'Fracking': The potential for shale gas in the north west
The British Geological Survey estimates there could be 1,300 trillion cubic feet of shale gas in the north of England.
Three energy companies have been granted permission to explore at sites across the North West to test the Bowland shale gas reserves for the viability of hydraulic fracturing - also known as fracking - a technique of extracting gas from shale rock.
Francis Egan, chief executive of Cuadrilla, the region's main player with licences to test drill at six sites, estimates the shale gas reserves in Lancashire alone are worth £140bn.
Fracking was blamed for causing two earth tremors near Blackpool in May 2011 and has met with stern opposition.
Numerous anti-fracking groups have formed and protests have been staged at several sites over fears of further earthquakes, water pollution and environmental damage.
BBC News gives a guide to the potential of fracking in the North West.
Elswick site, owned by Cuadrilla
The site was hydraulically fractured in 1993 by a previous owner.
It was bought by Cuadrilla in 2010 and is the company's only permanent working site as all their other locations are leased.
The firm said the plot was "visually unobtrusive, with many people in the area unaware of its existence".
When operational, it generates 1MW of electricity which is sent to the national grid via an onsite generator and underground cables.
The firm said it has "no new intentions" for the Elswick site.
Preese Hall, Weeton, owned by Cuadrilla
Drilling started in August 2010, to a depth of 9,200ft (2,800m). However, the gas extraction activity there was linked two earth tremors near Blackpool in May 2011.
It resulted in the government temporarily banning hydraulic fracturing in the UK. In June 2012, a government review concluded fracking was safe if adequately monitored.
A study by the British Geological Survey placed the epicentre for both earthquakes about 500m (1,600ft) away from a well at Preese Hall and a report commissioned by Cuadrilla said it was "highly probable" the activity triggered the earthquakes.
When Preese Hall was fracked, the Environment Agency found traces of naturally-occurring uranium and thorium. It also found levels of radium up to 90 times higher than those naturally occurring in drinking water.
Previous regulations classed the waste water, known as "flowback water", as industrial effluent and Cuadrilla was legally authorised to discharge two million gallons into the Davyhulme treatment works in Trafford which went into the Manchester Ship Canal after being processed.
But changes to the agency's guidelines which came into force in October 2011 means this "flowback water" would now be classed as radioactive waste.
Another report - by the Chartered Institution of Water and Environmental Management (CIWEM) indicated that treatment capacity should not represent a problem in the UK - a view shared by Caudrilla.
Protesters from Frack Off held demonstrations at the site following the tremors.
Cuadrilla announced in December it was pulling out of the site and will apply to extend current planning permission to allow time to seal the well and return the site to its previous condition.
The firm said it was "prioritising" new horizontal wells in Lancashire that will provide better data about the amount of gas that can be recovered from the shale rock.
Singleton, also known as Grange Hill, Cuadrilla
Temporary planning permission was granted by Lancashire County Council to do test drilling at Grange Hill in 2010 and the well was drilled to 10,700ft (3,300m) in 2011.
Cuadrilla said it will not apply for permission to carry out hydraulic fracking there.
The firm said it would be installing monitoring equipment at Singleton to test for disturbances in the earth.
Following the tremors near the Preese Hall site, which had magnitudes of 2.3 and 1.5, Cuadrilla said it will suspend operations if records anything over 0.5.
Westby, also known as Anna's Road, Cuadrilla
Planning permission was granted for exploration activities in April 2010.
The Defend Lytham group formed in opposition to any drilling and said the site was "totally inappropriate for a shale gas development given its proximity to housing and the inadequacy of the local infrastructure".
The company announced it was abandoning the site and would not seek consent to frack there in October 2013, saying it had encountered "technical constraints related to wintering birds".
The location is close to the wetlands of the Lancashire coast, with pink-footed geese and whooper swans returning to the area for winter.
However, a spokesman confirmed drilling was suspended at the site in September 2012 when a tool got stuck.
The company said it was now working to restore the plot to its former condition.
Site: Banks (also known as Becconsall), Cuadrilla
D'Arcy Oil Exploration Company started drilling the Banks site at Formby Oil Field, in 1939 and produced 72,000 barrels of oil until abandoning the site in 1965.
In 2010, Cuadrilla was granted temporary planning permission for the Becconsall site and drilled for three months to a depth of 10,500ft (3,200m).
Cuadrilla was granted a 36-month extension of planning permission in 2012.
Three anti-fracking protesters who halted test drilling by invading the site in 2012 were found guilty of trespass, assault and obstruction.
The firm said fracking will now not take place at the site. Like Singleton, it will become a monitoring site for movement below the ground, and some small-scale diagnostic well tests.
Proposed site: Clifton, owned by Cuadrilla
There are no plans to frack at the south of Clifton Business Park, off the A584.
The well was proposed in order to allow rock samples to be extracted from the shale deep below the surface for analysis.
However, the well is to be sealed and returned to its former condition once exploratory work is completed, the firm said.
Planned Site: Roseacre Wood, near Elswick, Cuadrilla
The energy firm announced plans for a proposed exploration site in Roseacre Wood, located in between the villages of Roseacre and Wharles, on 4 February.
The firm said it was currently undertaking detailed assessments of the site before applying for planning permission to test drill for shale gas with the intention of eventually fracking at the site.
In order to hydraulic fracture at the site, it would require further planning permissions from Lancashire County Council.
Mr Egan said: "We're committed to being a good neighbour and to talking with the community at every stage of the process."
Planned Site: Preston New Road, near Little Plumpton, Cuadrilla
A second proposed site to test for shale gas resources was announced by the energy firm on the same day in February.
Like Roseacre Wood, the firm applying for planning permission to test drill there with the intention to go on to frack at the site, too.
Barton Moss, Salford, owned by IGas
Energy company IGas has permission to build a vertical test well, near the M62 and Barton Aerodrome.
It began exploring potential energy reserves beneath the area in November and is expected to finish at the end of March.
The firm said "if the data proves positive" it could do further exploration of the formations including the use of hydraulic fracturing providing it obtains the necessary planning permission.
An IGas spokeswoman said: "Should the data prove positive, we may wish to explore these formations further, which could include the use of hydraulic fracturing."
"However, this would require a separate planning permission and additional permits from the Department of Energy and Climate Change and the Environment Agency, and most importantly further consultation with the community," she added.
About 60 protesters have been camped on Barton Moss Road since test drilling started.
The landowner, Manchester Ship Canal Developments Ltd and Peel Investments (North) Ltd, has launched legal action at Manchester Civil Justice Centre to remove campaigners from the site.
Greater Manchester Police has estimated the cost of policing the demonstrations at the site will be around £750,000.
Planned site: Cheshire, owned by Dart Energy
Australian company Dart Energy is involved in the exploration and production of unconventional hydrocarbons, in particular, shale gas and coal seam gas.
The chief executive of the company with shale gas test drilling rights for Cheshire said he "can't wait to drill in [Wayne] Rooney's backyard".
John McGoldrick told shareholders in Western Australia that "shale gas [under] where all the Man U [Manchester United] players live" was worth "billions".
However, he said the company had still to find the "sweet spots" in the county.