How could you possibly recover 100%?  Dig shafts into Ghawar and mine the stuff?
Mike Lynch floated that idea once. Oil mines were common in the early days of oil. That was Colonel Drake's original plan too. He was going to mine the oil, but then he decided to drill for it instead.
http://www.sjgs.com/history.html
There was a thought to do this with some of the shallow oil sands around Bakersfield.  The problem comes from the volatiles that would escape and have a quite negative environmental impact.