I don't pretend to know - but I'm curious enough to consider the possibility and make up my own mind based on the information available.
Economists are funny people. A lot of them seem to believe that the textbook theory of supply and demand translates perfectly into the real world. The reason I mention this is that there seems to be a belief that when crude oil becomes scarce enough, more drilling will occur, increasing the supply - and there's nothing wrong with that, in theory.
The problem is that in practice we are dealing with a finite resource. Ask an economist about the market for dodo bird feathers is doing, and he'll give you a blank look. Adam Smith didn't live in a world of limited resources - he lived in a world of immature industrialization.
With that in mind I found some fascinating charts regarding North Sea oil production at The Oil Drum. This is very thought-provoking stuff! There is a clear double production peak and decline in spite of frenzied exploration and development effort.
The original posting can be found here. I would have re-posted the charts, but the pix wouldn't show up on blogger for some reason :-(