Brazil’s Libra oilfield may hold 15 billion barrels, which is almost twice initial estimates, making it the biggest discovery in the Americas in over three decades.

Brazil’s national petroleum regulator ANP found oil below a layer of salt at the first exploration well of the government’s field.

The well was drilled to a depth of 5,410m (3.4 miles) and may reach 6,500m by December 2010, according to Bloomberg.

A deposit of 15 billion barrels would be almost twice the size of state-controlled Petroleo Brasileiro’s nearby Tupi field and would eclipse Brazil’s total current reserve base.

Libra and Tupi are located in the Santos Basin’s pre-salt region, which runs 800km off the coast and holds oil deposits beneath a layer of salt 3,000m beneath the ocean surface and another 5,000m below the seabed.

Petrobras will operate all new concessions in the pre-salt region under a proposed regulation.

The Brazilian Government intends to keep control of Libra as the nation tightens its grip on its reserves.

Pre-Sal Petroleo, a new state oil company, will own at least 40% of Libra, while 30% will be held by Petrobras.