Marcellus Shale


Q.    What is the Marcellus shale?

The Marcellus shale is an organic-rich black shale that was deposited in an oxygen-deficient marine environment during Middle Devonian time (~390 million years ago). Long known to be a source rock for many conventional oil and gas reservoirs in the Appalachian basin, it is now being explored as an unconventional reservoir. The formal name for this rock unit is the Marcellus Formation.

Additional Resource(s):
Pennsylvania Geology, Vol. 38, No. 1
http://en.wikipedia.org/wiki/Marcellus_Shale

Q.    Where is the Marcellus shale located?

The Marcellus shale is prevalent throughout much of the Appalachian basin and most of Pennsylvania, although its depth and thickness are variable.

Additional Resource(s):
Pennsylvania Geology, Vol. 38, No. 1
http://en.wikipedia.org/wiki/Marcellus_Shale

 

Q.    How deep and thick is the Marcellus shale?

The depth to the top of the Marcellus shale ranges from 0 feet where it crops out in central Pennsylvania to over 9,000 feet in parts of southwestern and northeastern Pennsylvania. The gross thickness of the Marcellus shale ranges from less than 20 feet along the Lake Erie shoreline in northwestern Pennsylvania to several hundred feet in central and northeastern Pennsylvania.  The net thickness of organic-rich Marcellus shale varies from less than 10 feet in western Pennsylvania along the Ohio border to over 250 feet in northeastern Pennsylvania.

Additional Resource(s):
Pennsylvania Geology, Vol. 38, No. 1
http://en.wikipedia.org/wiki/Marcellus_Shale
Full-color Marcellus isopleth map

Q.    What has been published previously regarding the Marcellus shale?

The most comprehensive research regarding the Marcellus and other organic-rich shales in the Appalachian basin was published as part of the U.S. Department of Energy’s Eastern Gas Shales Project (EGSP) in the late 1970s and early 1980s and by the Gas Technology Institute in Chicago during the mid 1980s to late 1990s. The EGSP reports can be obtained at no cost from the U.S. DOE through its National Energy Technology Laboratory website (“Archive of Unconventional Gas Resources Program”). The GTI reports can be obtained at cost from the GTI website. Another valuable resource is USGS Bulletin 1909 published by Roen and Kepferle (see citation, below).

Additional Resource(s):
Pennsylvania Geology, Vol. 38, No. 1
http://www.netl.doe.gov/publications/descriptions.html
GTI Reports
Roen, J.B., and Kepferle, R.C., eds., 1993, Petroleum geology of the Devonian and Mississippian black shale of eastern North America: U.S. Geological Survey, Bulletin 1909, 14 chapters individually paginated, at http://pubs.er.usgs.gov/usgspubs/b/b1909.

 

Q.    How does the current Marcellus shale play relate to the history of petroleum exploration and production in the Appalachian basin?

A recent issue of PA Geology provides an excellent primer on the evolution in thinking regarding the exploration of the Marcellus shale. Operators in the Appalachian basin have known the Marcellus Formation to be a reservoir rock for over 75 years. It was originally considered a nuisance, however, as tapping this formation would yield large volumes of natural gas just long enough to shut down drilling operations for a number of days. Once the gas had drained from “pockets” in this shale, drillers continued drilling to deeper targets (mainly, the Oriskany Sandstone). With current oil prices and advancements in drilling and completion technology, the Marcellus shale has become an important gas play.

Additional Resource(s):
Pennsylvania Geology, Vol. 38, No. 1

Q.   What kinds of maps does the Pennsylvania Geological Survey have that illustrate the extent, thickness, or other characteristics of the Marcellus shale?

The Survey has prepared an isopleth map showing the net thickness of organic-rich shale in the Marcellus Formation based on data collected as part of the Eastern Gas Shales Project (EGSP) . In addition, a digital version of EGSP Series No. 13 entitled “Black shale and sandstone facies of the Devonian “Catskill” clastic wedge in the subsurface of Pennsylvania” (Piotrowski and Harper, 1979) may be downloaded from the Survey's website (see link below). As the Survey continues to receive drilling and completion data for Marcellus shale wells, the number and variety of digital maps and data regarding this reservoir will increase.

Additional Resource(s):
Pennsylvania Geology, Vol. 38, No. 1
Full-color Marcellus isopleth map
EGSP plat-sized maps

Q.   How are Marcellus shale wells being completed, and how is this technology different from the development of conventional oil and gas reservoirs?

The two most notable ways in which Marcellus shale well completion differ from those completed in conventional reservoirs are (1) directional drilling and (2) hydraulic fracturing (“frac”) methods. As many as half of the Marcellus wells in Pennsylvania are expected to be completed as horizontal wells; this approach allows the operator to access more footage of reservoir than would be possible in a traditional, vertical hole. In addition, the use of “slick-water frac” methods enables operators to recover gas more efficiently and in larger quantities. Such approaches require as much as 20 times the water volume as that used in conventional well completions (see Pennsylvania Geology article, weblink below, for a more detailed discussion of these methods).

Additional Resource(s):
Pennsylvania Geology, Vol. 38, No. 1

 

Q.   How many wells are known to have been completed in the Marcellus shale in Pennsylvania, and where are they located?

As of the posting of this FAQ sheet, as many as ~450 Marcellus shale wells have been formally completed ( i.e., the Survey has received completion reports for these wells) across the state. Known drilling and/or completion activities exist in 14 counties (see map, weblink below).

Additional Resource(s):
Pennsylvania Geology, Vol. 38, No. 1
Drilling activity map

Q.   How much gas is in the Marcellus shale?

The answer to this question ultimately depends upon natural gas prices and state-of-the-practice technology at the time of the assessment. Having said that, the U.S. Geological Survey has reported estimates of recoverable gas reserves in the Marcellus shale at 1,925 billion cubic feet (Bcf) (Milici and Sweezey, 2006). Industrial and academic assessments vary from 50 to 500 trillion cubic feet (Tcf). A recent study by Kuuskraa and Stevens (2009) report Marcellus shale gas recoverable reserves at 100 to 200 Tcf. These estimates will continue to vary as the play is explored further.

Additional Resource(s):
Pennsylvania Geology, Vol. 38, No. 1

 

Q.   If the Marcellus shale is such a good, prospective reservoir, why hasn’t it been explored by the industry until now?

Simply put, current petroleum prices (oil in excess of $130 per barrel and natural gas in excess of $10 per thousand cubic feet;) and the availability of technology needed to successfully drill and complete a Marcellus gas shale well are the driving forces behind current exploration work in the Appalachian basin (see Pennsylvania Geology article, weblink below, for more information).

Additional Resource(s):
http://tonto.eia.doe.gov/oog/info/ngw/ngupdate.asp
Pennsylvania Geology, Vol. 38, No. 1

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