The find, known as Carioca, "would further boost Brazil's prospects as an important world oil province and the source of new crude in the Americas" Haroldo Lima, head of the government's oil and fuel market regulator, said.
But a spokesperson for the National Petroleum Agency stressed that the data was "unofficial" and based on unconfirmed sources.
Petrobras said "more conclusive data" about the potential of the discovery would only be known after further evaluation.
"But if this is confirmed, it's really huge," said Sophie Aldebert, associate director with Cambridge Energy Association in Brazil.
Felipe Cunha, an analyst with Brascan bank in Rio de Janeiro, said further tests would be required to assess the scale of the find.
"It's a very relevant number, [it] basically triples the reserves. But it still seems a little premature to have a precise number while they are drilling a second well," he said.
According to the US Energy Department, Brazil's existing proven oil reserves total 11.8 billion barrels, while the US holds 21.8 billion.
If the reserves are confirmed, Brazil could jump into the top 10 oil countries by reserves, surpassing nations like Nigeria.
Even if the reserves are proven, it is likely to take ten years before the latest discovery can be turned into significant supplies.