Statoil, along with its partners at Exxon Mobil, said its Lavani well off Tanzania has a reserve estimate of 3 trillion cubic feet of natural gas in place.
Tim Dodson, executive vice president for exploration at Statoil, said the find is significant given its proximity to other recent discoveries in the area.
"The results so far mark an important step toward a possible natural gas development in Tanzania," he said in a statement.
The well was drilled in waters around 7,800 feet.
Lavani marks the second significant find for Statoil in Tanzania and the seventh in 14 months following major finds in the Barents and North seas and off the coast of Brazil.
The World Bank announced last year it approved an International Development Association credit of $27.8 million to back Tanzania's energy expansion projects. U.S. officials had said a lack of energy is one of the main reasons for slow development in the region.