Are numerous production centers spread across vast areas or are they more geographically concentrated?
How vulnerable are they to intermediate-range missile attack? Are Patriot missile batteries currently in place to protect them?
I have no idea about defenses, but who would be shooting missiles at Saudi Arabia? It would have to be a nation like Iran or Iraq and if they did, there would be a whole lot of explaining to do by them I would think.
I don’t think anyone is handing a terrorist a ballistic missile.
My SWAG: Not very. Sure, you could do some damage, but remember Saddam? He had months for his men to place individual demolition charges, well-by-well, facility-by-facility, and even then, despite all the “expert” predictions of a recovery lasting years, in less than one year, all the fires were out, and production resumed very shortly thereafter. Short of nuking the Saudi fields, I doubt a missile attack could do half as much damage, even with more missiles than are available in all the inventory of all the nations in the Mid-East.