Loading of Venezuela's heavy crude at its main oil ports slowed this week after the U.S. imposed a tariff on trade with countries buying the South American nation's oil and producer Chevron began reducing its tanker fleet there, according to shipping data and a document seen on Tuesday. On Monday, U.S. President Donald Trump's administration published an executive order declaring that any country buying oil or gas from Venezuela will pay a 25% tariff on trades with the U.S. starting in early April.