Oil explorers in the U.S. state of Texas say they’ve discovered the largest deposit of shale oil in a region known as the Permian Basin. The so-called Wolfcamp formation, explorers believe, could hold up to 20 billion barrels of oil, worth up to $900 billion. The find could be three times bigger than the state of North Dakota’s Bakken rock formation, the largest find of unconventional oil ever discovered. “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, coordinator for the U.S. Geological Survey’s energy resources program, in a statement. The find also gives credence to those who think the Permian Basin could hold up to 75 billion barrels of shale oil. That would be the second largest in the world, behind Saudi Arabia’s Ghawar field. The land is currently controlled by five energy companies: Chevron, Occidental Petroleum, Apache, Pioneer Natural Resources and Concho Resources. According to The Motley Fool, a financial services company, Chevron owns the largest portion of land. The Permian has been a source of oil since crude was discovered in the 1920s. According to the Ft. Worth Star Telegram newspaper, the region’s shale layer has been largely untouched.