An underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, or silt) from which groundwater can be usefully extracted using a water well.

A thinly bedded shale that is rich in carbon, sulphide and organic material, formed by anaerobic (lacking oxygen) decay of organic matter. Black shales occur in thin beds in many areas at various depths and are of interest both historically and economically.

Conventional gas is natural gas that is produced by a well drilled into a geologic formation in which the reservoir and fluid characteristics permit the natural gas to readily flow to the wellbore.

The ratio of useful energy output versus the energy required to perform the process.

The European Commission's report outlining plans to ensure an affordable and secure energy supply.

A term used to refer to the reliability of future energy supply. This depends on a number of different factors, including the availability of energy supplies, their affordability, and the capacity to extract them in an environmentally sustainable manner.

Artificial ponds with very large surface areas that are designed to allow the efficient evaporation of water through exposure to sunlight and ambient surface temperatures.

The method used for drawing out resources from the ground. For unconventional gas, this is mainly hydraulic fracturing.

See ' Hydraulic fracturing '.

Natural gas that is trapped within spaces in the rock, making it more accessible.