A growing realization has set in among companies operating in the West African offshore sector – despite having been a producing region for more than 50 years, its participants have only scratched the surface in terms of its reserves potential.

The continent's western coastline has been dominated by the established oil-producing giants of Nigeria and Angola since Shell first found crude in the Niger Delta and started producing in 1958.

But the whole of the region is now emerging from the shadows as a presalt province in its own right, inspired by the outstanding success being experienced across the Atlantic offshore Brazil and assisted by improved seismic technology, better reservoir understanding, and more efficient deepwater exploration drilling techniques.

The region's presalt potential appears to stretch 1,500 km (932 miles) further north along the coast from the traditional oil-prone areas such as Angola, with companies such as Anadarko Petroleum, Tullow Oil, and Kos-mos Energy seeking to extend the so-called Jubilee trend in the West African transform margin from emerging oil superpower Ghana to frontier areas offshore Sierra Leone and Liberia.

Approximately 120 million years ago, Cobalt’s Kwanza basin blocks offshore Angola would have been around 80 km (50 miles) from the Whale Park Complex in Brazil’s Campos basin and in the same depositional basin. (Map courtesy of Cobalt International Energy)

Angolan presalt potential recognized

The most recent licensing round offshore Angola was a clear illustration of the industry's awareness of its presalt potential, with companies such as Statoil, BP, Total, and Eni picking up key operatorships and interests in blocks in the Kwanza and Benguela basins.

Statoil paid US $1.4 billion for an operating interest in presalt Blocks 38 and 39 and partner positions in Blocks 22, 25, and 40.

Angola already plays a bigger role in the Norwegian company's current portfolio than many realize. The Angolan continental shelf is the largest contributor to its production outside Norway, with equity production from Blocks 15, 17, and 4/05 yielding 173,000 boe/d in 2010 – 34% of its total international oil and gas output.

"With 20 years of experience in Angola and a position as one of the largest producers in the country, becoming an operator of two presalt licenses is an important milestone for Statoil. The Angolan presalt is a frontier play with high potential, believed to be analogous to presalt Brazil," said Tim Dodson, executive vice president for exploration. "Early access to a multiple-block portfolio in exploring this new play gives Statoil exposure to significant upside potential should the play be proven."

BP also expanded its position, gaining access to five deepwater exploration blocks to result in interests in nine blocks offshore Angola covering 32,650 sq km (12,600 sq miles). It signed production-sharing contracts (PSCs) for four blocks covering 19,400 sq km (7,490 sq miles), including operatorship of Blocks 19 and 24 as well as separately farming in to a 40% stake in the Petrobrasoperated, 4,840 sq-km (1,869 sq-mile) Block 26 in the Benguela basin. The five new blocks cover a total area of 24,000 sq km (9,267 sq miles) in 200 m to 2,500 m (656 ft to 8,202 ft) water depth, and they increase BP's total Angolan acreage by 275%.

Total collected three new deepwater blocks in the Kwanza basin. The company will operate Blocks 40 and 25, and it also has a minority partnership interest in Block 39. Block 40 covers an area of 7,604 sq km (2,936 sq miles), Block 25 spans 4,842 sq km (1,870 sq miles), and Block 39 covers 7,831 sq km (3,024 sq miles).

Total said its main objective is to explore for presalt deposits in the company's offshore West Africa blocks, which it believes share promising geological similarities with the Santos and Campos basins offshore Brazil. Over an initial five-year exploration period, the company will acquire 14,000 sq km (5,405 sq miles) of 3-D seismic and plans to drill five exploration wells with presalt objectives in aggregate over the three blocks.

Eni, meanwhile, signed a PSC as operator for Block 35, which covers 4,900 sq km (1,892 sq miles) in the northern Kwanza basin. The contract encompasses the drilling of two presalt wells and the acquisition of a 3-D survey of 2,500 sq km (965 sq miles) in the first five-year period.

Anadarko, Tullow, and Kosmos are working to extend the Jubilee trend from Ghana more than 1,120 km (700 miles) along the coast to the frontier waters of Sierra Leone and Liberia. (Map courtesy of Anadarko Petroleum)

Cameia discovery derisks region

The optimism of these companies has been boosted with recent success by Cobalt International Energy, which reported that it has substantially derisked Angola's presalt play.

