The Marcellus Shale, also referred to as the Marcellus Formation, is a Middle Devonian-age black, low density, organic rich shale that occurs in the subsurface beneath much of Ohio, West Virginia, Pennsylvania and New York. Throughout most of its extent, the Marcellus is nearly a mile or more below the surface. These depths make the Marcellus Formation a very expensive target. Successful wells must yield large volumes of gas to pay for the drilling costs that can easily exceed a million dollars for a traditional vertical well and much more for a horizontal well with hydraulic fracturing.

However, some especially interesting areas have been located. These are where thick Marcellus Shale can be drilled at minimum depths. Although this is a great oversimplification, it correlates with the heavy leasing activity that has occurred in parts of northern Pennsylvania and western New York.
(description Marcellus formation from Wikipedia)

How productive is the Marcellus Shale Formation?

Before 2000, the yields of many of the Marcellus gas wells were unimpressive. However, others were found to have a sustained production that decreased slowly over time and many continued to produce gas for several decades. For wells drilled with the new horizontal drilling and hydraulic fracturing technologies the inital production can be much higher than what was seen in the old wells. Then as production rates decline the well might receive new hydraulic fracturing treatments to boost the production. The wells will deplete over time but the fracturing treatments help to produce a more complete recovery of the natural gas over what may now be expected to be a 20 to 30 year period.

High yield wells in the Marcellus Shale have been built using the horizontal drilling technique. Some horizontal wells in the Marcellus Shale have initial flows that suggest that they are capable of yielding millions of cubic feet of gas per day, making them some of the most productive gas wells in the eastern United States. Although some experts are very optimistic on the long-term production rates of these wells, it is too early to determine their productive life or long-term yield.