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Optimism high for new wells in West Africa

The results of wells due to be drilled this year could be make-or-break for the four-country Transform Margin

Five years ago, a new interpretation of West Africa’s petroleum geology suggested the deep waters lying off Sierra Leone, Liberia, Ivory Coast and Ghana could emerge as a world-class oil province. Extending nearly 1,500 km from west to east, the lightly-drilled West African Transform Margin – an area between two tectonic plates – had been shown to hold oil at both extremes, but in complicated structures.

Exploration since then has yielded discoveries – although none on the scale of Ghana’s Jubilee field, now a 100,000 barrels a day (b/d) producer, which sparked the interest in what had been viewed as an unpromising area. There have also been a number of disappointments, and none of the discoveries west of the Jubilee area has been large enough for a development to be launched.

Drilling plans

But drilling this year could be a turning-point. Anadarko is scheduling two wells off Liberia, while Total – encouraged by a discovery off Ivory Coast in April – will drill two more wells there by end-year, and Chevron has plans to drill off Liberia. Tullow is planning an appraisal well at its Paon discovery off Ivory Coast before end-year, while Australia’s African Petroleum is planning two wells off Ivory Coast and possibly one off Sierra Leone. Anadarko will drill two wells off Ivory Coast by end-year and Lukoil, also having made an Ivory Coast discovery this year, is planning another well there.

The geological complications of the Transform Margin have proved to be considerable. Kosmos, the then-small company which discovered Jubilee, was set up to focus on a single geological structure – the upper cretaceous trap system, which it felt had been overlooked by other explorers. While previous explorers had preferred to look for structural traps, because they can be identified most readily from seismic surveys, Kosmos set out to find more-subtle traps – stratigraphic traps, in which oil is held in place by changes in porosity and permeability of overlying rocks, and combination traps with stratigraphic and structural features.

Traps in the area are complex, with numerous intersecting faults and compartments, and need special seismic techniques for their analysis. Explorers say the search is made more difficult by the limited nature of existing seismic surveys, which were not designed to provide a basin-wide understanding of petroleum systems.

Exploration of the eastern part of the Transform Margin, offshore Ghana, has shown the best results so far. The Tullow-operated Jubilee field, now with over three years of production history, holds reserves of more than 600 million barrels and is surrounded by other development prospects.

Jubilee is being developed in phases – an approach which limited the initial capital cost and allowed production to start quickly, through a leased floating production, storage and offloading (FPSO) vessel. Tullow is due to complete Phase 1A of the development this year, increasing the number of wells to 25 producers and water-injectors. The firm is forecasting that production will average 100,000 b/d this year, as in 2013, and says recent water-injection problems have been resolved.

Gas-handling problems continue, however, because of a long delay in the completion of Ghana National Gas Company’s onshore processing plant at Atuabo, to which Jubilee gas is due to be supplied. Injection wells have been drilled, but oil production is still limited by the volume of gas which can be injected – Jubilee was planned to flow 120,000 b/d. According to Tullow, the plant will be fully operational in the second half of this year.

Meanwhile, Tullow and the government are discussing further development options. When gas can be landed, well deliverability is likely to exceed the capacity of the FPSO, so debottlenecking is one option. Another could be installation of a second or larger FPSO, to allow outlying fields to be tied back to the Jubilee hub. Interests in the Jubilee unitised area are Tullow, 35.48%, Anadarko, 24.08%, Kosmos, 24.08%, PetroSA, 2.72% and the Ghana National Petroleum Corporation, 13.64%.

Next to flow off Ghana will be Tullow’s Tweneboa-Enyenra-Ntomme (Ten) development, lying about 20 km west of Jubilee in the Deepwater Tano licence. The three fields will produce from up to 24 wells, tied back to an FPSO moored in 1,500 metres of water. Capacity will be 80,000 b/d, with first oil due in mid-2016.

East of Jubilee, in the Kosmos-operated West Cape Three Points licence, there are the Mahogany, Teak and Akasa discoveries, which have seen a total of 10 wells so far. Kosmos says the finds are likely to be developed as tie-backs to Jubilee.