Cobalt has a new phased floating production project on the drawing board for Angola's deepwater presalt province after it confirmed its latest frontier wildcat had made a world-class light oil discovery that "far exceeded predrill expectations." The operator's Cameia-1 well in Block 21 was drilled in 1,682 m (5,518 ft) water depth to a total depth of 4,886 m (16,030 ft). According to the company, an extensive wireline evaluation program was conducted that confirmed the presence of a 360-m (1,180-ft) gross continuous oil column, with more than 75% net-to-gross pay estimate.

An extended drillstem test also has been performed on the well to provide additional information. The test flowed at an unstimulated sustained rate of 5,010 b/d of 44°API gravity oil and 14.3 MMcf/d of associated gas (approximately 7,400 boe/d) with limited drawdown. The flow rate confirmed the presence of a very thick, continuous, high-quality reservoir saturated with light oil.

Internal preconcept development plans are being progressed by Cobalt, with the development model following a phased approach similar to Tullow Oil's strategy on its Jubilee field offshore Ghana. Subsea concepts and design specifications have been developed and are being validated. The company is now exploring opportunities for accelerating vessel access for early production with third-party facility providers, although this will be better defined once results from the next appraisal come in.

Joseph H. Bryant, Cobalt chairman and CEO, said the Cameia discovery, which has the potential to flow at more than 20,000 b/d of oil, has increased the company's confidence in its West Africa presalt exploration inventory.

Cobalt and its partners have initiated an appraisal drilling program to further assess the size and extent of the discovery and to drill deeper objectives. Other multiple presalt prospects in Block 21 and the adjacent Block 20 include Lontra and Cameia North, and these, along with the Bicuar prospect, are expected to be drilled in 2H 2012 through 2013. Cobalt's Loengo prospect in Block 9 further south offshore Angola also is being targeted during the same period.

Cobalt officials also said the discovery has increased their understanding of the deepwater Gabon presalt play, which already is proven onshore and near-shore, and that they plan to drill a wildcat well in the Diaba block in late 2012 or early next year.

West Africa-Brazil link

Experts at Cobalt long have been believers in the presalt link between West Africa and Brazil, which has a single geologic history and setting. Go back 120 million years, and Brazil's prolific 2.5-3.5 Bboe Whale Park Complex and surrounding area in the Campos basin (containing discoveries such as Jubarte, Cachalote, Itaipu, and Wahoo) existed some 80 km (50 miles) from Cobalt's Kwanza basin Blocks 20 and 21 in the same depositional basin.

The company's analysis has confirmed that the Campos and Kwanza basins share similar presalt histories and characteristics, with the exploration history of the Cam-pos basin serving as an appropriate analog for the Kwanza basin.

Meanwhile, Anadarko, Kosmos, and Tullow are striving to establish the presalt trend farther north, working closely to extend and replicate the success achieved offshore Ghana with their Jubilee field, now fully producing from its first phase. The $1.1 billion Phase 1A is expected to come online later this year.

To the north is Sierra Leone, where Anadarko as operator with partners Kosmos and Tullow had encouraging success in 2009 with the deepwater Mercury discovery and, more recently, the Venus find in SL Block 07-10. These discoveries have endorsed and enhanced their strategy to extend the Jubilee play around the Gulf of Guinea to the Liberian basin, where the consortium has other multiple prospects.

Tullow also recently confirmed a hydrocarbon discovery at its Jupiter prospect in the same block as Mercury. The Transocean Discoverer drillship will return to complete drilling the Mercury-2 well by the end of March 2012.

Ghana: The jewel in the crown

Ghana remains the crown jewel for all three companies at present. They are already forging ahead with the next fast-track development in the Deepwater Tano Block after hitting better-than-expected oil reserves in the license, and like Jubilee, it will be developed using an FPSO.

The latest appraisal well, Ntomme-2A, firmed up further commercial reserves, with Tullow reaffirming that it envisages the Greater Tweneboa, Enyenra, and Ntomme (TEN) fields will be most efficiently developed in an integrated development scheme via a single production facility.

A plan of development for the TEN cluster is expected to be submitted to the government of Ghana in 3Q 2012 alongside a formal declaration of commerciality. First production from the project is anticipated 30 months after approval.