The sedimentary basin of Gabon lies on the West coast of Africa. South of the city of Lambarene, the gabonese Southern Onshore basin is an oil producing area with major discoveries such as Gamba, Rabi Kounga and the recently put on stream Avocette field. Total proven recoverable reserves exceed 1 billion barrels. All the discoveries have been made beneath a salt sequence of Aptian age. Hydrocarbon occurrences are linked to the efficiency of a single "pre-salt petroleum system" whose major characteristics can be summarized as follows:
- Reservoir-rocks correspond to sandstone layers deposited either during the rift period or above a regional post-rift unconformity (Gamba Formation). The thick, high sand/shale ratio late rift deposits (Dentale Formation) provide, together with the Gamba sandstones, the best and thickest pay zones.
- Sealing mechanisms are related to:
-- lacustrine shale layers within the syn-rift succession
-- Aptian salt (Ezanga Formation) overlying a thin immature shale interval with poor sealing efficiency on top of rift sediments (Vembo Formation).
- Source-rock potential is provided by thick Neocomian to Aptian lacustrine shales deposited during the rifting phase of the incipient Atlantic Ocean. The highest organic contents correspond to the lower Barremian shales (Melania Formation).
- Hydrocarbon migrations are favored by both the extensive faulting of rift deposits and interlayering of sandstone drains and source-rocks.
- Trap formation seems to be mostly associated with post-rift reactivation of rift structural features. Maturation and fluid expulsion occurred in both pre and post-salt times.
AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France