So far only one deepwater field is in production off West Africa. It is Mobil's Topacio subsea field off Equatorial Guinea, which is tied back to the FPSO shown in the photograph, lying in the shallower Zafiro field. Photo courtesy of Mobil.As a hydrocarbon province, West Africa, with 220 fields in production already, bears itself well in comparison with other areas of the world.
But it is the immense promise of the giant deepwater fields that is setting hard-pressed oil executives' pulse rates pounding.
In a recent study of future global deepwater activity, we rated West Africa as being the region with probably the best long-term prospects. This article examines the reasons behind our view.
GeologyTo reach the point at which we are today in our understanding and exploration of the geology of the deep waters off West Africa has been a journey of scientific endeavor.
It began 70 years ago, when Alfred Wegener first proposed that the amazing geographic fit of the eastern coast of South America and the western coast of Africa seemed to indicate that in the far distant past the two continents had in fact been joined together.
It is now recognized that the two continental land masses separated in the latter half of the Mesozoic era, as rifting and oceanic suturing split the super continent of Gondwanaland into five main pieces.
In the jargon of plate tectonics, both the western margin of Africa and the eastern margin of South America are "passive margins."
As the split between them developed from the late Jurassic into the Cretaceous period, similar and contemporaneous pre- and syn-rift deposits were laid down on what are now both sides of the South Atlantic, producing near mirror images of one another with regard to developing basin types, source rocks, and salt deposits.
The latter are of particular importance because of their effect on the post-depositional sediments that blanketed them as the ocean opened and a massive basinward sediment influx of turbidites and other closely related Tertiary clastic deposits occurred.
The salt deposits were laid down in the region of poorly circulating waters lying north of the volcanic rocks that now constitute the Walvis Ridge in the eastern South Atlantic and the Rio Grande Rise in the west.
The location of these two volcanic ridges effectively delineates the southern margin of the hydrocarbon-bearing basins to the north. On the western side of the Atlantic, these are the Santos and the prolific Campos basins of Brazil, and on the eastern side, the lower Congo basin offshore Gabon, Congo, Angola, and perhaps the northernmost part of Namibia.
Following the period of evaporite deposition in the Albian and Aptian, further rifting led to the opening up of the South Atlantic and the deposition of a series of post-Aptian clastic post-rift sediments as the marginal shelves subsided.
These turbidites, particularly those of Oligocene and Miocene age, now form the bulk of the reservoir rocks in the deepwater fields that have been found in the Congo basin to date. These sediments lie off the continental shelf, below the continental slope, at water depths of between 330 m and 1,405 m.
Recent seismic survey work in the Angolan portion of the basin has shown that post-depositional salt movements have created various trap types in the overlying sediments.
The salt thickens towards the African shore and has produced different structures in the extensional tectonic environment that lies in the upper margins of the basin from those that occur in the area affected by compressional tectonics in the western and deeper parts of the basin.
DiscoveriesOffshore exploration began in the early 1960s and was soon crowned with success in the shallow waters off the Niger Delta, off the Port Gentil region of Gabon, and with the discovery of the Emeraude field off the Congo.
Development soon followed, and the Okan field in Nigerian waters started up on Mar. 21, 1965.
Since then 220 fields have been brought into production, utilizing the services of nearly 700 platforms to produce up to 2.8 million b/d of oil from estimated accessible reserves of 23 billion bbl of oil equivalent.
The first deepwater field to be found in West African waters was Girassol field on Block 17 off Angola in April 1996. Since then a total of 25 further fields have been discovered in water depths ranging from 330 m to 1,405 m, and the region has risen to be the world's hot-spot in terms of new reserves found Table 1 [10,007 bytes] and Fig. 1 [ 56,537 bytes].
At the present moment, only one deepwater African development is on stream Table 4 [12,174 bytes] and Table 5. That is the Topacio subsea template tied back to Mobil's Zafiro floating production, storage, and offloading (FPSO) vessel in the northwesternmost part of Block B off Equatorial Guinea, against the median line with Nigerian Blocks OPL 223, 100, and 224.
