Jim Holt lays it out so even I can understand it in the London Review of Books:
"It has been estimated, by the Council on Foreign Relations, that Iraq may have a further 220 billion barrels of undiscovered oil; another study puts the figure at 300 billion. If these estimates are anywhere close to the mark, US forces are now sitting on one quarter of the world’s oil resources. The value of Iraqi oil, largely light crude with low production costs, would be of the order of $30 trillion at today’s prices. For purposes of comparison, the projected total cost of the US invasion/occupation is around $1 trillion."