By Fabio Teixeira

RIO DE JANEIRO, March 11 (Reuters) - Brazil's Petrobras PETR4.SA has obtained approval from the country's environmental agency to clear corals from the underside of the drilling vessel it plans to use in the Foz do Amazonas region, documents seen by Reuters showed.

The firm's February 10 request was granted on Monday for the vessel the state-run firm will send to drill off the coast of the northern Amapa state if it obtains a long-sought license to explore the environmentally sensitive region.

Due to the corals being a potentially invasive species if moved to another biome, it must be cleared in a process supervised by Ibama, Brazil's environmental agency.

Once the corals are cleared in Rio de Janeiro, the vessel could sail to the so-called Equatorial Margin, considered the most promising frontier for oil exploration by Petrobras.

If the license is granted, Petrobras still hopes to begin drilling this year.

Last month, Petrobras faced another setback to its plans to drill in the area, as Ibama's technical staff recommended the body deny the drilling license for a well in the northern end of the Equatorial Margin, which shares geology with nearby Guyana, where Exxon Mobil XOM.N is developing huge fields.

On Tuesday, Petrobras' exploration and production head Sylvia dos Anjos said at the CERAWeek conference in Houston that the firm is "optimistic" it will get the license.

In May 2023, Ibama denied Petrobras' request for an offshore drilling license for the Foz do Amazonas area off the coast of Amapa state, citing environmental concerns. The oil company appealed soon after and a final Ibama decision is pending.

(Reporting by Fabio Teixeira in Rio de Janeiro; Additional reporting by Marianna Parraga in Houston; Editing by Bill Berkrot)

((Fabio.Teixeira@thomsonreuters.com; +55 21 99603-4782;))