Peak Oil or the peak of any virgin resource field refers to a point where extraction rate peaks and begins to decline. Peak does not mean running out. Oil will continue to come until it declines to a trickle.
Extracting oil, gas or almost any resource is not like pumping crude from a tank that drains empty. Instead petroleum may initially be concentrated in a pool under pressure so that it surfaces without pumping. As the pressure subsides, much more remains, but trapped in sand, crevices, and so on. Extracting this takes more work.
A petroleum field then goes into secondary recovery, first forcing water down to push crude to the surface; then steam to decrease the viscosity of the crude; finally perhaps even using explosives to break up rock and release more oil from crevices. Today many wells are drilled with horizontal boring, fanning out runner holes into the rock to allow oil to drain into a much bigger exposure area.
The more dispersed and blocked the crude is, the more work required to bring it out. As a field goes into secondary and tertiary recovery, its return on energy drops until it’s finally not worth working any more. Advanced recovery technology can boost energy yield and delay the closure of a field, but it can’t offset basic process physics.
Those interested can find extensive technical discussion of this at: http://www.theoildrum.com/.