you're comparing apples and oranges the cia world factbook is talking about proved reserves which are oil that has already been found whereas campbell i believe the article doesn't cite its references completely enough to identify them is talking about ultimate reserves which is oil we might find but we haven't found it yet neither is particularly useful unless you state what price you expect to get and what technology you expect to use and then there's the oil sands wild card canada has or so barrels of the stuff of which only billion barrels are established which is yet another definition and venezuela has similar sized reserves so basically it's comparing apples oranges bananas and watermellons