April 17 (Bloomberg) -- Petroleo Brasileiro SA, Brazil's state-controlled oil company, will need to complete more wells and studies to determine the size of oil reserves in fields off the country's southeastern coast.

Petrobras, as the Rio de Janeiro-based company is known, has drilled 15 wells in so-called ``pre-salt'' offshore fields between Espirito Santo and Sao Paulo states, Edison Lobao, Brazil's mines and energy minister, told reporters today in Rio de Janeiro. Pre-salt fields lie beneath as much as 10,000 meters of ocean and seabed and represent a new Brazilian oil province beneath shallower existing fields.

``This is a very sensitive issue,'' Lobao said. ``Petrobras will not release information on these fields until it is absolutely sure of its technical assessment.''

Brazil, which said in November that its Tupi pre-salt field has 5 billion to 8 billion barrels of recoverable oil, may have as much as 70 billion barrels of oil in similar fields along its Atlantic coast between Espirito Santo and Santa Catarina states, O Estado de S. Paulo newspaper said today, without saying where it got the information.

Interest in the so-called ``pre-salt'' fields soared on April 14, when Haroldo Lima, head of Brazil's national petroleum regulator, said Petrobras' Carioca pre-salt field had 33 billion barrels of oil. Petrobras jumped 5.6 percent.

A field that size would make it the third-largest field ever discovered and would be big enough to supply every U.S. refinery for six years.

The largest estimates of Brazilian pre-salt would place Brazil among countries with the 10 largest oil reserves, the newspaper said.

Petrobras preferred shares, the company's most-traded class of stock, rose 99 centavos, or 1.2 percent at 4:07 p.m. in Sao Paulo.

To contact the reporter on this story: Jeb Blount in Rio de Janeiro at jblount@bloomberg.net; Romina Nicaretta in Sao Paulo at rnicaretta@bloomberg.net

Last Updated: April 17, 2008 15:38 EDT