To free the oil and gas from the rock, drillers crack it open by pumping water, sand and chemicals into the ground at high pressure, a process is known as hydraulic fracturing, or "fracking."
While expanded use of the method has unlocked enormous reserves of oil and gas, it has also raised concerns that contaminated water produced in the process could leak into drinking water.
The surge in oil production has other roots, as well:
â¿¿ A long period of high oil prices has given drillers the cash and the motivation to spend the large sums required to develop new techniques and search new places for oil. Over the past decade, oil has averaged $69 a barrel. During the previous decade, it averaged $21.
â¿¿ Production in the Gulf of Mexico, which slowed after BP's 2010 well disaster and oil spill, has begun to climb again. Huge recent finds there are expected to help growth continue.
â¿¿ A natural gas glut forced drillers to dramatically slow natural gas exploration beginning about a year ago. Drillers suddenly had plenty of equipment and workers to shift to oil.
The most prolific of the new shale formations are in North Dakota and Texas. Activity is also rising in Oklahoma, Colorado, Ohio and other states.
Production from shale formations is expected to grow from 1.6 million barrels per day this year to 4.2 million barrels per day by 2020, according to Wood Mackenzie, an energy consulting firm. That means these new formations will yield more oil by 2020 than major oil suppliers such as Iran and Canada produce today.
U.S. oil and liquids production reached a peak of 11.7 million barrels per day in 1970. It nearly reached that level again in 1985 when Alaskan fields were producing enormous amounts of crude, then it began a long decline. From 1986 through 2008, crude production fell every year but one, dropping by 44 percent over that period. The United States imported nearly 60 percent of the oil it burned in 2006.