The Marcellus Shale primarily underlies New York, Ohio, Pennsylvania, Virginia, West Virginia and Western Maryland, with a negligible share also found in Kentucky. Only a small portion of the formation is located in Maryland. Production wells have been drilled in New York, Ohio, Pennsylvania and West Virginia, and several companies have expressed interest in drilling into the formation in Maryland.

Several states -- all with high numbers of fracking wells -- have experienced problems in and around well sites.

• Water contamination

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  A Duke University study concluded that homes around fracking wells in Pennsylvania and New York State had abnormally high concentrations of methane. Concentrations in drinking wells within one kilometer of an active natural gas well were 17 times higher than those farther away. One well tested more than twice the government standard to reduce the risk of explosion.

• Water shortage

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  Officials restricted water use for fracking in Pennsylvania and Texas during the spring 2012 drought. Four to seven million gallons of water, chemicals and sand are pumped into the ground to frack a single well. One well can be fracked more than a dozen times.

• Earthquakes

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  Seismic activity in the central U.S. increased 168 percent from 2009 to 2011. The number of earthquakes has been increasing since 2001, but the accelerated pace in this region, which began in 2009 — including Oklahoma's strongest earthquake on record — has been linked to oil and gas production, including fracking, in areas in Oklahoma, Arkansas, Colorado and New Mexico.

• Road damage

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  Trucking traffic to fracking sites is the equivalent of adding 3.5 million cars to the road. Oil and gas companies are bonded to repair roads along their trucks' route, but states say they're still picking up part of the tab. Texas officials approved $40 million to repair roads in the Barnett Shale region while Pennsylvania's transportation department estimates about $265 million will be needed to repair roads in the Marcellus Shale region.

The development of shale resources has been called a "game changer" for the U.S. gas market. From 2006 to 2010 shale gas production increased by about 380 percent and now comprises nearly one-quarter of domestic gas production.

Bringing one Marcellus Shale well on line results in about 420 direct jobs across 150 occupations.

A state-commissioned report estimates that 45-120 billion cubic feet of shale gas could be extracted from 2016 to 2025, depending on the productivity scenario. Under the highest tax scheme proposed — 5 percent — the state could bring in $61.7 million in revenue. Peak drilling activity could result in more than 1,800 jobs for Marylanders, a report prepared for the Maryland Petroleum Council said.

Data: U.S. Energy Information Agency