Eni said it made a "high impact" discovery while working in its Agulha exploration prospect in the deep waters off the coast of Mozambique.
"Preliminary estimates show that the Agulha structure could contain 5 [trillion] to 7 trillion cubic feet of gas in place," the company said in a statement.
The well is located approximately 50 miles off the Mozambique coast. The company said the discovery marks a new phase for Mozambique, with three more wells planned for next year.
Eni in April said it estimated there are 80 trillion cubic feet of natural gas at its Mamba Complex and Coral discoveries in Mozambique.
Tullow Oil, which has headquarters in London, said Monday its Buzio-1 exploration well offshore Mozambique was a "dry hole." The company said it would combine information from Buzio-1 with other prospects in Mozambique to determine its next steps.