Companies that provide oilfield services declined to comment on operations in Venezuela. Many have taken write-offs on Venezuelan holdings in recent years and some have opted to reduce operations in the South American country.

PHILLIPS 66

Phillips 66 said in an email it is confident it could obtain alternative sources of oil to lessen any disruption to its operations. The company said it complies with all U.S. laws, and noted that Venezuelan crude historically has made up a small percentage of its oil supply.

CHEVRON

Chevron Corp said it actively manages its crude supplies to be able to furnish customers with fuels and lubricants, and continues to comply with U.S. laws.

VALERO

Valero Energy Corp is reviewing the new U.S. sanctions and "will re-optimize" its oil purchases to minimize any impacts on its operations, the San Antonio, Texas-based company said in a statement. It also plans to aid the United States to make the nation's refining system operate more efficiently as a result of the sanctions, it said.

CITGO PETROLEUM

Citgo Petroleum, the U.S. refining arm of PDVSA, did not reply to a request for comment. The Houston-based company received some 175,000 barrels per day of Venezuelan crude last year, more than any other U.S. refiner.

PBF ENERGY

PBF Energy Inc did not respond to a request for comment. The Parsippany, New Jersey-based company was the fifth largest receiver of Venezuelan crude with 9,505 barrels per day.

HALLIBURTON

Oilfield services provider Halliburton Co declined to comment on its operations in Venezuela. Last year, the company said it would write off a remaining investment of $312 million in Venezuela.

WEATHERFORD

Weatherford International Plc declined to comment on its operations in Venezuela. In the fourth-quarter of 2017, Weatherford took a $230 million write-down on its Venezuelan receivables.

SCHLUMBERGER

Top oilfield services provider Schlumberger did not immediately respond to a request for comment on its operations in Venezuela. In 2016, the company began reducing activity in Venezuela amid overdue bills. In 2017, the company took a $938 million write down on its Venezuelan holdings and unpaid bills.

BAKER HUGHES

General Electric's Baker Hughes declined to comment on its operations in Venezuela. In its latest annual filing, the company said it was actively managing the relationship with its customer in Venezuela as they "transition through a difficult period" and had increased its allowance for doubtful accounts in the fourth quarter of 2017 by $55 million to offset exposure from that unnamed customer.

(Reporting by Collin Eaton; Editing by Leslie Adler and Marguerita Choy)