The Marcellus Shale, a geological formation that holds vast deposits of natural gas, is located underneath most of West Virginia, as well as parts of Pennsylvania, New York, Maryland, and Ohio. The potential of the Marcellus was known for decades; however, drilling and extraction technology improved to the point that the gas could be economically extracted, opening up new reserves for development.

The Marcellus is the oldest of the Devonian black shales, formed during a period when West Virginia was inundated by a vast inland sea. Black shale develops in aquatic settings, such as salt marshes and swamps, where water has stagnated to the point that organic matter will not decay. That organic matter is combined with the clays accumulating in these settings to produce shale. The organic matter is compressed and compacted and heated through burial by overlying sediment.

Heating refines the organic matter into natural gas. Some of this gas is expelled from the shale, and it migrates upward through the overlying strata and accumulates in reservoirs. Gas has been extracted for a century from these reservoirs, but the remaining gas remained trapped in the spaces within the shale until technology was developed to reach and extract it. Wells in the Marcellus Shale are drilled vertically and then horizontally. In a process called hydraulic fracturing, water mixed with sand and chemicals is injected into the well at high pressure to fracture the shale, freeing the gas to flow into the well. The gas travels through pipelines to the consumer.

In 2002, the state issued the first Marcellus Shale permit for a well in Kanawha County. Since then, drilling has taken place in 45 of 55 West Virginia counties, mostly in the southwestern and north-central areas of the state. Since 2006, drillers have completed nearly 1,600 Marcellus Shale wells in the state, with more than 1,200 wells in development.

The development of the shale has raised concerns about water quality, landowners' rights, and the use of local roads. Marcellus Shale operations have drawn the scrutiny of federal and state regulators as well as the West Virginia Legislature.

To view an interactive map of the Marcellus Shale, go to West Virginia Geological and Economic Survey website