Operators follow offshore Ghana trend for new prospects

credit: Eldon Ball
Senior Editor, Technology & Economics

The center of interest off West Africa is on the exploration successes off Ghana and the implications they have for prospects offshore neighboring and nearby countries.

Production began last December at Tullow Oil’s Jubilee field – Ghana’s first deepwater oil development – and output has climbed to around 50,000 b/d since the field came onstream.

Currently four wells are in service and full capacity of 120,000 b/d should be attained within six months as the remaining five producer wells are completed and come on line.

Water is being injected into two of the producing wells at a rate of 75,000 b/d, and a further four water injectors will be completed this year to maintain production levels. Gas compression commissioning has started onboard the FPSO, with gas injection to the reservoir due to start next month.

Even more significant than that milestone are the new exploration efforts popping up along the upper West Africa coastline, led by Tullow Oil, Kosmos Energy, Anadarko, and others, as operators follow what they hope is a trend that mirrors the deepwater geology offshore South America.

Last July, Tullow Oil made another potentially major oil discovery in Ghana’s deepwater play fairway. Owo-1, drilled in 1,428 m (4,685 ft) of water in the Deepwater Tano license offshore Ghana, intersected “a significant column of excellent quality light oil.” Analysis of drilling, wireline logs, and reservoir fluid samples suggest this is a large new oil field warranting further appraisal.

The deviated well, drilled by the semisubmersible Sedco 702 from a location around 6 km (3.7 mi) west of the Tweneboa wells, encountered a gross vertical reservoir interval of 154 m (505 ft), including 53 m (174 ft) of net oil pay in two zones of good-quality, stacked reservoir sandstones.

Pressure data suggest that these zones form part of the same accumulation; samples indicate a light oil (33-36º API).

Once logging operations are complete, the well will be side tracked 0.6 km (0.4 mi) to the east in an attempt to determine lateral reservoir distribution, and also to intersect a deeper part of the Owo channel system.

Owo-1 was drilled to a final depth of 3,891 m (12,766 ft). On completion of the side track, the rig will drill the Onyina-1 exploration well elsewhere in the Deepwater Tano block, targeting a large fan/channel system between the Tweneboa and Jubilee fields.

Meanwhile, Tullow’s Enyenra-2A appraisal well in the Tano license area has encountered oil in high-quality sandstone reservoirs.

The well reached a TD of 4,234 m (13,891 ft) in a water depth of 1,674 m (5,492 ft). The location is down-dip of Owo-1 (since re-named Enyenra), 7 km (4.35 mi) to the north, the aim being to appraise the field’s upper and lower channels.

Wireline logs, reservoir fluid samples, and pressure data indicate that Enyenra-2A intersected 21 m (69 ft) of net oil pay in the upper channel and 11 m (36 ft) of net oil pay in the lower channel.

Pressure data show that the oil is in communication with the Owo-1 well, and oil pressures in the lower channel suggest it may also be in communication with the deeper pools evidenced in Owo-1 and its side track.

Additionally, Enyenra-2 tested a deeper Turonian fan, intersecting 5 m (16.4 ft) of gas/condensate bearing sandstones. The extent and thickness of this zone away from the well is undergoing interpretation.

Once operations have been completed, the well will be suspended for later use. Prior to flow testing, pressure gauges will be deployed to measure reservoir connectivity.

Deepwater Millennium will stay in the block to complete drilling of the Tweneboa-4 well. The next appraisal well will be Enyenra-3A, up-dip to the north of Owo-1.

Angus McCoss, Tullow’s exploration director, said: “This represents a major step forward in the appraisal of the Enyenra-Tweneboa area and is highly encouraging for our target to declare commerciality later this year. We are now looking forward to leveraging our expertise in the Jubilee play across the Atlantic through drilling the high-risk, high-impact, Zaedyus prospect in French Guiana in March.”

Tullow has a 49.95% operated interest in Ghana’s Deepwater Tano license, and 22.9% of the adjacent West Cape Three Points (WCTP) license, extending across a total of 2,605 sq km (1,006 sq mi). It is also unit operator of the Jubilee development, with a 34% interest, while Kosmos Energy is the technical operator. Kosmos, Anadarko, Ghanaian National Petroleum Co. (GNPC), and Sabre are partners in both exploration licenses, with EO Group also present in WCTP.

Between June 2007 and September 2010, Tullow has operated or participated in 15 deepwater exploration and appraisal wells across these permits, in water depths ranging from 1,079-1,428 m (3,540-4,685 ft), with only one failure, Dahoma-1 in WCTP. Aside from Jubilee and the South-East Jubilee extension which includes Mahogany Deep, the company has achieved commercial discoveries in Turonian and Campanian intervals on Tweneboa and Owo in Deepwater Tano, and on Odum in WCTP.

More off Ghana

In February, Kosmos Energy reported its Teak-1 exploration well offshore Ghana discovered hydrocarbons. The well, in the Cape Three Points block, encountered oil, gas/condensate, and natural gas over a 576-m (1,890-ft) interval.

Logs and samples indicate the well found pay in five Campanian and Turonian sandstones with 26 m (85 ft) of oil. Samples show the Campanian reservoirs with 40º API and the Turonian with 32º API. The well is updip and northeast of Jubilee field.

The Atwood Hunter semisubmersible rig drilled the Teak-1 well in a water depth of 868 m (2,847 ft) to a total depth of 3,170 m (10,398 ft) and will now drill Teak-2. The partners plan to drill other Upper Cretaceous prospects over a sequence up to mid-2011, when the exploration period for the license expires.

Farther northwest along the West African Transform margin, Tullow is a participant in the recent Mercury-1 light oil discovery off Sierra Leone, drilled by the Deepwater Millennium rig.

Meanwhile, Anadarko is designing an appraisal program to be combined with the potentially high-impact Montserrado exploration well (Cobalt prospect) in the partners’ deepwater acreage offshore Liberia. First, however, rig capacity has to be secured.