Smaller independent oil companies are successfully drilling deep prospects off western Ghana, finding light sweet crude and natural gas in Cretaceous sandstones that may extend from Benin westward to Sierra Leone.
Dallas-based independent Kosmos Energy LLC discovered two deepwater fields off Ghana. In 2007, it used the Bedford Dolphin, a fifth-generation drillship to drill the Mahogany-1 discovery well at Jubilee field. In early 2008, Kosmos drilled the Odum-1 discovery well 13-km away on a separate structure. Six wells have intersected oil-bearing sands (Table 1).
Jubilee field
Jubilee field straddles two offshore license blocks (Fig. 1):
- West Cape Three Points Block covers 1,761 sq km (435,200 acres) in water depths ranging 50-1,800 m. It is operated by Kosmos Energy (30.875%), with working interest partners Anadarko WTCP Co. (30.875%); Tullow Ghana Ltd., an affiliate of Tullow Oil PLC (22.896%); E.O. Group (3.5%); and Sabre Oil and Gas Ltd. (1.854%). Ghana National Petroleum Corp. (GNPC) has a 10% carried interest. The block carries a 7-year exploration agreement.
- Deepwater Tano Block covers 1,106 sq km in water 200-2,060 m deep. It is operated by Tullow (49.95%), with working interest partners Kosmos Energy (18%); Anadarko (18%), and Sabre Oil and Gas (4.05%). GNPC has a 10% carried interest.
Tullow is Jubilee field operator.
Rigs
The first three wells at Jubilee field and the Odum-1 discovery well were drilled with the Songa Saturn drillship in 2007-08.
So far, the partners have leased four deepwater drilling units to drill off Ghana: Blackford Dolphin, Eirik Raude, Atwood Hunter, and Aban Abraham. Anadarko said the semisubmersible Blackford Dolphin arrived in Ghana during the second week of October 2008. It drilled the Hyedua-2 appraisal well and was moving to drill a Mahogany development well.
The Eirik Raude semisubmersible arrived in Ghana in fourth-quarter 2008 and drilled the Mahogany-3 well on West Cape Three Points Block. It will drill the Tweneboa-1 exploration well on Tano Deep Block during first-quarter 2009.
Anadarko said these two rigs will continue the exploration, appraisal, and development well drilling program in the two adjacent license blocks.1
On June 24, 2008, Kosmos contracted with Atwood Oceanics Pacific Ltd., a wholly owned subsidiary of Atwood Oceanics Inc., to use the Atwood Hunter moored semisub for 27 months beginning April 2009. Kosmos said its share of the drilling contract commitment is more than $400 million and that Kosmos and Noble Energy have an option to extend the drilling contract for 1 year.
The Aban Abraham and Atwood Hunter are expected in Ghana soon and will focus on exploratory and appraisal work. In January, Atwood Oceanics’ Atwood Hunter, a third-generation semisub rated to drill to 28,000 ft, was drilling in 5,500-ft water off Israel for Noble Energy. Aban Offshore’s Aban Abraham drillship, now rated to drill in water to 6,900 ft to a depth of 19,350 ft, left the Sembawang shipyard in Singapore on Dec. 17 after extensive modifications to extend its drilling depth capacity and was upgraded to fourth-generation capability from second generation.2
2007 wells
Kosmos drilled the Mahogany-1 well in 1,320 m (4,330 ft) of water, 63 km (39 miles) from the Ghanaian coastline and 132 km southwest of the port city of Takoradi.
The well reached 3,826 m TD and on June 18, 2007, the company announced, discovering a 270 m gross hydrocarbon column with 95 m of net stacked pay in a Cretaceous sandstone reservoir.
Kosmos and partners drilled the first appraisal well, Hyedua-1, on the adjacent deepwater Tano Block, about 5.3 km southwest of the Mahogany-1 discovery well.
Hyedua-1 was drilled in water 1,530-m deep and reached a TD of 4,002 m. It encountered a gross reservoir interval of 202 m, including 108 m of stacked reservoir sandstones and 41-m of net hydrocarbon-bearing pay. The discovery was announced August 2007.
2008 wells
Kosmos drilled the Odum-1 exploration well 13 km east of Mahogany-1 well and Jubilee field, West Cape Three Points Block. Kosmos and partners used the Songa Saturn drillship to drill in 955 m water to a TD of 3,386 m.
The Odum-1 intersected 60-m gross (22-m net) of 29° gravity oil pay.
The second appraisal well, Mahogany-2, was drilled on the West Cape Three Points license, in 1,080 m water with the Songa Saturn drillship. It is 6.1 km northeast of the Mahogany-1 well and 11.3 km northeast of the Hydeua-1 well.
