Sept. 29 (Bloomberg) -- Brazil’s so-called pre-salt oil region may hold between 25 billion and 100 billion barrels of oil, the country’s cabinet chief said.

“The reserves are significant,” Chief of Staff Dilma Rousseff said today in a speech in Brasilia, adding that the government doesn’t yet have a final estimate.

Brazil’s proved oil reserves totaled 12.6 billion barrels last year, according to London-based BP Plc. The pre-salt region located along the Brazilian coast includes the Tupi field, the biggest oil discovery in the Americas since 1976.

The estimate made today by Rousseff surpasses the 80- billion barrel forecast made in November 2008 by Haroldo Lima, head of Brazil’s national petroleum agency, or ANP.

Rousseff said a proposed shared-production plan would eliminate the government’s risks and guarantee the crude would belong to Brazil.

Under Brazil’s proposed new regulation, oil producers bidding to help develop the offshore fields will have to offer a percentage of production to the government. Brazil’s proposed new oil regulation requires congressional approval.

The government is seeking to make Petroleo Brasileiro SA, the country’s state-controlled oil producer, the sole operator of the pre-salt region. The company known as Petrobras will also hold a minimum 30 percent stake in all joint ventures set up to bid for licenses.

Reducing Costs

Petrobras Chief Executive Officer Jose Sergio Gabrielli, also speaking in Brasilia today, said a single operator of the pre-salt fields will help reduce costs.

The pre-salt area runs 800 kilometers (500 miles) and has oil deposits beneath a layer of salt resting as deep as 3,000 meters (9,843 feet) beneath the ocean surface and another 5,000 meters below the seabed.

Petrobras fell 12 centavos, or 0.3 percent, to 34.77 reais in Sao Paulo trading. The shares have risen 52 percent this year, less than a gain of 63 percent for the Bovespa index.

To contact the reporters on this story: Helder Marinho in Rio de Janeiro at hmarinho@bloomberg.netAndre Soliani Costa in Brasilia at asoliani@bloomberg.net

Last Updated: September 29, 2009 16:30 EDT