GEOLOGY OF THE RIO MUNI BASIN
The Rio Muni Basin forms part of the extensive West African margin basin system, formed during continental separation and creation of the South Atlantic Ocean through the Cretaceous and Tertiary. This basin system contains a thick wedge of Cretaceous to Tertiary sediments built over an early Cretaceous rifted terrane.
In the Northern Gabon Basin and extending into southern Rio Muni, the rift section comprises lacustrine and fluvio-deltaic faulted and tilted strata of Barremian and Neocomian age. In Gabon this section includes proven sandstone reservoirs, and the Kissenda and Melania lacustrine shales which are prolific source rock intervals. Overlying the syn-rift section is a thick section of Late Aptian salt and a well developed succession of Mid to Late Cretaceous and Early Tertiary marine limestone and sand-shale sequences. Movement of salt has formed a wide range of prospective traps including diapirs, turtle-backs, and rollover structures to growth faults on the margins of salt walls.
In northern Rio Muni, the syn-rift section comprises Late Barremian to Mid Aptian terrestrial clastics and lacustrine shales characterised by extensional rollover structures to mega-scale listric faults updip, and toe-thrust structures downdip. The syn-rift section is overlain by a ‘transitional’ sequence of well-developed salt and good quality marine oil-prone source rock intervals. An Albian (Madiela) carbonate platform developed over the area plus a Cenomanian-Turonian sand-shale sequence which contains a major source interval. This post-salt sequence commonly forms extensional rafts detaching on an Albian-Aptian shale or salt. A well developed Senonian section onlaps the earlier rafted topography. The latest drift sequence is dominated by a thick wedge of Miocene to Recent clastics.
Modelling indicates that the ‘transitional’ source intervals may be locally mature on the shelf and may have charged Albian to Turonian carbonates and clastics. The deep water equivalents are believed to have been generating hydrocarbons from Mid Tertiary times and provide the likely source to the deep water sandstones which provide the excellent reservoirs encountered at Ceiba.