Oil Minister Rafael Ramirez declared that the oil fields had reverted to state control just after midnight.
Television footage showed workers in hard hats raising the flags of Venezuela and the national oil company at a refinery and four drilling fields in the petrolific Orinoco River basin.
At a ceremony the following day, Chavez addressed thousands of red-clad oil workers.
“The nationalisation of Venezuela’s oil is now for real,” he said.
Chavez described the state takeover a historic victory for Venezuela after years of US-backed corporate exploitation and said the country must have control over its natural resources.
Venezuela’s state oil company, Petroleos de Venezuela, is taking at least a 60% stake in each of the operations. But the foreign companies have been invited to stay as minority partners and have until June 26 to negotiate the terms, including compensation and reduced stakes.
The companies ceding control included BP, ConocoPhillips, ExxonMobil, Chevron, Total SA and Norway’s Statoil.
All but ConocoPhillips have agreed in principle to state control, and Venezuela has warned it may seize that company’s assets if it does not fall in line.
Chavez also accused foreign oil companies of damaging oil fields through bad drilling practices and chasing quick profits, and said Venezuela could sue them for this damage.
It is unclear what will happen next.
Venezuela lacks the expertise for optimal production and refining of its heavy crude.
If Chavez scares off Western corporations, the Venezuelan oil industry could be severely handicapped.
He claims state-owned firms from China, India and elsewhere are ready to fill any gap, but analysts believe only the Western multinationals have the necessary skills and experience.
The big oil corporations would be reluctant to pull out of Venezuela.
They have invested more than $US17 billion ($A20.6 billion) in the projects, now estimated to be worth $30 billion. Venezuela has indicated it might only pay the original investment and use oil and tax forgiveness to further reduce the cash payout.
Even if the oil companies stay, do not expect to see new investment.
New investment in the Venezuelan oil industry dried up some time ago. Foreigners are not willing to take the risk and the country itself has mismanaged its own assets.
Energy output has slipped by one-third, corruption within the state oil complex is rampant and foreign assets are now being sold because the state cannot supply or service them effectively.
The country is still a major oil producer but Petroleos de Venezuela is seeing its output shrink month by month.
