Eni announced it made the discoveries while drilling in more than 9,800 feet of water approximately 10 miles off the Congolese coast.
The energy company said discoveries in the Nene Marine exploration site were significant.
"The company estimates the volume of the discovery proved with the two wells so far drilled at around 600 million barrels of oil and 700 billion cubic feet of gas in place," Eni said in a statement.
U.S. energy company Chevron said in March it expects separate development of deep-water projects off the Congolese coast will produce 140,000 barrels of oil by 2017.
Eni has been present in Congo since the late 1960s.
Eni said Thursday its second quarter net income was down more than 55 percent to $764 million. Eni Chief Executive Officer Paolo Scaroni attributed much of the loss to the troubled European economy and political unrest in the Middle East and North Africa.
"We expect a significant improvement in our second half results," he said in a statement.