Shale gas is an unconventional source of natural gas that has been known, and exploited to a limited extent, since the 19th century. However in the last decade or so, using a combination of directional drilling to penetrate rock formations sub-horizontally and hydraulic fracturing, shale gas has been exploited in North America where it has been developed as an alternative to imported conventional gas. Large deposits of gas-bearing shales have also been discovered in many other parts of the world, notably in Russia, China, Argentina and Libya.

Other smaller sources of unconventional gas also exist, principally coal bed methane, but they are not considered further here.

Shales are fine-grained hard rocks with a pronounced layered fabric which are already naturally fractured or are brittle enough to be fractured artificially. Shale gas is mainly methane, formed by either prolonged heating over millions of years, like conventional oil and gas, or by the action of bacteria on organic carbon within the shales. The most productive shales are dark-coloured because of their high content of organic carbon. The gas lies trapped within the relatively low-porosity (up to ~12%) and impermeable shales, i.e. it has not been able to escape and migrate from a source rock to a ‘trap’ in a reservoir rock as has conventional natural gas. This means that the best shale gas prospects lie in thicker beds of rock. The gas has to be ‘released’ by hydraulic fracturing, a technique which involves artificially fracturing the rock . A greater amount of gas is released when a well is drilled sub-horizontally along a shale bed rather than vertically through it.

Last updated Dec 2013

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