Marcellus Shale Formation

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The Marcellus Shale, also known as the Deveonian Marcellus formation, was formed approximately 380 million years ago during the middle Devonian age. The organic material decayed, forming methane and other natural gases. About 300 million years ago, the pressure of the gas caused fractures to form in the shale. These fractures run from the northeast to the southwest. According to research done by Dr. Terry Engelder of Penn State University, there are an estimated 363 trillion cubic feet of recoverable gas in the shale, enough to meet U.S. demand for about 19 years. About 75-80% of this recoverable gas is located in Pennsylvania. Peak production isn’t expected for approximately 10 more years, and drilling can be expected to last about 30 years.

Source: http://naturalgas.extension.psu.edu/Devonian-Marcellus-Formation-Maps.htm

Horizontal Drilling and Increased Interest in the Shale

Photo and text from http://geology.com/articles/marcellus-shale.shtml

Although the gas reserves in the Marcellus Shale are not a recent discovery, increasing gas prices, coupled with the development of horizontal fracturing technology, has led to the rapidly increasing interest. In the horizontal drilling process, a vertical well is drilled, possibly several thousand feet deep, and special joints are then used to turn the shaft 90 degrees. Horizontal shafts of up to 5,000 feet are then drilled. Because the gas containing fractures are vertical, traditional vertical drilling would only be expected to contact a few fractures. Horizontal drilling should maximize the number of fractures intersected. To release the gas held in the shale, a portion of the drilled well is sealed off, and a combination of water, chemicals, and sand (fracturing fluids) are injected under high pressure to that portion. The casing of this portion of the well is broken, exposing the well to the Marcellus Shale, and the high pressure fluid opens fractures in the shale. The sand is pushed into the fractures, expanding and holding them open and allowing for continued gas flow. This process is known as hydraulic fracturing or hydrofracing.

Sources:
Marcellus Shale - Appalachian Basin Natural Gas Play, http://geology.com/articles/marcellus-shale.shtml (last accessed 4/14/09)

Conversation with Dr. Terry Engelder, Penn State University, January 2009.