One portion of the giant field, known as the Wolfcamp formation, was found to hold 20 billion barrels of oil trapped in four layers of shale beneath West Texas. That's almost three times larger than North Dakota's Bakken play and the single largest U.S. unconventional crude accumulation ever assessed, according to the U.S. Geological Survey. At current prices, that oil is worth almost $900 billion.

The estimate lends credence to the assertion from Pioneer Natural Resources CEO Scott Sheffield that the Permian's shale could hold as much as 75 billion barrels, making it second only to Saudi Arabia's Ghawar field. Irving-based Pioneer has been increasing its production targets all year as drilling in the Wolfcamp produced bigger gushers than the company's engineers and geologists forecast.