The Oil Fields are Conveniently Near the Coasts

If you study this map, you'll see that the large oil fields of Iran and Saudi Arabia are located conveniently near the Persian Gulf (also called the Arabian Gulf).  There are large oil fields in Iran, under the Gulf itself, and in Saudi Arabia.  They're all part of the same oil patch.

Coincidentally, CENTCOM commands the 5th fleet, whose amphibious capabilities currently include the Bataan Amphibious Ready Group(ARG), whose flagship is the USS Bataan.  I'll bet these guys have better maps of the Persian Gulf, and it's oil fields, than we do. This is an image of the Persion Gulf from outer space.

And this a fascinating more detailed version of that same image (3 megabytes).  Iran has a rougher territory than Saudi Arabia.