An offshore find by Brazil's state-run oil firm Petroleo Brasileiro SA, or Petrobras, in the Atlantic ocean may be the largest oil discovery in the past 30 years, the head of the Brazil's National Petroleum Agency said on Monday.

Petroleo Brasileiro's American depository shares were recently up $8.41 to $121.26 on composite trading on the New York Stock Exchange, and hit a high of $125.29 during intraday trading.

The discovery was made at its BM-S-9 block, known as Carioca, and may hold as much as 33 billion barrels of oil.

Haroldo Lima, the director of the agency, cautioned that his information on the field off the coast of Rio de Janeiro was unofficial and still needed to be confirmed.

Lima's comments were confirmed by Fernando Manso, a spokesman for the agency, and were reported earlier by Folha de S. Paulo. Petrobras declined to confirm the discovery.

If the report is confirmed as true, the oil in the Carioca exploration area would be five times larger than the recent giant Tupi discovery in Brazil, whose estimated reserves of 8 billion barrels were announced by Petroleo Brasileiro SA in November.

Tupi was the largest discovery since Kazakhastan's Kashagan field, which has 12 billion barrels of recoverable reserves.

The discovery will further boost Brazil's prospects as an important world oil province and the leading source of new crude in the Americas.

The discovery by Petrobras was in partnership with Repsol-YPF and BG Group.

Brazil's current proven oil reserves are 11.8 billion barrels, according to the U.S. Energy Department. The U.S. has 21.8 billion barrels in proven reserves. If the reserves are confirmed, Brazil could jump into the top 10 countries by reserves, surpassing countries like Nigeria.

The largest oil fields today include Saudi Arabia's Ghawar and Kuwait's Burgan fields. Ghawar holds an estimated 75 billion to 83 billion barrels of crude; Burgan has as much as 72 billion.

Energy