Not all crude oil is the same — some oil is physically lighter and easier for refineries to process. Venezuela's oil is heavy and dense, and requires special refineries. Burning any type of oil contributes to climate change, but Venezuela's oil is "among the dirtiest oils in the world to produce when it comes to global warming," says Paasha Mahdavi, associate professor of political science at the University of California, Santa Barbara.

What matters is that this kind of oil is something that a bunch of US refineries are set up to process, while the US exports light crude.