Here is a very powerful excerpt from another article about oil.

The condition of Saudi fields, and those of other OPEC nations, is a closely guarded secret. That's largely because OPEC quotas, which were first imposed in 1983 to limit the output of member countries, were based on overall reserves; the higher an OPEC member's reserves, the higher its quota. It is widely believed that most, if not all, OPEC members exaggerated the sizes of their reserves in order to have the largest possible quota -- and thus the largest possible revenue stream.

Here is the link to the whole article. It's very well done.Oil:The Breaking Point Part 1 & 2 by Peter Maass