It’s the size of the tap...

I think it is really important to understand that Peak Oil is not about oil 'running out' (which, of course it will do one day, eventually). Rather, as the good folk at the Oil Drum like to put it it: "It’s the size of the tap, not the size of the tank."

Gail Tverberg states the case with clarity and precision:
"On a worldwide basis, the phenomenon of peak oil can be thought of as a crisis in resources needed to produce oil. It’s the size of the tap, not the size of the tank. As we deplete the large, easy-to-produce fields and move to ever-more-difficult fields, it takes more and more oil rigs, more petroleum engineers, and more investment dollars.

Eventually we reach a point where we are out of equipment, out of trained personnel, and the investment cost for expanding production becomes prohibitive. When production begins to drop because of all of these pressures, we reach peak oil'." 
I highly recommend reading the full article ' What is Peak Oil?'.