Marcellus Shale - Appalachian Basin Natural Gas Play
New research results surprise everyone on the potential of this well-known Devonian black shale.
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.
Early Marcellus Estimates by USGS
As recently as 2002 the United States Geological Survey in itsAssessment of Undiscovered Oil and Gas Resources of the Appalachian Basin Province, calculated that the Marcellus Shale contained an estimated undiscovered resource of about 1.9 trillion cubic feet of gas.
 That's a lot of gas but spread over the enormous geographic extent of the Marcellus it was not that much per acre.
The First Hints of Big Production Range Resources - Appalachia, LLC may have started the Marcellus Shale gas play. In 2003 they drilled a Marcellus
well in Washington County, Pennsylvania and found a promising flow of natural gas . They experimented with
horizontal drilling and hydraulic fracturingmethods that worked in the Barnett Shale of Texas. Their first Marcellus gas
production from the well began in 2005. Between then and the end of 2007 more than 375 gas wells with suspected
Marcellus intent had been permitted in Pennsylvania .
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 aMiddle 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.
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 inital 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
How Does the Gas Occur in the Rock?
Natural gas occurs within the Marcellus Shale in three ways: 1) within the pore spaces of the shale; 2) within vertica
l fractures (joints) that break through the shale; and, 3) adsorbed on mineral grains and organic material. Most of the
recoverable gas is contained in the pore spaces. However, the gas has difficulty escaping through the pore spaces
because they are very tiny and poorly connected. Most historic wells in the Marcellus produced gas at a very slow rate
because of the low permeability mentioned above.This is typical for a shale. However, some of the most successful
historic wells in the Marcellus share a common characteristic: they intersect numerous fractures. These fractures allow
the gas to flow through the rock unit and into the well bore. The fractures intersecting the well also intersect other
fractures and those fractures intersect still more fractures. Thus, an extensive fracture network allows one well to drain
gas from a very large volume of shale. A single well can recover gas from many acres of surrounding land.