What is the Marcellus Shale?

February 23, 2011

What is the Marcellus Shale?

By Taylor Kuykendall Register-Herald Reporter The Register-Herald Wed Feb 23, 2011, 12:02 AM EST

Despite all the excitement and fear surrounding the development of the Marcellus shale, few seem to understand what the Marcellus shale is or how it got there.

Shale is a fine-grained sedimentary rock formed by clay- and silt-based mud. The Marcellus shale is a particular rock layer of middle-Devonian age. The Devonian is a geologic period spanning from 416 to 359.2 million years ago.

The Marcellus formation is named after an outcrop of the shale near the village of Marcellus, N.Y. The Marcellus shale is primarily beneath West Virginia, Pennsylvania, New York, Ohio, Maryland, Tennessee, Virginia and Kentucky. The most easily accessible and gas heavy portions of the shale are located mostly in New York, Pennsylvania and West Virginia.

In several eastern regions of West Virginia, the Marcellus shale actually crops out from the surface, where the rock is exposed and visible. The depth and thickness of the shale varies across the state and throughout the entire shale.

Most of the Marcellus shale lies a mile or more underneath the ground. It rests on top of another geologic layer known as the Onondaga Limestone. In eastern West Virginia, the Marcellus shale lies over the Needmore Shale, a dark gray to green rock that belongs to the Onesquethaw group.

Knowing the elevations of these other formations and the surface elevation of a given area can assist in estimating the depth of a successful Marcellus shale well.

Fossil fuels, including natural gas, coal and oil, are formed when organic materials, such as decayed plant and animal matter, are built up in thick layers. Over time, the decayed organic matter is compacted and becomes further covered by sand and other inorganic debris. Then, heat and pressure turned the material into fossil fuels.

The Marcellus shale, a relatively porous structure, holds natural gas in its pores or fractures in the rock. Some is also absorbed in grains of minerals in the shale. Because of the mostly impermeable layers above and below the Marcellus, natural gas deposits are trapped in various sections of the formation.

The gas encapsulated in the porous shale is reached by fracturing the shale so gas may escape and travel into the well. In some cases, natural fracture patterns in the shale will network, allowing gas from a large volume of shale, under several acres of land, to be collected from one well.

Some of the thickest sections of the Marcellus shale in West Virginia are estimated to be about 200 feet thick.

Recent estimations have pushed estimated volume of the natural gas supply beneath the Marcellus shale to as high as 500 trillion cubic feet.

There are numerous other shales in West Virginia as well. Some are more shallow than the Marcellus, and are expected to attract the attention of the incoming gas industry.

Future interest may be directed at the Utica shale, a giant shale bed beneath the Marcellus shale that is much thicker and much more geographically extensive than the Marcellus shale. The commercial viability of the Utica shale has been proven in some areas, but it is not known if the potential source rock is as viable throughout the shale.

The main obstacle in developing the Utica shale is that it much deeper than the Marcellus shale, between 3,000 and 5,000 feet deeper than the already hard-to-reach Marcellus shale.

— E-mail: tkuykendall@register-herald.com

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