Factbox: Shale gas stirs energy hopes, health concerns

(Reuters) - The boom in shale natural gas drilling has raised hopes that the United States will be able to rely on the cleaner-burning fuel to meet future energy needs, but concerns about its impact on water quality could slow the industry's ability to tap this resource.

The U.S. Environmental Protection Agency said on Thursday it will begin to take a closer look at the environmental and human health impact of shale gas drilling, which could mean new regulations on a booming sector of the energy market.

Following are some shale gas facts.

* Shale gas is natural gas -- largely methane -- produced and stored in shale formations a mile or more underground in many of the lower 48 U.S. states.

* Together with other "unconventional" natural gas sources such as tight sands and coalbed methane, shale gas accounts for 60 percent of technically recoverable U.S. onshore reserves, according to the U.S. Department of Energy. At least half of new reserves growth is expected to come from shale gas by 2011. In all, shale reserves are estimated to contain enough gas to meet total U.S. demand for 30 years.

* The U.S. Energy Information Administration calculated proven natural gas reserves at 244 trillion cubic feet (6.9 trillion cu meters), or about 11 years' supply, up from the agency's 2006 estimate of 211 trillion cubic feet (6.0 trillion cu meters).

* A separate estimate from the Potential Gas Committee, an industry group whose biennial estimates are seen as a benchmark, concluded in June 2009 that the U.S. has 1,836 trillion cubic feet (52 trillion cu meters) of technically recoverable natural gas reserves, up sharply from 1,321 tcf in 2007. Of the new total, 33 percent is shale gas.

* Combined with the U.S. Department of Energy's latest estimate of 244.7 tcf in proven reserves at the end of 2008, the PGC's data gives the U.S. total available future supply of 2,081 tcf, an increase of 549 tcf from the committee's previous survey in 2007. The U.S. consumption rate of about 23 tcf a year. The latest estimate represents some 90 years' supply.

* One trillion cubic feet of gas is enough to heat 15 million homes for a year or to fuel 12 million natural gas-powered vehicles for a year, according to DOE figures.

* The abundance of shale and other forms of natural gas may allow the U.S. to reduce its dependence on overseas energy sources while reducing greenhouse gas emissions. Natural gas produces about half of the carbon dioxide emitted by coal, and about a third less than oil, and so is seen as a "bridge" fuel between petroleum and renewable fuels such as wind and solar. Natural gas also emits lower levels of other pollutants such as sulfur dioxide and nitrous oxide.

* A recent boom in shale gas development in states including Texas, Wyoming and Pennsylvania has been driven by advances in hydraulic fracturing in which a mixture of water, sand and chemicals are forced underground at pressures sufficiently high to open gas-bearing fissures in the shale, releasing the fuel.

* Exploitation of shale "plays" has also been aided by horizontal drilling, enabling much wider coverage of shale formations than with traditional vertical drilling, and with less surface disturbance.

* The "big four" U.S. shale plays are the Barnett in Texas, currently the most productive, with about 50 percent of total U.S. shale gas output; the Haynesville in Louisiana/Texas; the Fayetteville in Arkansas; and the Marcellus in Pennsylvania and surrounding states.

* The Marcellus is likely to become the biggest producer of shale gas, according to Chesapeake Energy Corp., the second-largest U.S. producer of natural gas overall.

* The Marcellus could contain as much as 489 trillion cubic feet of gas, according to Terry Engelder, a Penn State University geoscientist. Its value is enhanced by the high quality of its gas and the fact that it is close to the major U.S. Northeast market, keeping transmission costs relatively low. More than 800 Marcellus wells have been drilled in Pennsylvania since 2005, most of them in 2009.

* Energy companies are expected to apply for 5,200 Pennsylvania drilling permits in 2010, about triple the number in 2009.

(Reporting by Jon Hurdle in Philadelphia; Editing by Lisa Shumaker)