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 1.7 or so barrels of the stuff (of which only 175 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.