Fifteen companies have preregistered for Brazil’s third auction of oil exploration rights in the presalt area while 10 companies have expressed interest in the second round, oil regulator ANP said Sept. 11.

ANP told Reuters that all the companies were oil majors.

The two auctions will take place on Oct. 27.

Through production-sharing contracts, firms will be able to bid on eight blocks in the Santos and Campos basins in the presalt area, where hydrocarbons are trapped under thick layers of salt beneath the ocean floor.

The fields are considered among Brazil’s most prized energy assets. Those already under production are showing above-average productivity.

The companies had a deadline of Sept. 8 to fill out an initial form expressing interest in the bidding process, but will not officially be considered registered until they have been approved by a committee.

No more companies can sign up to participate. In August, oil regulator ANP published details of how the auction will work, including bidding arrangements for the three prospects for which state-controlled oil-giant Petrobras exercised its preferential rights.

The numbers of companies preregistered for the bidding rounds first appeared in a column in Brazil’s O Globo newspaper on Sept. 11.