The discovery, which was confirmed on Wednesday, "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 director of the USGS Energy Resources Program.

His team has uncovered an estimated 20 billion barels of oil, 16 trillion cubic feet of natural gas and 1.6 billion barrels of natural gas liquids at the Wolfcamp Shale geologic formation in the Midland area of West Texas. The find is the largest source of shale oil, three times larger than a shale discovery in 2013 in North and South Dakota and Montana, officials said.

Guidroz hailed "changes in technology and industry practices" that allowed them to estimate a huge increase in resources that are technically recoverable.

Texas Governor Greg Abbott tweeted that his state had just become about a trillion dollars (934 billion euros) richer.

Geologists said that the oil from shale rock is classed as unconventional compared to other oil found in the ground, because extracting it requires advanced drilling or recovery methods, such as hydraulic fracturing.

Record discovery

Ken Medlock, director of an energy-studies program at Rice University in Houston, said it seems "likely that we're seeing the birth of a new Permian Basin," referring to the energy-rich area of West Texas and southern New Mexico, which is one of the most productive oil and gas regions in the US.

The advent of horizontal drilling, hydraulic fracturing and other advancements will allow for the removal of shale oil at a volume that will make the basin "the dominant onshore platform for oil production," he said.

Chris Schenk, a Denver-based research geologist for USGS said it's been known for years that the region could yield new bountiful oil production, but it took the USGS time to assess the Wolfcamp Shale and estimate the volume of that production.

"We think the potential is there for the future, and it's not going to be realized overnight," he said.

Some analysts think the discovery could lead to a resurgence of the oil and gas industry in Texas, following a severe downturn when energy prices plummeted and tens of thousands of jobs were cut.

mm/bw (AP, USGC)