Oil demand for plastics is pretty minor. Any demand for oil for plastics can be supplied from non-middle eastern countries with ease if we stop burning the stuff.
Natural gas is used for fertiliser production, not oil. Thanks to fracking and coal seam gas there is plenty of natural gas outside the middle east for fertiliser production.
OPEC can jack up the price to anything they like, but if nobody is buying because there is no demand then the middle east can revert to the backwater it was 100 years ago.