Brazilian state-run energy giant Petrobras (PBR) expects to bring up to four more drilling rigs to prospect in the offshore Santos Basin in the second half of the year.

The ultra-deepwater rigs will be used to "attack" areas in the subsalt region in the Santos Basin, Petrobras' Mario Carminatti told the local Estado news agency. The Santos Basin is home to the Tupi field, the Western Hemisphere's largest oil discovery in more than 30 years.

The rigs will be used to complete studies of the Tupi, as well as other subsalt drill targets, Petrobras' executive manager for exploration said.

"We have other areas of interest in the Santos Basin, all in the subsalt," Carminatti added.

Tupi is estimated to hold recoverable reserves of between 5 billion and 8 billion barrels of oil equivalent. It's located in the BM-S-11 block, part of a cluster of exploration blocks loaded with oil prospects hidden more than 7,000 meters under water, shifting salt and rock.

Carminatti's comments followed the announcement earlier Friday that Petrobras had reached a deal with Exxon Mobil (XOM) and Norwegian driller Seadrill (SDRL.OS) to take over control of Seadrill's West Polaris rig.

The West Polaris was drilling for Exxon in the BM-S-22 block, site of the Azulao and Guarani prospects.

Similar deals could be reached with other foreign oil majors with drilling rigs operating offshore Brazil, Carminatti said. The idea is to utilize downtime between contracts to squeeze in some drilling.

"This was already done with Repsol (REP), with Anadarko (APC) and now with Exxon," Carminatti said.

-By Jeff Fick, Dow Jones Newswires; 55-21-2586-6085; jeff.fick@dowjones.com