Nice piece from Econbrowser on the effect of sweet and sour crude and oil prices.
Light sweet crude is easier and less expensive to refine and, not surprisingly, not every refinery is equipped to handle the heavier, sour crudes. Why not? Well, rather than repeat Professor Hamilton here, I suggest you read his post.
P.S. (the government is involved)
UPDATE: Other factors: tanker shortage, refining capacity and more lack of refining capacity/
Oil shale and other exploration here.