Drilling successes extend to the adjoining block to the east, Offshore Cape Three Points, where Eni has the Sankofa oilfield. The company is planning a development using an FPSO, to recover 150 million barrels out of 450 million barrels in-place. Preliminary development studies are also under way by Hess for its series of seven discoveries in the Deepwater Tano/Cape Three Points Block. The company is planning three appraisal wells and a well-test in the block this year.

Ivory Coast promise

Deep-water discoveries this year by Total and Lukoil have raised hopes for Ivory Coast’s part of the Transform Margin – particularly Total’s find, which shows that upper cretaceous oil potential extends a considerable distance west of Ghana’s Jubilee area.

In April, Total said it discovered oil in an upper cretaceous trap with the first well drilled in the CI-514 Block, off the western part of Ivory Coast’s coastline, marking the first discovery in the San Pedro basin. The Saphir-1XB well, at a water-depth of 2,300 metres, found a 40-metres oil column in 350 metres of thick sands, showing light oil of 34° API.

Total says two wells will be drilled in the blocks to the east of the discovery, CI-515 and CI-516, by year-end. Block CI-514 is held by Total with 54%, Canadian Natural Resources (CNR) with 36% and the state’s Petroci with 10%. In April last year, Total found oil with the Ivoire-1X well in the CI-100 Block, adjacent to the border with Ghana, where the company holds 60% and is joined by local Yams Petroleum with 25% and Petroci with 15%.

Lukoil’s recent find was with its first well in the CI-101 Block, in the eastern offshore, where the Capitaine East-1X well passed through 140 metres of sandstone and confirmed the presence of hydrocarbons. Water-depth at location was 2,000 metres. Lukoil also has a discovery, named Independence, in the adjacent CI-401 Block, where an appraisal well late last year confirmed oil and another appraisal is under consideration. Interests in CI-101 and CI-401 are Lukoil, 56.66%, PanAtlantic, 28.34% and Petroci, 15.0%.

Meanwhile, there are plans for further development of the country’s two main producing fields, Espoir and Baobab, both flowing through FPSOs and both operated by CNR. The company says it will start a 10-well drilling programme at Espoir in the second half of the year and a six-well programme at Baobab around the turn of 2014-15. It is indicated that the work is targeted to add about 10,000 b/d of oil-equivalent to Espoir production and about 19,000 b/d-equivalent to Baobab production, providing a significant increase to Ivory Coast’s present 35,000 b/d output.

Sierra Leone, Liberia

Off Sierra Leone, Anadarko’s Venus discovery of 2009 – an upper cretaceous structure matching Jubilee in the east – established the Transform Margin’s potential, but not many wells have been drilled in the country’s offshore since then. Anadarko followed Venus with discoveries at Mercury and Jupiter, but an appraisal well at Mercury was unsuccessful. Talisman had a disappointment with its Djembe well and Lukoil claimed a discovery with its Savannah well last year, although little information has been provided.

Liberia has seen more activity. African Petroleum, 100%-holder of the LB-08 and LB-09 blocks, found oil shows with the country’s first deep-water well, Apalis-1, and then made a significant discovery with its second well, Narina-1, which showed a net 32 metres of oil reservoir in two zones. The firm’s third well, Bee Eater-1, also found oil reservoir although the permeability was lower than hoped-for. The company is considering 3-D seismic acquisition over two prospects near the Narina discovery, Narina West and Night Heron, to firm them up for drilling.

Bee Eater lies close to the border of LB-09, prompting Anadarko, operator of the adjoining LB-10 licence, to schedule a well nearby. Iroko-1 is due to spud in the second quarter, and will be followed by a second well in LB-10, Timbo-1. Earlier, Anadarko found an 8-metres hydrocarbons zone with its Montserrado-1 well in the LB-15 licence.

Further west, prospects for a well off Guinea were raised in May when Tullow lifted its force majeure notice on activity in its large licence area, which it had declared after US authorities said they were investigating the activities of a participant, US-based Hyperdynamics, in obtaining the licence. The first well had been about to be drilled.

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