At least 11 oil firms paid more than $900,000 each to join an auction next month for rights to explore Brazil\'s huge deepwater Libra field, authorities said Thursday. The National Petroleum Agency did not identify the companies involved but Britain\'s BP and BG Group, as well as American firm ExxonMobil are not on the list, according to ANP director Magda Chambriard. The oil regulator chief said she expected about 40 companies to bid for Libra. \"Now, there is an international context,\" the oil regulator\'s chief said. The enormous Libra oil prospect, found in 2010, marked the largest oil discovery in Brazilian history. It is believed to hold between eight and 12 billion barrels of recoverable oil, and covers an area of 1,500 square kilometers (600 square miles) in ultra deep oil fields. The auction, scheduled for October 21, will be for a 35-year non-renewable concession to work the field. In 30 years, the Libra field could bring in returns of $408 million -- $135 million in royalties and $273 million in oil participation -- according to Chambriard. Next month\'s auction will be the first such sale since the 2007 discovery of huge pre-salt deepwater reserves off southeastern Brazil. ANP estimates that those reserves, buried beneath several kilometers of ocean, bedrock and hot salt beds, could hold more than 100 billion barrels of high-quality recoverable crude and could turn Brazil into one of the world\'s top exporters. Brazil hopes to boost its oil output from around two million barrels a day currently to nearly five million by 2020, largely thanks to the pre-salt reserves. State-run energy giant Petrobras will have a mandatory 30 percent participation in the Libra concession. ANP will auction the remaining 70 percent, in which Petrobras can also participate alone or as part of a consortium with other firms.