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 are not sources 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 permeability. Shale gas has been produced for years from shales with natural fractures; the shale gas boom in recent years has been due to modern technology in creating 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 in 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.