An audit of Nigeria's oil industry has found giant gaps between the amount of tax that firms say they paid and what authorities say they received.
Oil and gas accounts for 80% of the government's revenues
The discrepancies - totalling hundreds of millions of dollars - have been discovered by a government-backed anti-corruption agency.
Nigerian Extractive Industries Transparency Initiative (NEITI) looked at payments from 1999 to 2004.
Its report stopped short of saying what might have happened to the cash.
"We found some strange situations," said Chris Nurse, managing director of audit firm Hart, which carried out the study for the NEITI.
"For the year 2002, there were $250m (£142m) that the companies say they paid to the central bank that do not appear in central bank records."
Mr Nurse added that there were also discrepancies between figures provided by different arms of the government, and that accounting standards were low across the board.
Nigeria is Africa's largest oil producer, and oil and gas revenues account for more than 80% of the country's annual budget.
The country is the sixth most corrupt in the world, according to independent global anti-corruption watchdog Transparency International.
The main multinational oil firms in Nigeria are Shell, Exxon Mobil, Chevron, Total and Agip.
Foreign oil facilities in the main Niger Delta oil-producing region have recently suffered attacks from anti-government militants, who say the region should get a bigger share of oil revenue.