Peak Oil as a concept for production in a conventional reservior makes sense. It's on a time frame/size that makes sense. Reserves are not explicity capacity based, reserves are heavilly economically based. As price increases you get more reserves despite having produced oil. This, in addition to technological changes, allows for increased reserves and is why global peak oil as a concept is a bad predictor. Global peak oil was originally predicted to be 1970's.

Edit: grammer/spelling