* Bidding round for 269 blocks scheduled for Oct. 7
* Lower oil prices force Brazil to "adjust calculations"
* Country seeks to double output and exports by 2025
By Vladimir Soldatkin
LONDON, June 2 (Reuters) - Brazil said on Tuesday that lower crude prices had presented a major challenge to its plans to sell dozens of oil and gas fields and that it had been forced to "adjust all calculations".
The Brazilian oil industry has been struggling with erratic production levels, tough regulation and a major corruption scandal involving the state-run oil company Petrobras .
The bidding round, scheduled for Oct. 7, will offer 269 blocks in 22 sectors of 10 sedimentary basins. But Brazilian authorities, who aim to double national oil production and exports by 2025, acknowledged the auction may not go as smoothly as they had envisaged a year ago.
"Companies are telling us that they are conservative because of the current level of oil prices ... From August to February oil prices fell from $110 per barrel to $48. And this is the real difference for all the petroleum industry," said Magda Chambriard, director of Brazil's state oil agency ANP.
"This is something that made us adjust all calculations related to this bidding round that were ready in 2014," she told reporters after the bidding round presentation in London.
She did not specify what the agency had been forced to adjust regarding the sale.
In 2013, the ANP held three auctions in a single year, including one for the massive "subsalt" prospect Franco that was won by a Petrobras-led consortium with France's Total, Royal Dutch Shell and Chinese oil companies.
"Of course, doing a bidding round (now) you cannot expect the same kind of results."
Giant untapped offshore oil deposits known as subsalt or pre-salt could play a major role if Brazil is to meet its target of doubling oil production and exports in a decade.
Such deposits have not been included in the current bidding round. Chambriard said the "pre-salt" auction could be held next year or at the beginning of 2017.
These high-quality but technically challenging crude resources get their name from a layer of mineral salts that trap oil thousands of metres beneath the seabed off Brazil's coast.
Announced in 2007, the discoveries are some of the world's largest in decades. Estimates range from 30 billion to 100 billion barrels of oil and gas, enough to supply all the world's needs for one to three years. (Editing by Pravin Char)