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| Equatorial Guinea territory overlies parts of two world-class
petroliferous sedimentary basins. Both are highly prospective
for hydrocarbons with proven petroleum systems including marine
oil-prone source rocks and high quality sandstone reservoirs. In the north of the country around Bioko, the offshore sector overlies the distal parts of the Niger Delta-Rio del Rey basin system. The Niger Delta is one of the world’s largest petroleum provinces with estimated proven oil reserves of 48 billion bbls. The Equatorial Guinea part of the basin has established oil and gas production from the Zafiro and Alba fields plus a number of other discoveries. The southern part of Equatorial Guinea’s offshore sector overlies the Rio Muni Basin. This is contiguous with the Kribi-Campo (Douala) Basin of Cameroon to the north (which hosts the Sanaga Sud - Kribi oil and gas fields) and with the North Gabon Basin to the south which contains numerous oil and gas discoveries. The excellent potential of the Rio Muni Basin has been demonstrated by the discovery of commercial oil at Ceiba field. The sedimentary section beneath both the Bioko and Rio Muni areas extends oceanward to the territorial limit of Equatorial Guinea. The sediments of the distal Niger Delta Basin and those of the deep water Douala and Rio Muni basins merge in the region south of Bioko, giving significant petroleum potential to the whole of Equatorial Guinea’s offshore areas. Industry attention is now extending to the ultra-deepwater Gulf of Guinea around the islands of the Cameroon volcanic trend which include the Equatorial Guinean island of Annobon. Here it is postulated that the extensive offshore economic zone could contain a thick sedimentary section with petroleum potential, as evidenced by oil seeps on the neighbouring volcanic islands of Sao Tome and Principe. |
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| 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. |
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| Offshore Bioko contains an active petroleum system comprising
reservoir sands that lie structurally above voluminous marine
shales containing oil-prone source rocks. The most important of
these are the marine shales of the basal Pliocene Qua-Iboe and
the Miocene Akata (Isongo) Formations which contain mixed Type
II and Type III kerogens. A thick sequence of Tertiary to Quaternary sands and shales was deposited as a wedge of clastic sediments in the distal Niger Delta basin setting. Shelf edge sedimentation has led to deposition of delta slope and basinal clastics by a variety of gravity driven processes including gravity sliding, debris flow and turbidity currents. Progradation of the delta has been accompanied by shale diapirism and growth faulting with associated rollover structures, and a zone of compressional toe-thrusting has developed at the foot of the delta slope. The reservoirs at the Zafiro field complex occur in debris flow and slump sandstones of the Qua-Iboe Formation, comprising large, channelised sandbodies deposited in deep water settings. The Alba field produces from a deep water sand in the Isongo Formation. Simple four-way dip structural closures and toe-thrust structures are common within the Isongo Formation whereas the Qua Iboe Formation contains a number of structurally enhanced stratigraphic traps. Good seals for all traps are provided by the interbedded Qua-Iboe and Isongo shales. Regional seismic lines show that Mid to Late Cretaceous strata extend far into the ultra-deep water and certainly beneath the present-day volcanic intrusives such as Bioko Island. This is evidenced by the oil seeps on the volcanic trend in Sao Tome and the occurrence of Turonian ammonites on Bioko. The presence of volcanics in this setting therefore need not preclude hydrocarbon occurrences in the vicinity of Bioko and the other islands in the chain, including Annobon. Exploration is at an immature stage in offshore Bioko and holds considerable potential for future discoveries. The deep water areas to the south and west of Bioko hold potential, particularly within large fan-like mound features recognised on seismic data in front of the toe-thrust limit, which are as yet untested by drilling |
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