Stretching from southern New York through the western portion of Pennsylvania and into the eastern half of Ohio and northern West Virginia, the Marcellus shale deposit covers an area between 140,000 to 250,000 square kilometres, and is estimated to contain up to 500 trillion cubic feet of gas – as much as 24 years worth of US gas production at today's rates.

Currently extraction of Marcellus shale gas is in its infancy, but its huge future potential is attracting attention. To investigate the climate-change impact of extracting this gas, Chris Hendrickson, from Carnegie Mellon University in Pennsylvania, and colleagues, calculated the likely greenhouse-gas emissions produced by all aspects of the gas extraction and burning.

Within their model the researchers included a pre-production phase (well-site investigation), site preparation (access roads, well-pad construction) and drilling to produce an operational well. They also considered post-production processing of the gas (removing water, for example), transmission and distribution to end users, and the eventual burning of the gas.

For a typical Marcellus shale well the team estimates overall greenhouse-gas emissions of 68 g of CO2 equivalent for every megajoule of gas produced. This figure is dominated by combustion of the gas, which accounts for 74% of the total emissions. "We estimate that shale natural gas has slightly higher emissions than conventional gas for production, but the production emissions are typically small compared to combustion emissions," said Hendrickson. The findings are published in Environmental Research Letters.

Overall the researchers estimate that extraction of Marcellus shale gas would produce 3% more greenhouse-gas emissions than extraction of conventional natural gas. When weighed up next to coal, the Marcellus shale is positively glowing, with up to 50% fewer greenhouse-gas emissions, assuming carbon capture and storage is not in place. "No US coal-fired electricity generation plant has carbon capture and storage right now," Hendrickson told environmentalresearchweb.

But of course greenhouse-gas emissions are not the only consideration when it comes to extracting shale gas. "Marcellus Shale has other environmental issues, including disruption of natural habitats, the use of water, creation of wastewater and truck transport impacts in rural areas," said Hendrickson.

But given that coal extraction carries a similar environmental cost, Hendrickson and his colleagues believe that Marcellus shale gas extraction is preferable. "Energy supplies provide substantial social benefits, so we favour extraction of Marcellus Shale natural gas as long as the extraction is managed to minimize adverse economic, environmental and social impacts," said Hendrickson.

Every shale gas field is different, so it is impossible to generalize to other parts of the world, but for the US at least shale gas looks set to be the new black gold.