Officials of the Nigerian National Petroleum Corporation said the two companies merely declined to sign agreements for the operatorship of the two blocks.

Many industry watchers believe the firms might have rejected the Federal Government's decision to hold 30 percent equity in the blocks under the Production Sharing Contract (PSC) arrangement guiding the awards of the acreages.

While the American oil giants are heading for the exit, Petrobras and the Nigerian Petroleum Corporation signed an agreement to explore deepwater block OPL324 offshore Nigeria. Petrobas would serve as operator under the 30-year contract and hold a 75 percent stake in the block. The agreement heralds Petrobras' first venture as an operator in Nigeria. Late last year, Petrobras acquired a 15 percent stake in block OPL250, which is operated by ChevronTexaco.