I work in the oil industry, on the 'get it out of the ground' side of things and I can tell you that there is absolutely no shortage of oil. They will be taking oil out of the North Sea long after I'm pushing daisies and I'm only in my 20's ...
You know what the threshold for profit on most of the installations in the north sea is? $12 a barrel
Some of the newer installations that are extracting 'difficult' oil are higher at $35-40.
The reservoirs of old had to be abandoned when about 30% - 40% had been extracted and the technology now exists to get and get most of the rest out. Of course this technology has a cost but its not why oil is so expensive today. In fact its mostly a mystery, speculation would be my best guess. All those reckless bankers have moved into commodities I think!