RIO DE JANEIRO, Dec 3 (Reuters) - Brazilian prosecutors

asked state-run oil firm Petrobras to do more studies

before drilling off the coast of the Amazon rainforest, citing

potential effects of extreme weather on tides, according to

documents filed on Monday to environmental agency Ibama.

An oceanographer's technical assessment commissioned by the

Federal Prosecutor's Office and dated Oct. 1 said extreme

weather events such as recent droughts in the Amazon could

create "unpredictable scenarios."

On that basis, prosecutors argued that an oil dispersion

model used by Petrobras in its divisive bid for a drilling

license near the mouth of the Amazon River is "insufficient."

Prosecutors said in a letter to Petrobras on Nov. 19 that if

the company fails to comply with the "minimum technical

requirements" for obtaining a license, they could file a lawsuit

against Petrobras due to the threat of environmental harm.

Petrobras will respond to all the questions raised by

prosecutors, the firm said in a statement, adding that it

offered "infrastructure, resources, plans and

socio-environmental programs that are much more robust" than

those required by law.

Petrobras is trying to explore for oil fields off the far

northern coast of Brazil in the so-called Equatorial Margin,

which is considered the country's most promising oil frontier

due to shared geology with nearby Guyana, where Exxon Mobil ( XOM )

is developing huge fields.

The Petrobras plan to drill in the environmentally sensitive

region has opened a rift in the government of President Luiz

Inacio Lula da Silva between some allies with environmental

concerns and others trying to expand the oil and gas industry.

It is not the first time federal prosecutors have threatened

to block the drilling plans, after they recommended Ibama deny a

license to Petrobras due to possible harms to indigenous

communities.