By Gary Duffy
BBC News, Sao Paulo
Brazil has developed expertise in deep-sea drilling in recent years
The Brazilian government says huge new oil reserves discovered off its coast could turn the country into one of the biggest oil producers in the world.
Petrobras, Brazil's national oil company, says it believes the offshore Tupi field has between 5bn and 8bn barrels of recoverable light oil.
A senior minister said Brazilian oil production had the potential to match that of Venezuela and Saudi Arabia.
Petrobras delivered its estimate after analysing test results.
The state-controlled company says the results show high productivity for gas and light oil - the best quality oil - which is more valuable and cheaper to refine.
Petrobras says the find has the potential to move Brazil into a position where it is one of the top ten oil reserves in the world.
Brazil currently has proven oil reserves of 14 billion barrels, over half of which have been discovered in the past five years.
The news, which led to a sharp rise in company shares, was also given an enthusiastic welcome by the government.
The senior minister in charge of the cabinet, Dilma Rousseff, said if the deposits turned out to be as significant as first thought, it would place Brazil in the same league as Venezuela and countries in the Arab world.
With a reserve like this, the country could be transformed into an exporter of petroleum, she said.
"This has changed our reality," she said.
Most of Brazil's oil is heavy and found at great depth but even so its reserves have almost doubled in the last ten years, as has output.
Some analysts say this latest find raises the interesting scenario of offering a new source of supply to the United States, reducing its dependence on Venezuela, a country with which it has such a fraught relationship.
With the Tupi field potentially equal to 40% of all oil ever discovered here, it seems by any standards a significant moment for Brazil.
"If the best-case scenario happens, this discovery would make Petrobras' reserves overcome those of Shell and Chevron," said Felipe Cunha, an analyst with brokers Brascan.
Petrobras controls 65% of the firm which has the exclusive licence to explore and extract oil from the Tupi field in the Santos basin, about 180 miles south of Rio de Janeiro.
British firm BG and Portugal's Galp Energia hold minority stakes in the business.
Reserves have also been found in the more northerly Campos and Espirito Santo basins.