NY Times article Cliff notes version : US and Chinese demand has put a pinch on available resources with little excess refining or pumping capacity to make up for it.
There is plenty of oil still in Saudi Arabia but it's mostly higher sulphur oil that costs more to refine and at the moment, is undesired.
The general security situation in Iraq has put off companies from exploring there for the time being.
Personally, I still blame George W. .......
