New data shows the Grangemouth refinery contributed over £400 million to the economy last year.
It is due to shut next year - with 400 jobs at risk.
A report for Scottish Government ministers says the "relatively well-paid technical jobs" there will be difficult to replace.
Data shows the average salary across over 500 jobs at the site is 45% above the local average.
It says the refinery supports the full-time equivalent of nearly 3,000 jobs across the area.
The assessment was completed before Petroineos made its closure announcement in September 2024.
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