The company, and others, have made big discoveries offshore Ghana, namely the multi-billion barrel Jubilee field, and an oil strike in Sierra Leone that suggests the finds extend all along the coast.

Tullow’s Zaedyus well in French Guiana, drilled in more than 2,000 meters of water and at a cost of some $200 million, was tapping rock formations laid before Africa and South America separated millions of years ago.

The company, and partners Royal Dutch Shell, with 45 percent and France’s Total with 25 percent, believed the oil-rich west African geology would be replicated in South America.

“Our Atlantic twin basin concept has proven to be valid,” Tullow’s exploration director Angus McCoss said on an investor call.

Tullow’s geological theory is not without precedent. In the past decade, fields with tens of billions of barrels of oil have been found offshore Brazil in reservoirs that were formed when the two continents were attached.

The high-risk nature of the venture was reflected in the terms the companies received from the government, McCoss said, suggesting any oil produced will yield good margins.

BoA Merrill Lynch raised its net asset value of the company by 55 pence per share.

“We could see the resources at Zaedyus growing over time,” it said in a research note.

Tullow operates the Guyane Maritime licence and holds a 27.5 percent stake. Northpet — a company owned by Northern Petroleum and Wessex Exploration — has a 2.5 percent holding.

Northern Petroleum shares traded up 29.2 percent while Wessex soared 36.3 percent.

Tullow, Europe’s largest listed explorer by market capitalization, plans to drill wells in neighboring Guyana and Suriname next, with the French Guiana find making success there look more likely.

“We are pleased with the preliminary results of this first deepwater well offshore French Guiana,” said Dave Lawrence, executive vice-president of exploration and commercial at Shell.

“We are early in the evaluation, but the initial results are encouraging for this new play.”

The well was drilled to a depth of 5,711 meters and drilling will continue beyond 6,000 meters.