A Review of the Geology of the Rio Del Rey Basin, Offshore Cameroon

A review of the tectonic evolution, stratigraphic and structural framework, petroleum systems and hydrocarbon potential of the offshore portion of Rio Del Rey (RDR) Basin in Cameroon is presented in this paper. The RDR basin is a passive margin basin present in the Gulf of Guinea and it is located southeast of Niger Delta over an area of about 7,000 km offshore. Its stratigraphic components, like those of Niger Delta, comprise of a diachronous succession of prodelta shales (Akata Formation), overlain by delta front sands and shales (Agbada Formation), in turn, overlain by fluvial sands (Benin Formation). The age of this basin ranges from Eocene to Recent and it consists of four structural provinces namely; the growth fault province in the north, the Cretaceous Onshore province in the southeast, the Shale Ridge province in the southwest, and the Delta Toe-thrust in the south-central region. The RDR Basin features three stages of tectonic development: Pre-rift phase (Late Proterozoic to Late Jurassic); Syn-rift phase (Late Jurassic to Early Cretaceous) and Post-rift phase (Late Cretaceous to Holocene). It presently stands out as the major contributor of the petroleum in the country.


INTRODUCTION
The sedimentary basins in Cameroon have been classified into two namely; coastal and continental basins [1]. The coastal basins are the Douala/Kribi-Campo and the Rio Del Rey Basins. These are passive margin basins formed in the Cenozoic following the opening of the South Atlantic Ocean when the Gondwana supercontinent broke up in the Late Jurassic. They were filled with sedimentary rocks of predominantly marine origin [2].
The RDR Basin (Figure 1), is one of the coastal basins in the southwest region of Cameroon and represents the southeastern portion of the Niger Delta in the Gulf of Guinea [3][4][5][6][7]. This basin is located between latitude 4°and 5° north and longitude 8°20 ′ and 9°10 ′ east ( Figure 2).
The offshore portion of RDR Basin (which is approximately 7000 km 2 ) represents one of the major petroliferous regions of Cameroon [8]. It is bordered by Niger Delta Basin of Nigeria to the west and northwest, and by Rio Muni Basin of Equatorial Guinea to the south. To the north, by the Rumpi Hills and to the east by the Cameroon volcanic line that separates RDR basin from the Douala/Kribi Campo Basin [7]. Figure 1: Map of Cameroon outlining the Cameroon Volcanic Line (with volcano ages) and the sedimentary basins [3][4][5]. Despite the high potential of Cameroon's sedimentary basins for significant hydrocarbon reserves, they remain underexplored [9]. Therefore, it is important to review the geology of this basin in order to reveal its tectonic evolution, stratigraphic and structural framework, petroleum systems and hydrocarbon potential this would further highlight its petroliferous nature and call for more exploration activities. Since previous work had mostly been done by the government agencies and remained classified.

Tectonic Evolution
The tectonic evolution of the RDR Basin, as well as most basins found along the West African coast (Gulf of Guinea), is strongly related to the processes of rifting and drifting that resulted in the opening of the South Atlantic Ocean, hence leading to the separation of the South American plate from the African plate. The three stages marking the tectonic evolution of RDR Basin are: • Pre-rift stage. It ranges in age from Late Proterozoic to Late Jurassic, characterized by continental deposits of clastic rocks which are Carboniferous to Jurassic in age [10].
• Syn-rift stage. Ranges in age from Late Jurassic to Early Cretaceous, consisting of thick sequences of fluvial and lacustrine deposits.
• Post-rift stage. Its age ranges from Late Cretaceous to Holocene. It constituents younger rocks which are deposited as transgressive units made of shelf clastic and carbonate rocks accompanied by progradational units along the continental margins and as open-ocean water units [9].

Structural Framework
The structural style of the Niger Delta extends into the Rio Del Rey Basin, with both growth fault and overpressured clay diapir zones being recognizable ( Figure 3) [11,12]. Four structural provinces make up the RDR Basin [13]. These include the Cretaceous Onshore province, the Growth Fault province in the north, the Shale Ridge province in the southwest, and the Delta Toe-thrust zone in the south-central area ( Figure 4) [14,15]. The Rio Del Rey Basin is characterized by three main structural styles from north to south: • Detachment-based growth fault zone; • Highly faulted, mobile shale cored domes and ridges; • Toe thrusts and folds.
Almost all individual hydrocarbon traps are fault-dependent, and lack of sufficient internal seals in sand-rich wave-dominated deltaic sequences can limit the objective window.
Downthrown fault traps in hanging-wall reservoirs are effectively sealed against older prodelta and marine shales and are present in most fields. All large oil fields are located in the dome and ridge province and are potential analogues for similar structures now being explored on the upper slope offshore Nigeria. Marine shales of Paleocene to Eocene are the best quality source rocks identified so far.

Stratigraphy of the RDR Basin
Short and Stauble reported that the subsurface offshore of RDR Basin has a stratigraphic succession broadly similar to that of the Niger Delta [16] ( Figure 5). There is a diachronous succession of pro-delta shales (the Akata Formation), overlain by delta front sands and shales (the Agbada Formation), in turn, overlain by fluvial sands (the Benin Formation). These deposits range in age from Eocene to Recent (Table 1).

Petroleum System
Petroleum occurs throughout the Agbada Formation in the RDR clastic wedge. Although the distribution of hydrocarbons is complex, there is a general tendency for the ratio of gas to oil to increase eastward within individual depobelts [17].
In the onshore part of the basin, the Agbada Formation has reservoir rocks consisting of sandstones and paralic non-consolidated sand. The depth of burial and depositional environments of these reservoirs are the two main factors controlling the characteristics of the reservoirs. Nevertheless, the major target in deep water are the turbidite sands in the Akata Formation. The eastern portion of the RDR Basin is made up of alternation of sand and shale thus confirming the existence of thick and effective seals.
The Akata Formation constitutes the source rocks consisting of upper prodeltaic marine shales. There is also the presence of possible source shales at the base of the Eocene Agbada Formation [18]. According to Stacher, the organic matter are predominantly  The four provinces shown on the regional structure map of Rio Del Rey Basin [14,15].  of terrestrial origin consisting of Type III/I kerogens (humic to mixed type) [19].
Seals are more efficient in the turbiditic sands. These clays form top seals for some important fields in the eastern portion of the RDR Basin [17]. Structural traps are more common, although there are pinch-outs and turbiditic fan lenses [20]. Studies from Evamy and Haremboure, shows that the development of structural traps was during syn-sedimentary deformation of the Agbada paralic sequence [21]. Hydrocarbon migration in the basin is probably favoured by the presence of minor faults ( Figure 6).