There is a law that has been in place since the 1970s prevents most U.S. oil from being sold abroad.
It all dates back to the 1973 oil embargo by several Arab nations. World oil prices were soaring, and Congress was trying to limit U.S. exposure to the global crude markets. So lawmakers enacted a bunch of conservation measures (including fuel standards for cars and trucks) as well as restrictions on exports. The idea was to keep as much crude oil at home, limit the nation's reliance on imports, and sidestep the volatile global markets.