Shale deposits occur through the United States and represent significant potential sources of fossil fuel. When sedimentary mud hardened into shale, it often trapped organic particles that were later converted to oil or natural gas. Accessing the fossil fuel deposits involves shattering the shale by pumping in liquids under high pressure. This process may result in the contamination of adjacent water tables and is therefore highly controversial. Major shale deposits are in New England, Texas and the Midwest.
The Marcellus Shale - The Marcellus shale runs along the southern edge of Lake Ontario and Lake Erie, spilling into Quebec at its northern end. It reaches into Ohio in the west and as far south as Kentucky, with non-petroleum-bearing deposits reaching into Tennessee. In the east, it reaches into New York and runs south along the Virginia border. Beside it, across the southern edge of Lake Erie, is the Antrim shale, taking up most of Michigan east of Lake Michigan.
The Barnett Shale - The Barnett shale is in the Fort Worth Basin in Texas and is the most productive gas shale in 2011, with over 14,000 producing gas wells. It was one of the earliest developments, with drilling starting in 2002. Relatively small, it reaches from the Oklahoma border in the north to Burnett County in the south and from Dallas in the east to Taylor County in the west. Most of the petroleum deposits and the drilling are along the central eastern part.
The Bakken Formation - The U.S. Geological Survey estimated in 2008 that there were about 4 billion barrels of recoverable oil in the Bakken formation and classified it as a key resource for development. It is the largest assessment of any region in the lower 48 states. The United States part of the Bakken is the southern end of a formation in the southern Canadian prairies and it extends into North and South Dakota and Montana.
Other Significant Shale Deposits - A small but active gas shale field is the Fayetteville shale in Arkansas. It is of an age and structure similar to the Texas Barnett shale and stretches east to west across the state, just north of Little Rock, with an average width of 50 miles. Other significant shale deposits are Eagle Ford in Texas, running across the southern part of the state about 100 miles inland from the Gulf coast; Haynesville Bossier on the northern Texas-Louisiana border; Avalon-Bone Spring on the Texas-New Mexico border; Woodford in Oklahoma between Fayetteville and Barnett; and a number of smaller deposits, such as Niobrara, Mowbry, Lewis and Mancos scattered through the states of Colorado, Utah and Wyoming.