It might be a lot easier to stop oil production by targeting the oil workers rather than field facilities the way terrorist tactics work. One for one, there are more terrorists than oil engineers. Not a good trade for those who most need Saudi oil--Europe and Japan.
based on conversations with my dad, the production facilities are vulnerable to people with knowledge of the system- not necessarily engineers, but even bright techs.
The Saudis, believe it or not, have a lot of their own folks running ARAMCO. Maybe not efficently...but if you can keep the plant going without blowing it up, you probably have enough knowledge/skill to blow it up if you want to.
An out and out attack by a squad/platoon sized element unfamiliar with the layout/technology might not do that much damage, but an inside job by four or five guys who smuggled in a truck/car bomb or two or four, and placed them where they would do the most good (bad) could.