The most important area by far is north and south of the mouth of the River Congo. Off Angola, 17 fields have been found in the last 21/2 years with reserves currently estimated to be in the region of 7 billion bbl of oil equivalent, of which at least 80% is oil.
This is just as well, because there is really no infrastructure in place for dealing with large volumes of associated gas at present. The finds to date have been made in Tertiary reservoirs in Blocks 14 through 17, operated respectively by Chevron Corp., Exxon Corp., Royal Dutch/Shell, and Elf Aquitaine SA.
North of the Cabinda enclave, in the narrow Haute Mer zone of the Republic of the Congo, Elf has a series of what appear to be three finds smaller than those made in Angolan waters, but their proximity to the border may lead in time to the discovery that they do in fact extend into Angolan waters or vice-versa.
The 400 million bbl Moho field has been touted as a possible tie-back to the fixed facilities at N'kossa, but the relatively poor quality of the first discovered Cretaceous reservoir rocks in the area has allowed the focus of attention to move southwards.
In 1999, drilling may begin in the deeper waters of the Mer Profonde North by Exxon, and later the Mer Profonde South and Tres Profonde South by Elf, and by Agip SpA on the Tres Profonde North permit.
Immediately north of the Congo acreage, in southeastern Gabon, the Anton and Astrid Marin permits of 6,600 and 6,000 sq km respectively were farmed into by Total SA 28%, Unocal Corp. 25%, Kerr-McGee Corp. 14%, and R.B. Falcon Corp., Houston, 11%. The original license owner was Vanco Energy Corp., Houston, which retained a 22% interest. Water depths range from 1,000 m to 3,000 m, and 20-32 prospects have been identified, with a combined potential as great as that of the Angolan blocks.
To the north the pickings are less certain. Deepwater drilling off Nigeria has been successful in parts-for instance, Shell's Bonga field-but no other really large and worthwhile finds have been made so far that would be worth developing under today's extremely tight financial conditions.
The exception is on the border between Nigeria and Equatorial Guinea, where the Zafiro field and its associated satellites show good promise for a developing play exploration program.
On the strength of its activities in Equatorial Guinea, Mobil Corp. has been given exclusive rights to explore and technically evaluate 22 blocks in the neighboring deep waters of Sao Tome and Principe for 18 months. Following this, Mobil will be given first refusal on a production sharing contract for the blocks in the year 2000.
The overall situation is, therefore, very strong and very promising. Elf and Chevron operate the blocks in which most of the reserves have been found to date, but as Table 6 [9,299 bytes] clearly shows, equity holdings spread across the various fields by companies tell a slightly different story. While Elf still holds its premier position in West Africa, the other major players are much more evenly matched, and in fact Exxon overtakes Chevron on percentage of total reserves held across the whole of the region.
DevelopmentsAt the moment there is only one development scenario being considered: floating production with subsea tiebacks.
Table 7 [9,980 bytes] shows the potential demand for subsea completions off West Africa compared to the existing situation. West Africa is very much the coming place for subsea work, and Angola Blocks 14 to 17 will lie at its heart.
On Block 14, Chevron is currently at work on the initial phase of Kuito field in 400 m of water. This will involve tying back 12 subsea wells to an FPSO vessel, currently under conversion in Singapore, for production and the injection of both water and gas. The intention is to flare no gas. The extensive use of gas flaring offshore to allow higher rates of oil production, in sight of hills being rapidly deforested for cooking fuel, has led to an outcry amongst West African environmentalists.
After a period of production, a decision will be made as to whether to go for a second phase of development in 3-5 years.
Other fields in this block are Kuito South, Benguela, Landana, and the most recent find, Belize.
Exxon operates Block 15 to the south. Here four finds-Hungo, Kissanje, Marimba, and Dikanza-may contain about 1 billion bbl of oil equivalent and could be developed together in an integrated scheme.
Shell's Block 16 is the poor relation at the moment. The Bengo and Longa finds have not been reported as commercial on a stand-alone basis, but as exploration is at an early stage further success is to be anticipated. From the experience of Petrobras in the Campos Basin, turbidite reservoirs pay better on further exploration.
