Brazil has started drilling a second well at its offshore Libra oilfield to assess the size of its deepwater reserves.

Drilling of the new well 370m from the first began last week. Brazil is exploring a pre-salt region as part of a plan to swap oil reserves for new shares in state-controlled Petrobras, Bloomberg reports.

A previous attempt was thwarted by a mechanical problem.

According to seismic data, Libra is expected to be larger than Franco, which has an estimated 4.5 billion barrels of recoverable oil.