Oh, I think I know this one. Venezuela crude is really heavy and dirty, but a lot of refineries, including ones in Texas and I imagine Iran, are designed to mix it with some lighter crude to derive a decent gasoline yield. These refineries only work like this however, and have basically become dependent on dirty crude sources like Venezuela. The USA I think is dealing via Alberta tar sands, but maybe they decided to just steal the oil from Venezuela directly (Trump is unhinged like that).