The information on this page is from the www.geology.com website. For more information on the Marcellus Shale, please go to their website.
Super Giant Field in the Appalachians?
A few years ago every geologist involved in Appalachian Basin oil and gas knew about the Devonian black shale called the Marcellus. Its black color made it easy to spot in the field and its slightly radioactive signature made it a very easy pick on a geophysical well log.
However, very few of these geologists were excited about the Marcellus Shale as a major source of natural gas. Wells drilled through it produced some gas but
rarely in enormous quantity. Few if any in the natural gas industry suspected that the Marcellus might soon be a major contributor to the natural gas supply of the United States – large enough to be spoken of as a “super giant” gas field.
Recent Surprise Estimates
In early 2008, Terry Englander, a geoscience professor at Pennsylvania State University, and Gary Lash, a geology professor at the State University of New York at Fredonia, surprised everyone with estimates that the Marcellus might contain more than 500 trillion cubic feet of natural gas. Using some of the same horizontal drilling and hydraulic fracturing methods that had previously been applied in the Barnett Shale of Texas, perhaps 10% of that gas (50 trillion cubic feet) might be recoverable. That volume of natural gas would be enough to supply the entire United States for about two years and have a wellhead value of about one trillion dollars! 
What is the Marcellus Shale?
The Marcellus Shale, also referred to as the Marcellus Formation, is a Middle Devonian-age black, low density, carbonaceous (organic rich) shale that occurs in the subsurface beneath much of Ohio, West Virginia, Pennsylvania and New York. Small areas of Maryland, Kentucky, Tennessee, and Virginia are also underlain by the Marcellus Shale. See the map of the Marcellus Shale above.
How Deep is the Marcellus Shale?
Throughout most of its extent, the Marcellus is nearly a mile or more below the surface. The map at right shows the depth of the Marcellus Shale. These great 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.
Using the two maps together, some especially interesting areas can be seen. 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.
Where is the Highest Production Potential?
Rock units are not homogeneous. The gas in the Marcellus Shale is a result of its contained organic content. Logic therefore suggests that the more organic material there is contained in the rock the greater its ability to yield gas. John Harper of the Pennsylvania Geological Survey suggests that the areas with the greatest production potential might be where the net thickness of organic-rich shale within the Marcellus Formation is greatest. A map showing this distribution for the state of Pennsylvania is shown at right. Northeastern Pennsylvania is where the thick organic-rich shale intervals are located.
This map shows the approximate depth to the base of the Marcellus Shale. It was prepared using the map by Robert Milici and Christopher Swezey above and adding depth-to-Marcellus contours published by Wallace de Witt and others, 1993, United States Department of Energy Report: The Atlas of Major Appalachian Gas Plays.
Well Production Rates
Before 2000, many successful natural gas wells had been completed in the Marcellus. The yields of these wells were often unimpressive upon completion. However, many of these older wells in the Marcellus have a sustained production that decreases slowly over time. Many of them continued to produce gas for decades. A patient investor might make a profit from these low yield wells with slowly declining production rates.
For new wells drilled with the new horizontal drilling and hydraulic fracturing technologies the initial production can be much higher than what was seen in the old wells. Early production rates from some of the new wells has been over one million cubic feet of natural gas per day. The technology is so new that long term production data is not available. As with most gas wells, production rates will decline over time, however, a second hydraulic fracturing treatment could restimulate production