The U.S. Geological Service (USGS) has assessed the Wolfcamp shale formation in the Midland Basin portion of Texas’ Permian Basin province as the largest unconventional oil field yet discovered in the United States—nearly three times larger than Bakken-Three Forks in North Dakota.

USGS says Wolfcamp shale 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. The estimate is for continuous (unconventional) oil and consists of undiscovered, technically recoverable resources.

Midland Basin map. Image credit: DOI/NPS/USGSMidland Basin map. Image credit: DOI/NPS/USGSUndiscovered resources are those that are estimated to exist based on geologic knowledge and theory. Technically recoverable resources are those that can be produced using currently available technology and industry practices. Whether or not it is economically profitable to produce these resources has not been evaluated.

“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,” says Walter Guidroz, program coordinator for the USGS Energy Resources Program. “Changes in technology and industry practices can have significant effects on what resources are technically recoverable, and that’s why we continue to perform resource assessments throughout the United States and the world.”

Since the 1980s, the Wolfcamp shale in the Midland Basin has been part of the “Wolfberry” play that encompasses the Mississippian, Pennsylvanian, and Lower Permian reservoirs. Oil has been produced there largely using vertical well technology.

However, more recently, oil and gas companies have been using horizontal drilling and hydraulic fracturing. More than 3,000 horizontal wells have now been drilled and completed in the Midland Basin Wolfcamp section.

Continuous oil and gas are dispersed throughout a geologic formation rather than existing as discrete, localized occurrences, such as those in conventional accumulations. Because of that, continuous resources commonly require special technical drilling and recovery methods, such as hydraulic fracturing.

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