Bad news to Castros:Petrobras has withdrawn from an offshore oil exploration block

(Reuters) - Brazilian oil giant Petrobras (PETR4.SA) has withdrawn from an offshore oil exploration block in Cuba's waters that it leased amid great fanfare in 2008, a Brazilian official said Thursday, citing poor prospects.

Marco Aurelio Garcia, foreign policy adviser to President Dilma Rousseff, told reporters exploratory work off Cuba's northern coast did not "give results."

When asked if Petrobras had abandoned the block, he said: "Yes, that was already decided some time back. Petrobras withdrew from that (block). We're sorry, but the truth is you have to work with tangible elements and there wasn't any security of that in this block."

The decision appeared to be a blow to Communist-ruled Cuba's hopes for an oil bonanza from still-untapped offshore fields it says hold 20 billion barrels of oil.

But Spanish oil company Repsol YPF (REP.MC) is expected to bring a drilling rig to Cuba later this year to drill at least one exploratory well.

The U.S. Geological Survey has estimated Cuba has about five billion barrels of oil offshore. (Reporting by Marc Frank, Jeff Franks; Editing by Pascal Fletcher)