Tuesday the U.S. Geological Survey announced the largest ever assessment of “continuous oil” ever made in the United States. The Wolfcamp shale in the area of Midland, Texas is estimated to contain three times the oil and gas of the Bakken shale formation in Montana and North Dakota. From the USGS:

The Wolfcamp shale in the Midland Basin portion of Texas’ Permian Basin province contains an estimated mean of 20 billion barrels of oil, 16 trillion cubic feet of associated natural gas, and 1.6 billion barrels of natural gas liquids, according to an assessment by the U.S. Geological Survey. This estimate is for continuous (unconventional) oil, and consists of undiscovered, technically recoverable resources.

The estimate of continuous oil in the Midland Basin Wolfcamp shale assessment is nearly three times larger than that of the 2013 USGS Bakken-Three Forks resource assessment, making this the largest estimated continuous oil accumulation that USGS has assessed in the United States to date.

“The fact that this is the largest assessment of continuous oil we have ever done just goes to show that, even in areas that have produced billions of barrels of oil, there is still the potential to find billions more,” said Walter Guidroz, program coordinator for the USGS Energy Resources Program.

Continuous oil refers to resources that are spread out rather than concentrated into a single area. So why is this massive amount of oil and gas just being assessed as recoverable now? Because technology has changed:

Oil has been produced using traditional vertical well technology. However, more recently, oil and gas companies have been using horizontal drilling and hydraulic fracturing, and more than 3,000 horizontal wells have been drilled and completed in the Midland Basin Wolfcamp section.

The Fort Worth Star Telegram reports this backs up industry claims that the area could be the 2nd largest oil and gas field in the world:

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.

The USGS published this map showing the area of the new assessment: