The article specifies that historical volume of the basin's oil deposits would be worth $2T at current market prices of over $100/bbl. That's a neat trick, projecting today's prices backwards in time onto oil that was originally pumped in the 1920s. The article admits that the basin's proven reserves are currently 4.5B bbls. Multiply that figure by the WTI price of $105.68 for March 20, 2012 (the most recent one available from EIA) and we have a total present value of $475.56B for the basin's proven reserves. Hey, that's a lot less than $2T; in fact it's only 23.78% of the big number. Do the same math comparing the 20B bbls already pumped to the 4.5B remaining. It pays to understand that when oil producers talk about a basin that's over 76% depleted, the easily reached oil was pumped decades ago. The remaining stuff will cost more to extract, and the cost of production hasn't even been figured into the simple math in the article.
I get even more annoyed when reputable sites like PennEnergy pass on articles like this that originated as PR pieces. The originator, RainChief Energy (RCFEF), is a penny stock that hasn't made a profit. It was formerly known as "Black Diamond Brands Corporation" doing who knows what. The math for this company is even simpler than my math above, with only one non-producing well on their property. There's plenty of oil left in the Permian but right now RCFEF isn't pumping any of it.
Full disclosure: No position in RCFEF or other companies mentioned in the puff-piece.