El Presidente reckons a separate Scotland could put less than 10% of oil and gas revenues into a sovereign wealth fund every year for the next 20 years and end up with £30bn by 2035. That would imply more than £10bn revenues every year for the next 20 years. Seems pretty optimistic, given that even the Scottish Government's own figures for Scotland's "geographic share" of North Sea revenue for 2009-10 was less than £6bn. Not to mention that UK oil production peaked in 1999, and according to official figures, somewhere between 76% and 90% of all UK oil reserves had already been extracted by the end of 2010.
And £30bn sounds a lot, but compare it with the £59.2bn total public expenditure in Scotland for 2009-10 (which, of course, didn't include all the considerable costs that an independent country would incur) and it doesn't seem so impressive.