What is it?
Natural gas produced from a type of sedimentary rock called shale. It's an energy source that many experts believe will be able to substitute for other fossil fuels.
Shale gas has a long history but generally hasn't proved to be a very profitable business. Recent discoveries relating to the means of drilling for it, combined with rising fuel oil prices, have led many to argue that shale gas is now a much more viable energy option.
Where is it found?
It's found in shale "plays," which are formations of shale with a significant accumulation of natural gas.
from the U.S. Department of Energy shows that shale gas plays can be found worldwide. China has the largest reserves, followed by the United States, Argentina, and Mexico.
Fifty-four percent of the world's conventional natural gas is concentrated in Russia, Qatar, and Iran, and the Middle East has dominant reserves of fuel oil. Some argue that by expanding the worldwide energy supply, shale gas has the potential to alter the geopolitics of energy with less dependency on Russia and the Middle East.
How is it collected?
Shale gas is accessed through a combination of horizontal drilling and hydraulic fracturing, more colloquially known as "fracking."
is a handy explainer on fracking, with descriptions and illustrations.
What is its economic potential?
North America is so far the leading producer of shale gas. The Barnett shale play in Texas is the largest producing natural gas field in the United States and has encouraged discussion of expanding the extraction of shale gas worldwide.
The Shale Gas Market Report 2011-21 estimates that the global shale-gas market is currently worth $26.66 billion a year.
What are the environmental effects?
The Obama administration has been a proponent of shale gas as an alternative energy source. Environmentalists have been quick to advise caution, however, citing substantial environmental risks.
In a late 2010 report, the U.S. Environmental Protection Agency (EPA) concluded that shale gas emits higher amounts of methane, a potent greenhouse gas, than conventional natural gas.
As a greenhouse gas, methane is 25 times more potent than carbon dioxide, the primary greenhouse gas produced from the burning of coal, oil, and conventional gas.
The first comprehensive study of the greenhouse-gas footprint of shale gas was done this year at Cornell University. That study projects that over a 100-year time period, shale gas's greenhouse-gas footprint is comparable to coal's and worse than fuel oil's.
"Produced water" is what's left after the fracking process. Only 50-70 percent of that chemical-filled water is removed after fracking -- the rest is left in the ground, where it could come into contact with drinking water.
Shale gas has become a part of the balancing act between economic benefit and environmental harm that has been central to the energy industry for years.
-- compiled by Joanna Kinscherff