The Elevation Resources drilling rig is shown at the Permian Basin drilling site in Andrews County Texas Thomson Reuters The US Geological Survey said Tuesday that it assessed what could be the largest deposit of untapped oil ever discovered in America.

An estimated average of 20 billion barrels of oil and 1.6 billion barrels of natural gas liquids are available for the taking in the Wolfcamp shale, which is in the Midland Basin portion of Texas' Permian Basin.

Based on a West Texas Intermediate crude oil price of $45 per barrel, those deposits are worth about $900 billion.

US oil exploration companies have flocked to the superrich Permian Basin in recent years and used shale-drilling technology to create an oil boom that simultaneously helped trigger a price crash two years ago. The count of active oil rigs fell with prices, but has risen over the past few months, mostly in the Permian. Bloomberg noted that the Wolfcamp, where this deposit was found, has been one of the primary targets of shale drillers.

"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," Walter Guidroz, program coordinator for the USGS Energy Resources Program, said in a statement.

More than 3,000 horizontal oil wells have already been drilled and completed in the Midland Basin Wolfcamp section, according to the USGS. To get the oil, producers fracture, or "frack," the earth below with a high-pressure liquid mixture to untap oil and gas from shale rock.

This map from the USGS shows the area where the deposit was found: