Shale is a fine-grained sedimentary rock composed of mud that is a mix of clay and other minerals.
Many shale formations hold significant quantities of natural gas.
The gas was formed when the organic matter within shales was subjected to high temperatures and pressures.
Because shales have low permeability, hydraulic fracturing and horizontal drilling are required to make production of natural gas commercially viable.
Shale gas is usually held at depths of more than 2000 metres – well below the reservoirs used for agriculture and town water supplies.
The parts of Australia considered most likely to hold commercial deposits of natural gas in shales are in remote regions of Western Australia, South Australia, Queensland and the Northern Territory.