West African oil and gas basins are showing signs of growth, with Italian energy company Eni the latest to announce a discovery off the coast of the Congo.

The Italian energy company announced the discovery of natural gas and an ultra-light form of crude oil called condensate in the Nkala Marine basin about 12 miles off the coast of the Republic of Congo.

"The exploration of the pre-salt sequences continues to deliver new discoveries all along the West Africa's margin and confirms Eni's exploration technologies effectiveness, given the technical complexity of these play," the company said in a statement.

Pre-salt means the reserves are beneath a thick layer of salt at the ocean's floor, similar to those in Brazil, one of South Africa's largest producers. Eni estimated the size of the discovery at between 250 million and 350 million barrels of oil equivalent in place. The entire Marine basin may hold up to 5.8 billion barrels of oil equivalent.

Chevron's subsidiary in the region announced this week it started oil and gas production from the Lianzi field in a unified offshore economic zone straddling the borders of Congo and Angola. Located in the deep waters off the coast of western Africa, Lianzi is expected to produce an average 40,000 barrels of oil per day.

Eni said it's already discovered about 4 billion barrels of oil in basins off the Congolese and Gabonese coasts since it started work there more than four years ago.

Eni has been operating in Congo since the late 1960s, producing around 100,000 barrels of oil per day.