Venezuela's maverick President Hugo Chavez plans to slap a new tax on oil producers in a bid to grab windfall gains as crude trades above $100 a barrel.

Foreign oil companies in Venezuela "are earning money that they haven't counted on in any calculation", Chavez said, adding: "It's not the product of any extraordinary effort."

In 2006 and 2007, he threw out oil contracts and forced companies including Royal Dutch Shell, Exxon Mobil and Chevron to accept minority stakes in all oil projects.

Earlier this year, amid signs Chavez had been forced to rethink his hardline policy, Shell signed a deal with state oil company PDVSA for joint studies covering the development of an area the British company used to control.

Chavez says the law enabling the latest tax grab has been drafted but the rate is still being discussed.

Venezuela has already increased income tax rates, royalties and extraction taxes. It also has a clause in each joint-venture agreement guaranteeing that if taxes and royalties don't add up to at least 50% of every barrel, companies must pay the difference.