Demand is exceeding supply, to get your share you must pay more, with relatively new players like China increasing their share than demand is continuing to outstrip supply.
The cost of extraction is not increasing to any great extent, yet, so the oil companies are doing very well thank you.
The big problem is that it can only get worse. Not for the oil companies yet, but eventually they will have nothing to sell, and who will want their shares then.
Prospector