The state-led Brazil giant said the blaze at the P-20 unit had injured two workers and forced the company to shut in 22,000 barrels of production as a safety precaution.
One worker was treated for smoke inhalation and another for a slightly twisted ankle, Petrobras said.
“The company announced the fact to the competent authorities and started research with the aim of investigating the causes of the incident,” the company added.
It was unclear if production had been restarted.
Overall, Marlim is Brazil’s number-three producing field, with output of 207,000 barrels of oil equivalent per day recorded in October.
Petrobras has been in a push to avoid platform downtime amid a drive to add oil production of over 200,000 barrels per day in the prolific Campos basin by 2017.
The company is investing $5.6 billion to refurbish and repair ageing production platforms in the area and wants the units to be given maintenance and refurbishment overhauls regularly every two years.