(Bloomberg) -- The U.S.’s fastest growing oil field may have even more to give.

The Permian’s Delaware Basin, the less drilled part of the giant West Texas and New Mexico oil field, holds more than twice the amount of crude as its sister, the Midland Basin, the U.S. Geological Service said Thursday.

The Wolfcamp Shale and Bone Spring rock formations in the Delaware hold an estimated 46.3 billion barrels, the scientists said in their first assessment of the area. In addition, it holds about 281 trillion cubic feet of natural gas, about 18 times the amount in the Midland Basin, which is more heavily drilled and better known.

The Midland and Delaware estimates are the USGS’s “largest continuous oil and gas assessments ever released,” Dr. Jim Reilly, the organization’s director, said in a statement. The amount consists of “undiscovered, technically recoverable resources,” the USGS said.

The study only looked at the two rock formations, which are both well known to operators including Exxon Mobil Corp., Royal Dutch Shell Plc, EOG Resources Inc. and Occidental Petroleum Corp. Industry experts say there are as many as a dozen so-called ‘pay zones’ in the area.

To contact the reporter on this story: Kevin Crowley in Houston at kcrowley1@bloomberg.net

To contact the editors responsible for this story: Simon Casey at scasey4@bloomberg.net, Carlos Caminada, Christine Buurma

©2018 Bloomberg L.P.