UPDATE 1-Norway's PGS says wins biggest ever contract

(Adds details, share price)

OSLO, May 5 (Reuters) - Norwegian seismic survey group Petroleum Geo-Services ASA (PGS) (PGS.OL) won a contract worth $180 million to $200 million from Brazil's Petrobras (PETR4.SA), the biggest ever of its kind, PGS said on Monday.

PGS said the tendered value of the contract, which is for offshore surveys using its newest vessel, was $251 million but the value was cut after talks with Petrobras.

Shares in PGS rose 2.8 percent to 146.25 crowns by 1038 GMT, valuing the group at about $5.16 billion. The Oslo bourse's benchmark index .OSEBX was down 0.5 percent.

The contract is for a high-density, four-dimensional marine seismic survey work and is the largest such contract ever in the industry, PGS said.

The surveys in the Campos, Santos and Espirito Santo Basins are due to begin in October 2008, using PGS' latest vessel the Ramform Sovereign, PGS said.

"This contract is an important strategic milestone to PGS," PGS Chief Executive Jon Erik Reinhardsen said in a statement, adding that the contract was for "the area where the world's most significant recent discovery is made".

Last month the head of Brazil's National Petroleum Agency said a discovery by Petrobras at the Carioca prospect may be the world's biggest oil discovery in 30 years, though officials have subsequently toned down the optimistic comments.

Investor relations officer Baard Stenberg said the margin on the contract was in line with what the company was getting for other jobs but did not disclose a specific figure. (Reporting by Aasa Christine Stoltz and Richard Solem; Editing by David Holmes)