Mahogany, Hyedua set down roots of deepwater play offshore Ghana
Jeremy Beckman, Editor, Europe
Tullow Oil is continuing its run of successes with frontier plays in Africa. Following exploration breakthroughs in Uganda’s Lake Albert basin, the company recently discovered in a previously untested deepwater play fairway what may be Ghana’s largest oil find to date.
London-based Tullow has built up a wide spread of exploration and production interests offshore West Africa, mainly through acquiring Energy Africa in 2004, and more recently, Australia’s Hardman Resources.
Adding acreage
In June this year, the company also branched into Ghana, securing operating interests in the Shallow Water Tano and Deepwater Tano licenses. Through a farm-in agreement, Tullow also acquired 22.9% of the adjoining West Cape Three Points license, operated by Kosmos.
Shallow Water Tano contained three undeveloped oil and gas fields, while the deepwater blocks - previously unexplored - had prospective structures in the Albian and Upper Cretaceous intervals. All three licenses also were on trend with the producing Espoir oil field off Côte d’Ivoire, where Tullow partners CNR International.
Ghana is on the West Africa Transform Margin, formed during the rifting of the Atlantic Ocean basin. On the geologically corresponding other side of the Atlantic, Tullow has exploration acreage in Suriname, French Guiana, and Trinidad, picked up via its recent acquisition of Hardman Resources.
“We put geology ahead of geography when looking at new exploration opportunities,” says exploration director Angus McCoss, who has introduced Tullow to a process of global prospect ranking followed by rigorous capital allocation, which he applied for his previous employer, Shell. “In this case [the Ghanaian deepwater blocks], our geophysicists in Cape Town spotted an opportunity, which we followed through as a top quartile-prospect.”
This summer, Kosmos/Tullow contracted the fifth-generation drillshipBelford Dolphin, which had been working off India, to drill Mahogany, one of several targets imaged by a recently acquired 3D seismic survey over the two deepwater blocks. There were immediate dividends. The Mahogany-1 well, drilled to TD of 3,826 m (12,552 ft) in the Kosmos-operated West Cape Three Points permit, discovered what could be Ghana’s largest oil find to date in a previously untested play fairway in the Tano basin.
The well, which was targeting a large Santonian turbidite fan/stratigraphic trap, encountered a 270-m (886-ft) gross hydrocarbon column and net stacked pay of 95 m (312 ft), with no definitive oil/water contact. Because of the high porosity, there was no need for a flow test, says McCoss. Analysis of the pressure and sampling data indicated that a production well could achieve rates above 15,000 b/d of oil.
After re-mapping the trap and incorporating the well data, the partners confirmed a large seismic amplitude anomaly which suggested that the accumulation extended into the Deepwater Tano block. This prognosis was vindicated in August, when theBelford Dolphin found further light oil while drilling Hyedua-1 in the same Santonian turbidite sandstones, 5.3 km (3.3 mi) southwest of Mahogany-1 in 1,530 m (5,019 ft) water depth.
The well encountered a gross reservoir interval of 202 m (663 ft) - better than the pre-drill estimate - with good quality stacked reservoir sandstones and net hydrocarbon-bearing pay of 41 m (134.5 ft). Logging and pressure test results also suggest that Hyedua-1’s reservoir sands are in communication with Mahogany-1, indicating a continuous accumulation across the two licenses with combined oil columns of over 361 m (1,184 ft).
Tullow has since sidetracked the well to acquire more data over the reservoir interval and may suspend it for use as a potential producer. The drillship then will move to the Gulf of Mexico. No other deepwater drilling is ongoing offshore Ghana, McCoss adds.
During October, the license partners planned to acquire a new 910-sq-km (351-sq-mi) high-resolution 3D seismic survey over the combined structure, to be followed by three more appraisal wells toward year-end, subject to rig availability. Numerous other prospects have also been mapped from the existing 3D seismic over the two deepwater blocks, some of which could be drilled in 2008.
In September, Tullow also spudded an appraisal well on the North Tano oil and gas discovery in the Shallow Water Tano license, in 50 m (164 ft) water depth. The well, drilled by the GlobalSantaFe jackupAdriatic VI, was targeting a gas structure and a secondary oil zone, with potential to hold 88 bcf. Also in this license is the South Tano discovery in 95-131 m (311-430 ft) of water. Both structures are thought to be gas fields with oil rims.
“We want to see if modern drilling techniques, which have been so successful for Tullow in similar low-permeability reservoirs in the southern North Sea, will help to reduce damage to the formation during drilling and allow the hydrocarbons in North Tano to be extracted,” says McCoss. “At the same time, we don’t want to get sidetracked from the main story, which is the materiality in our Mahogany/Hyedua discovery.”
Estimates of recoverable oil from Mahogany-Hyedua at this stage range from 150-220 MMbbl at the low end, to a mid-range of 450-550 MMbbl, to a high-end case of over 1.3 Bbbl.
The partners are already thinking in terms of a large FPSO with subsea wells for the development.
“Our modeling of the economics of this discovery indicates it would attract financing as a very healthy, standalone, commercial development,” says McCoss.
Via Hardman Resources, Tullow is also a partner in the Chinguetti standalone development offshore Mauritania, which is another frontier play. Production rates at Chinguetti have been disappointingly low. According to McCoss, performance problems are due to low net/gross reservoirs, faulting, and compartmentalization. “In our deepwater blocks off Ghana, we have massive sands and a much higher gross interval and net pay.”
Lake Albert targets
Tullow participates in all three blocks in the Ugandan sector of the Lake Albert Rift basin system. The company is 100% equity holder in and operator of block 2, where all seven exploration and appraisal wells drilled to date have successfully tested light sweet oil with flow rates ranging from 2,000-12,000 b/d. Tullow holds a 50% interest in each of the Heritage Oil-operated blocks 1 and 3A.
The Kingfisher-1 well drilled in block 3A flowed earlier this year at 14,000 b/d from three different oil-bearing reservoirs. The upside potential of structures tested to date is in the order of several hundred millions of barrels of oil.
According to McCoss, “Tullow has leveraged its basin-wide presence, operatorship of block 2, and expertise in rift basin exploration to draw up a long-term exploration strategy for the Lake Albert Rift basin. This combines extensive land and lake 2D, 3D seismic surveys with high resolution satellite imagery to map prospects onshore, near-shore, and offshore for drill-out campaigns aimed at unlocking the full oil potential of this promising oil province.”
TheNabors 221 land rig, capable of drilling to 6 km (3.7 mi) in normal size hole, was scheduled to be on site for the beginning of October to set up for spud of the first deviated well into the largely offshore Ngassa mega-structure situated in block 2.
In addition to several intermediate targets, the primary objective of the Ngassa-1 well is basal sands at a sub-surface depth of around 3,500-4,500 m (11,483-14,763 ft), which the Kingfisher-1 well failed to test because of rig limitations. This well is to be followed by an appraisal well on the Kingfisher structure. Several wells are planned in 2008-2009 onshore the north block 2, in block 1, and offshore.
McCoss describes Tullow’s position in block 2 as the beachhead for the whole basin. The company hopes to have approval for an early production system by the end of the year.