a basal Albian, organic-rich claystone section. Further deformation by Santonian-Coniacian transpression caused uplift of the shelf area and deposition of an Upper Cretaceous slope fan sequence. This is overlain by a Tertiary passive margin wedge, deposition of which was punctuated by renewed uplift and erosion at the base of the Neogene. Present-day basin architecture is dominated by a series of NE-SW trending highs and lows that developed in response to the basal Senonian and Neogene events. The highs resulted initially from Santonian-Coniacian inversion of Aptian deposits underlain by the continental extension of NE-SW fracture zones. In addition, salt deformation of rafted deposits and the development of a base-of-slope compressional belt are evident in the deeper water portions of the basin.

Petroleum geology

Upper Cretaceous tectonism and base level change resulted in the development of turbidite reservoir sequences associated with a series of Upper Cretaceous sequence boundaries. These turbidites form the reservoir in the Ceiba discovery, which was tested at a rate of 12,400 BOPD. Both Upper Cretaceous and Tertiary turbidite reservoirs form the major exploration targets in the basin and are situated in a variety of structural and stratigraphic trap types. These exploration targets arc thought to be accessible to a deepwater source kitchen from which earlier shelf wells were shadowed.

Figure 2. Blocks F&G Location map.

End_of_Record - Last_Page 17---------------