The Mahogany-2 well encountered a 193-m-thick gross hydrocarbon-bearing interval with 50-m net hydrocarbon-bearing pay of high-quality, stacked reservoir sandstones. It reached 3,443 m TD and will be used as a potential development well (OGJ, May 26, 2008, p. 22).
Kosmos announced in June that the well flowed 5,200 bo/d of 36° gravity crude and 5.5 MMcfd of natural gas on a 40/64-in. choke with a flowing tubing pressure of 1,543 psi (Fig. 2). The test was from a single 17-m zone representing less than one-third of the oil-bearing pay intersected by the well. At that time, Kosmos estimated that a Jubilee production well should be capable of flowing more than 20,000 bo/d when completed with 5.5-in. tubing.
Following the first three wells, Tullow estimated Jubilee’s ultimate upside potential at 1.8 billion bbl.
In late 2008, partner Anadarko estimated the recoverable resources at 500 million to 1.8 billion boe.1
2009 announcements
Recent discoveries suggest that the Tano basin may hold more oil than expected.
Kosmos announced results Jan. 8 from the Mahogany-3 well, drilled by the Eirik Raude semisub more than 5 km southeast of Mahogany-1 and 5 km south of Mahogany-2, in water 1,236 m deep (Fig. 3).
The Mahogany-3 well penetrated 33 m (net) oil-bearing sands and reached TD of 4,028 m. Part of the net oil pay (16 m) were reservoir sands equivalent to those in other wells.
Mahogany-3 well discovered the Mahogany Deep reservoir, with 17 m of oil pay. This interval is stratigraphically deeper and separately trapped, according to Kosmos. Reservoir fluid samples indicated 35° gravity oil.
On Jan. 12, 2009, Tullow and partners announced results from the Hyedua-2 appraisal well in the deepwater Tano license. The Bedford Dolphin spud the well in late 2008 in water 1,246-m deep to a TD of 3,663 m. The well encountered 55-m of oil-bearing pay.
Production tests were run on a 41-m zone within the pay interval. Kosmos said the Hyedua-2 well produced 16,750 b/d of 37° gravity oil and 21 MMcfd during a drillstem test conducted on an 88/64-in. choke with a flowing tubing pressure of 1,387 psi.
Goodbody analyst Gerry Hennigan said Tullow and partners should be able to extract “more than the 300-350 mbo originally proposed” during Phase 1 of Jubilee, which is to be developed with 17 wells.3
The Bedford Dolphin moved off site in January to drill a Mahogany development well, after which it will drillstem test Mahogany-1, according to Kosmos.
The partners will also spud a well on the Tweneboa prospect this quarter, in Deep Tano Block, with the Eirik Raude semisubmersible, and an exploration well on Teak prospect later in the year.
Oil production
Tullow said on Jan. 8 that it’s targeting about 4 billion bbl of oil and gas resources in the Gulf of Guinea off Ghana and Ivory Coast. It plans to invest €2.3 billion to develop the Jubilee field and has other interests in shallow water.
Kosmos is targeting first production from Jubilee in 2010 using an FPSO with a processing capacity of 60,000 b/d of oil and 80 MMscfd of gas (OGJ, May 26, 2008, p. 22).
Anadarko planned to sanction the Jubilee field and said in late 2008 that, “a suitable FPSO has been selected and a letter of intent has been signed on the vessel.”
Adjacent drilling
Hess recently finished drilling the Ankobra-1 well in 1,732 m water to 3,963 m TD. It did not encounter commercially significant hydrocarbons. Hess is currently acquiring 1,619 sq km of new 3D seismic data and will drill a well in the unexplored western half of the license area (OGJ Online, Dec. 17, 2008).
Gas market
At the time of the Jubilee discovery in 2007, Ghana was working out technical and economic details related to new Nigerian natural gas expected to reach Tema and Takoradi, Ghana’s two international seaports via the West Africa Gas Pipeline (OGJ, Aug. 27, 2007, p. 68). Ghana was to take 75% of the line’s 134 MMcfd start-up volume, expected to begin at the end of 2007.
West African Gas Pipeline Co. completed construction of the 1,033 km-WAGP pipeline in April 2008, but water content of the gas remained above specifications. In December 2008, the pipeline was repressured and start-up appeared imminent, with initial capacity of 200 MMcfd and capability to expand to 600 MMcfd.
References
- Anadarko Petroleum, Third-quarter 2008 Operations Report, www.anadarko.com.
- “Aban Abraham Drill Ship Sets Sail from Singapore,” Dec. 17, 2008, www.erhc.com.
- “Tullow announces good news from Ghana,” Finfacts, Jan. 12, 2009, www.finfacts.ie.
