Shale oil, trapped thousands of feet below the North Dakota soil, is under extreme pressure. And in a way, the same is true of Continental Resources Inc. The Bakken-focused firm turned in 39 percent production growth last year and reaffirmed its goal of tripling production by 2017. But to get there -- and to do its part for the U.S. energy boom -- it will have to keep wringing oil out of increasingly stubborn rocks.