The boom in shale natural gas drilling has raised hopes 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 bountiful resource.

New York City urged a ban on natural gas drilling in its watersheds on Wednesday.

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 (tight sands and coalbed methane), shale gas accounts for 60 percent of technically recoverable U.S. onshore reserves, according to the 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 26 years.

* Estimates of total U.S. natural gas reserves have been rising in recent years. The Energy Information Administration calculated proven reserves at 244 trillion cubic feet, or about 11 years' supply, up from the agency's 2006 estimate of 211 trillion cubic feet.

* A separate estimate from the Potential Gas Committee, an industry group, in June 2009 concluded that the U.S. has 1,836 trillion cubic feet of technically recoverable natural gas reserves, the highest in the 44-year history of the organization. That represents about 80 years' supply at the current national consumption rate of 23 trillion cubic feet a year. The higher forecast largely reflects a reassessment of shale plays in the Appalachians, the Gulf Coast and elsewhere.

* 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 which then flows to the surface.

* 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 biggest U.S. shale play is the Marcellus Shale which underlies most of Pennsylvania and West Virginia, along with parts of Ohio and New York state. 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 northeast market. More than 800 Marcellus wells have been drilled in Pennsylvania since 2005, most of them in the last year as energy companies accelerate development of the field, state regulators say.

(Reporting by Jon Hurdle; Editing by Daniel Trotta)