Brazil's oil regulator ANP approved a plan by the country's state-run oil company Petroleo Brasileiro SA to renew its development plan for Brazil's Marlim offshore oil field, one of the country's largest, an ANP director said on Friday.

Petrobras, as the oil company is known, drew up the plan after the ANP told it about two years ago to come up with ways to limit declining output from several of the largest, but ageing, oil fields in the Campos Basin.

The Campos Basin is an offshore region northeast of Rio de Janeiro that was responsible for 76 percent of Brazil's oil output in May, down from 85 percent four years earlier. Output has stagnated as Petrobras has been unable to bring on new areas fast enough to make up for declines from older fields.

While the plan will require Petrobras to make substantial investments in Marlim, the ANP did not say how much Petrobras must spend on the upgrade, the ANP director, Florival Carvalho said.

"The ANP analyzed the plan from the point of view of increasing the capture of oil and they will have to make the investments to get this done," Carvalho said.

The new investments could further burden Petrobras, already the world's most indebted and least profitable major oil company, as it struggles to pay for a $221 billion, five-year investment plan, much of it over budget and behind schedule. Stagnant production and the need to subsidize fuel imports have squeezed Petrobras for cash and caused its debt to soar.

Petrobras officials were not immediately available for comment.

Last year, the ANP approved new development plans for the giant Roncador and Marlim Sul fields, also in the Campos Basin. Roncador, Brazil's top producing field produced 278,000 barrels of oil and equivalent natural gas per day (boepd) a third less than the 359,000 barrels a day it produced four years earlier.

In addition to Marlim, Roncador and Marlim Sul, Petrobras will have to boost investment in the Albacora Leste, Jubarte, Albacora and Barracuda-Caratinga fields, all also in Campos.

As a condition of the approval, the ANP made a series of additional requests and ordered the company to complete a field recovery planning document with firm investment commitments by the end of this year.

The main requirements focus on the need to increase water injection into the field to capture residual oil and the application of technology and equipment to boost recovery of oil mixed with water and sediments. Petrobras will also have to drill new wells and perform new seismic studies.

Petrobras preferred shares, the company's most-traded class of stock, rose 1.08 percent in afternoon trading in Sao Paulo on Friday, on track for a second day of gains.