Here's a provocative piece from the London Review of Books by Jim Holt, who has written for the New Yorker and the NYT. He argues that what seems like fumbling and missteps in Iraq have actually set things up nicely for western oil companies to gain effective control of much of the Iraq oil that hasn't yet been pumped , which is a lot. "How will the U.S. maintain hegemony over Iraqi oil? By establishing permanent military bases in Iraq." Five are in various stages of completion, well away from urban areas. As long as civil conflict simmers, there will be a rationale for keeping maybe 35,000 U.S. soldiers there. That could reduce the importance (and perhaps the revenue) of Russia, Venezuela and OPEC.
This is the most persuasive case I've seen for oil being a primary motivator behind the invasion. I'm skeptical to some degree because although almost nobody ackinowledges it, at some level almost everybody knows that there's no need to keep U.S. troops in the area to get Persian (or Arabian, depending which side of the gulf you live on) Gulf oil. Those countries have to sell it more than the U.S. has to buy it; otherwise their economic base is sand, and they'll have increasingly restive populations. And also, as Holt himself acknowledges:
"Still, there is reason to be sceptical of the picture I have drawn; it implies that a secret and highly ambitious plan turned out just the way its devisers foresaw, and that almost never happens."