The largest development in the near future is going to be that of Elf's Girassol B Oligocene reservoir, where some 40 subsea wells, including some 23 production wells, 14 water injection wells, and 3 gas injection wells are going to be tied back to the world's largest FPSO, by three novel riser towers. The FPSO's hull is under construction in South Korea.
The function of these towers is to ensure that the wellhead fluids rise smoothly and continuously from the seabed through the cold seawater to the production modules with minimum wax and hydrate formation in the pipes.
Adjacent to the field is the Girassol C reservoir, which may be tied back at a later date, and the nearby Dahlia 1 and 2 discoveries as well as the Rosa and Lirio finds, which will probably need separate production facilities for their full development in due course.
The southern tracts offshore Angola, Block 18-30 and the ultra-deepwater Blocks 31-34, look particularly prospective because of their potential to mirror Brazil's Campos basin on the other side of the oceanic divide, where the giant Albacora, Marlim, Marlim Sul, and Roncador fields all occur in similar reservoir rocks and tectonic environments.
Many prospectsWest Africa offers the most exciting prospects of all of the world's deepwater hydrocarbons plays.
Our study indicated that over the period to 2002, about $17 billion of capital expenditure will be invested there Table 8 [5,789 bytes]. This is a similar figure to Brazil, the Gulf of Mexico, and Europe. However, Europe is a high cost province due to its harsh operating environment. The Gulf of Mexico is highly explored and dominated by a series of small prospects-any major growth awaits decisions on the future use of FPSOs.
Brazil currently dominates deepwater activity. Petrobras has recently announced expenditure of $8 billion over the next 4 years-but it is by comparison a more highly developed deepwater domain. West Africa has only one deepwater field producing but a growing list of major finds awaiting development.
Among the discoveries likely to be developed on a stand-alone basis, as Fig. 2 [35,493 bytes] shows, in the Gulf of Mexico the average reserve size was 80 million bbl, compared with 500 million bbl in West Africa.
The great West African elephant hunt has bagged some major trophies, but there are many more prizes in prospect.
Note: The data of future prospects discussed above relate to fields under consideration for development. Unless stated otherwise, this is not a forecast of future activity.
- Fainstein, R., Gregory-Sloan, J., "South Atlantic Salt Basins: The deepwater challenge," Brasil Energy, No. 316, 1998, pp. 8-10.
- Pettingill, H.S., "World Turbidites-1, Turbidite plays' immaturity means big potential remains," OGJ, Oct. 5, 1998, pp. 106-112.
- Pettingill, H.S., "World Turbidites-2, Lessons learned from 43 turbidite giant fields," OGJ, Oct. 12, 1998, pp. 93-95.
- Keaveny, M., "Hydrocarbon prospectivity, salt tectonics offshore Angola," Offshore Magazine, October 1998, pp. 68, 70, and 170.
- "Sao Tome acreage boosts Mobil off West Africa," OGJ, Oct. 5, 1998, p. 112.
- "Rising to the top," Euroil, October 1998, p. 31.
- The World Deepwater Report (www.douglas-westwood.co.uk) provides an overview of global activity and forecasts the deepwater market over the period 1998-2002.
- Infield Systems database (www.infield.com) contains details of existing and proposed global offshore field developments.
- DeepWater Online (www.deepwater.co.uk ) is a new service. It provides detailed information on existing and proposed field developments in water depths of 300 m or greater.
Roger Knight has spent the past 11 years responsible for the collection, validation, and evaluation of the global offshore oil and gas data held on the Infield Database. He holds a PhD in geology and is a member of the Institute of Petroleum. Prior to the Islamic Revolution, Knight worked as a university lecturer in petrology and structural geology in Iran. His e-mail address is oil&[email protected].
John Westwood heads the industry analysts Douglas-Westwood Associates and has, over the past 13 years, been responsible for over 120 specially commissioned oil industry business studies. He previously spent 12 years working in the underwater contracting industry and has served on the boards of offshore industry companies in the U.K. and Norway. His e-mail address is [email protected].
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