Sediments of the Gabon basin, 16,000–18,000 m thick, range in age from Early Cretaceous, or perhaps latest Jurassic, to recent. A salt layer of late Aptian age separates the almost completely continental facies of the presalt or Cocobeach sequence from the largely marine postsalt sediment.

Since its inception, the basin has taken the form of a half graben whose eastern edge consisted of a series of three, or perhaps more, hinge zones which migrated successively farther west and which controlled the distribution of depositional environments and facies changes. The recognition of the hinge zones is of great importance in predicting reservoir trends. In its central part, the basin was bordered on the west by the Anguille basement high, and as the position of this high remained more or less fixed at the (present) continental margin, the basin became narrower with time. It was not until Miocene time that the Anguille high subsided strongly and ceased to influence deposition.

Mainly during late Cocobeach deposition the Lambarene-Ikassa Kongo-Gamba horst zone was formed. After the peneplanation of horsts and grabens alike, the sea invaded the Gabon basin for the first time. The relatively thin, transgressive, coastal-marine sequence between the unconformity and the overlying salt is called the “Gamba formation,” whose sandstones are important oil producers. The main productive trend is related to the structural configuration of the underlying horsts and grabens.

The third hinge zone (Atlantic hinge belt) was active during deposition of much of the postsalt sequence. This hinge zone probably extends over the whole length of the basin and controlled the separation of deeper marine (source) environments on the west from shelf (reservoir) environments on the east. The lack of shale members to cap potential reservoir rocks along at least parts of the hinge belt may explain why no important oil accumulations have been found so far.

The principal oil fields producing from the postsalt sediments are 60–100 km west of the Atlantic hinge belt, partly on the eastern slope of the Anguille basement high. The environments of deposition of the producing sediments vary from brackish-water estuarine to marine-distal deltaic; the largest oil fields are on the west not as a result of optimum reservoir conditions but because of the timely development of nonpiercing salt-induced domal structures of large areal extent. Steep salt piercements are present farther east in deeper parts of the depositional basin, and oil accumulations related to these piercements tend to be smaller.

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