By KEVIN BEGOS

PITTSBURGH — The U.S. Geological Survey said Tuesday that the Marcellus Shale region contains some 84 trillion cubic feet of undiscovered, recoverable natural gas, far more than thought nearly a decade ago.

Tuesday’s figure is much higher than the last government assessment in 2002, which suggested about 2 trillion cubic feet of recoverable gas.

The USGS said the estimate came from new information about the gas-rich formation underlying New York, Pennsylvania, Ohio and West Virginia, and from technical improvements in how wells are drilled.

Environmental groups have expressed concerns that the process of extracting the gas from deep underground could contaminate the water supply. But gas industry groups welcomed the independent government estimate.

“While some critics continue to question the viability of responsible domestic shale gas development, it is abundantly clear — as laid out by this new data — that the Marcellus Shale will continue to lead the way in meeting American’s energy needs for years to come,” said Kathryn Z. Klaber, president and executive director of the Marcellus Shale Coalition, an organization of energy companies that says it’s committed to the responsible development of natural gas from the shale formation and the enhancement of the region’s economy.

The agency also estimated there are around 3.4 billion barrels of undiscovered, recoverable natural gas liquids. That product attracts a premium price over the natural gas.

The USGS figures represent an average of several possibilities about the gas reserves, located thousands of feet beneath the surface and coaxed out of the ground through high-volume hydraulic fracturing, or fracking.

The new survey suggested that the gas reserves are 43 trillion cubic feet to 144 trillion cubic feet, and the gas liquids are 1.6 to 6.2 barrels, with a 95 percent probability of the low range and 5 percent of the high range.

More than 3,300 wells have been drilled across Pennsylvania in just the last few years. The boom has raised concerns about the use of fracking, which injects chemical-laced water to break up the shale and allow natural gas to escape into the shale to push out the minerals. Environmental groups and the Environmental Protection Agency worry that the process could damage water wells, poison groundwater or harm trout streams. But the industry insists it’s safe.

The USGS Marcellus assessment covered areas in Kentucky, Maryland, New York, Ohio, Pennsylvania, Tennessee, Virginia and West Virginia.

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Online: USGS Report: http://bit.ly/pDIv3M

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