FACTBOX: Major oil service companies in Venezuela

(Reuters) - Venezuela has begun seizing oil service projects from private companies include U.S.-based Williams Companies (WMB.N) as leftist President Hugo Chavez boosts his control over the country's oil industry.

The move follows months of disputes between state oil company PDVSA and oil contractors over billions of dollars in unpaid debts accumulated as last year's oil price tumble left the company with a cash flow crunch.

Venezuela's Congress on Thursday approved a law allowing the state to take over a group of mostly small local service companies, sparking concerns the government could expropriate assets of major global oil services players.

The following are major service companies that operate in Venezuela:

* Schlumberger Ltd (SLB.N): The world's largest oil service company is also the principal provider of services in Venezuela including drilling, measurement and well services designed to boost output of operations.

Sources say PDVSA's debts with Schlumberger are the largest among the service companies operating in the country.

* Halliburton Co (HAL.N): A major player in Venezuela's oil services market, Halliburton provides products such as drilling fluids and a range of oil well services. PDVSA owes it hundreds of millions of dollars in debts.

Supporters of leftist Chavez have frequently insulted the company for its association with former U.S. Vice President Dick Cheney despite it being a key PDVSA contractor.

* Petrex: Owned by a division of Italy's Eni (ENI.MI), Petrex operates around 30 land rigs throughout Venezuela, mostly concentrated in the eastern region.