That oil was important for Europe and most of all Japan and China. And that is still the case in many ways. Of course now the US maybe doesn't want China to have cheap oil anymore.
If it was about securing US oil, war in Venezuela and Canada would be far more sensible.
And even then the US didn't need to do anything, everybody would have happily sold the US oil without the need for any kind of war or US troupes of any kind.
The US increase intervention was required because of policy growth, where the US wanted an ever increasing control over these places for things other then oil. Once the Soviet Union was gone, the US started to dream of reshaping the the region and integrating it fully in the the US system.