BP PLC, Chevron Corp and Royal Dutch Shell PLC are among firms that will compete for exploration and production rights off the Brazilian coast on Thursday, in the first of three oil bidding rounds scheduled for coming weeks.

Combined, the minimum signing bonuses for the blocks in the so-called 16th post-salt oil bidding round in the Latin American oil powerhouse come to roughly $800 million.

That is significant by historical standards, but pales in comparison to Brazil's so-called transfer-of-rights bidding round and 6th pre-salt round, both scheduled for early November. Those rounds are expected to bring around $25 billion and $2 billion into government coffers, respectively.

The post-salt blocks are less prolific than the pre-salt blocks, where billions of barrels of oil are trapped beneath a thick layer of salt beneath ocean floor.

Still, there are some prized areas in the 16th round, particularly in the Campos and Santos Basins off the coast of Sao Paulo and Rio de Janeiro. One block in particular, the C-M-541 in the Campos Basin, has a minimum signing bonus of roughly 1.375 billion reais ($335 million) and directly borders the pre-salt region.

Seven relatively minor blocks off the northeastern state of Bahia are the subject of environmental litigation by federal prosecutors, authorities said on Wednesday, casting judicial uncertainty over parts of the auction.

Authorities and environmentalists are concerned that the seven blocks in question, which have a combined signing bonus of roughly $16 million, are located too close to the ecologically significant Abrolhos archipelago.

Ecopetrol SA, Karoon Energy Ltd, Qatar Petroleum, Equinor ASA, Total SA, China National Offshore Oil Corp Ltd, Repsol SA, Murphy Oil Corp, Petrogal, Petronas and Wintershall Dea GmbH are also registered to participate in the bidding.

Brazil's Petroleo Brasileiro SA, known as Petrobras, and Enauta Participacoes SA are also eligible to bid.

Karoon, Shell, Enauta, Ecopetrol and Petrobras declined to comment on the bidding round. All other companies did not immediately respond to a request for comment.

(Reporting by Gram Slattery and Marta Nogueira; editing by Richard Pullin)