1. The US was in no danger of remaining without oil, the price of oil skyrocketed after the US invasion, and the "access" was certainly not worth trillions of dollars that the war cost, not to US as a whole.
2. The US already had a military presence in Saudi Arabia, and without the vicious circle of violence it itself caused, the US had no reason to maintain a permanent military presence in Irak.
Just see the recent Soleimani debacle -- the US is one such debacle away from being kicked out of Irak after blowing those trillions, killing hundreds of thousands of civilians, helping spawn Daesh, causing economic devastation in Irak and the region.
In 2008, the US imported 600.000 barrels of oil per day from Irak -- that's when oil peaked at ~150 -- but if the whole year the oil stayed at 150$/barrel, and the US just stole it without paying Irak anything, that's just 32 billion USD.
It would take the US 60 years to recuperate all the money it would have spent on Irak war if it was to steal all the oil from Irak at the levels of 2008, or 380 years at todays import levels and valuations (assuming the oil was war booty -- it is not).
There is no way the war was started over oil, that would be beyond dumb.
My personal theory is it was just the industrial-military oligarchs who started the war as to forever saddle the US taxpayer.
1. Secured strategic access to Iraq's oil resources. 2. Establish a permanent military presence in the region. 3. Removed the the de facto government.