Devonian shales are formed from the mud of shallow seas that existed about 350 million years ago (during the Devonian period of the Paleozoic era). Shale is a very fine-grained sedimentary rock, which is easily breakable into thin, parallel layers. It is a very soft rock, but does not disintegrate when it becomes wet. These shales can contain natural gas, usually when two thick, black shale deposits "sandwich" a thinner area of shale. Because of some of the properties of these shales, the extraction of natural gas from shale formations is more difficult than the extraction of conventional natural gas. Most of the Devonian Shale containing natural gas in the United States is located around the Appalachian Basin. As of November 2008, FERC estimated that there are 742 Tcf of technically recoverable shale gas in the United States, representing a large and growing share of total recoverable resources. 

Quoted from: http://naturalgas.org/overview/unconvent_ng_resources.asp