The United States and Canada are the only countries so far to produce shale oil and gas in commercial quantities, but more and more countries are opening up to exploration, leaving resisting nations isolated.
Critics say extracting the precious resource is hugely harmful for the
environment, while supporters point to the energy boom it can help create.
Countries starting exploration
* Poland is one of the leaders in Europe on shale gas. Around 15 international groups are prospecting and have drilled several dozen test wells, although results are below industry expectations. Some wells are being tested for potential production that could begin next year.
* In Britain, the government supports the exploration of shale gas and drilling of a test well in the countryside of Sussex, southeast England, has begun. But it has become the focus of a national campaign against the practice of hydraulic fracturing, a controversial extraction method.
British energy firm Cuadrilla had to suspend its first drilling in northeast England in 2011 after small tremors were recorded. But in December 2012, the government authorised it to start again with tighter monitoring measures.
* Denmark has handed out two exploration licences, but the drilling that should have begun this year will no doubt only take place in 2014 to make time for environmental studies.
* Ukraine signed a contract in January with Shell.
* Spain and Romania have also handed out exploration licences.
* Argentina is seen as one of the countries with the greatest potential in the sector. It is seeking to tap shale gas and oil and the state-owned energy company has drilled extensively in the Vaca Muerta range.
A cooperation agreement with US group Chevron was signed in July and a deal with China's CNOOC is in the pipeline.
* Russia may be sitting on one of the world's largest reserves of shale oil and they are attracting interest from foreign oil companies. Shell signed a deal with a Gazprom subsidiary in April to drill for shale oil in the Khanty-Mansiysk area of central Siberia.
* China is encouraging exploratory drilling but results so far have not lived up to expectations.
* Australia has just started prospecting.
* Canada has already begun producing shale oil in Alberta. Quebec, which has imposed a moratorium on drilling for shale gas, has left the door open to the extraction of shale oil in the Gulf of St Lawrence, where exploratory drilling is due to start soon.
Those thinking about it
* Lithuania is on the point of signing an exploration accord with Chevron that has been months in the making.
* In Germany the Merkel government proposed in February the authorisation of production of shale gas subject to strict controls. But opposition within Chancellor Angela Merkel's own party resulted in the issue being shelved until after elections this weekend.
* A major producer of natural gas, Algeria is also thought have significant reserves of shale gas and oil. Recent legislation has authorised exploration for them.
* After a moratorium imposed because of environmental concerns, South Africa now envisages starting exploration after April 2014 elections.
* France banned fracking in 2011, closing the door to drilling for shale oil or gas on its territory. The ban was confirmed last year by the new Socialist government on the grounds that the environmental consequences are still unknown.
* Bulgaria and the Czech Republic both announced moratoriums last year.
* The Netherlands has issued two exploration permits but has blocked them from being used pending further study on the environmental impact.
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