Shale gas is natural gas produced from shale. Because shales ordinarily have insufficient permeability to allow significant fluid flow to a well bore, most shales have not historically been large producers of natural gas. Shale gas is one of a number of “unconventional” sources of natural gas; other unconventional sources of natural gas include coalbed methane, tight sandstones, and methane hydrates.

Shale has low matrix permeability, so gas production in commercial quantities requires fractures to provide flow pathways. Limited gas volumes have been produced for years from shales with natural fractures; the shale gas boom in recent years has been due to modern technology enabling the creation of extensive artificial fractures around well bores. Horizontal drilling is often used with shale gas wells.

Shales that host economic quantities of gas have a number of properties in common. They are rich in organic material, and are mature petroleum source rocks, having passed through the thermogenic gas window. They are sufficiently brittle and rigid enough to maintain open fractures. In some areas, shale intervals with high natural gamma radiation are the most productive.

Some of the gas produced is held in natural fractures, some in pore spaces, and some is adsorbed onto the organic material. The gas in the fractures is produced immediately; the gas adsorbed onto organic material is released as the formation pressure declines.

(Source: Wikipedia (see link - http://en.wikipedia.org/wiki/Shale_gas)

Haynesville Shale

The Haynesville Shale, is a black, organic-rich shale of Upper Jurassic age that underlies much of the Gulf Coast area of the United States. "Haynesville Shale" is a drillers term for shale rock units within the Haynesville Formation.

The Haynesville Formation is underlain by the Smackover Formation and overlain by rocks of the Cotton Valley Group. It was deposited about 150 million years ago in a shallow offshore environment.

Geologists have long known that the Haynesville Formation contained natural gas. However, because of its low permeability the Haynesville was originally considered to be a gas source rock rather than a gas reservoir.

Today, natural gas production from the Haynesville occurs from rocks about two miles beneath northwestern Louisiana, southwestern Arkansas and eastern Texas. The most productive areas have been Caddo, Bienville, Bossier, DeSoto, Red River and Webster Parishes of Louisiana plus adjacent areas in southwest Arkansas and east Texas.