The United States faces an awkward rival in its first attempts in 40 years to export crude oil — Iran.

Iran, whose economy has been throttled by Western sanctions that have halved its crude shipments, is now selling higher quality and cheaper oil to China that leaves little room for the U.S. crude to enter the world's top energy consumer.

While buyers in Japan and South Korea have been willing to try a U.S. grade of the super-light crude known as condensate, China has already locked in annual contracts with Tehran and is not expected to take any U.S. oil in the short-term.