There's been talk about spare capacity so here's a thread..
First, Euan Mearns has a good chart just freshened up this morning in his Oil Production Vital Statistics September 2015 post
If spare capacity is a gauge for the "risk premium" and so price, it is easy to see the low level around '05, the bounce after the recession and the rebound up to 5mbd in '04 that finally broke the price.
Of course also easy to see is the decline over the last year. Iran is really the only one with spare capacity aside from KSA.