The Marcellus Formation is part of the Devonian black shale, with the thickness of gas-producing rock being as much as 900 feet. It is an entity of marine sedimentary rock running through Appalachia in the United States–primarily located in Pennsylvania, New York, West Virginia and Ohio. This formation runs what is estimated to be 600 miles north to south, and potentially holds up to 500 trillion cubic feet of natural gas, 50 trillion cubic feet being recoverable through the use of current technology. The shale here consists of mostly-untapped natural gas reserves, and its inherent closeness to high-demand urban areas on the East Coast makes it ideal for companies invested in energy development as well as energy export. The Marcellus Formation is considered to be one of the richest gas fields in all of North America, therefore selling mineral rights contained within this area can be very profitable.