Lancashire's shale gas estimated at £136bn by Cuadrilla

In response to claims flowback water was radioactive, Francis Egan said it has low levels of radioactivity which is not harmful

Shale gas reserves in Lancashire could have a market value of £136bn, the firm applying to extract gas from the area estimates.

Cuadrilla Resources chief executive Francis Egan said the valuation of gas in Lancashire's Bowland basin was not "over optimistic".

In a BBC Radio Lancashire debate, he reckoned it could extract 10% of the 200 trillion cubic feet of gas there.

Opposition to fracking, a form of gas extraction, fear it may harm the area.

'Can cause cancers'

The firm has applied to Lancashire County Council to frack for shale gas at its Anna's Road site in St Annes and Banks, near Southport.

Cuadrilla is currently doing an environmental impact assessment for each of the sites.

Mr Egan said: "From experience elsewhere in the world, 10% is probably a reasonable estimate and not an over optimistic estimate."

Fracking is a technique involving pumping water and chemicals into shale rock at high pressure to extract gas.

Protesters say fracking could have "enormous implications" for the local environment particularly about the contamination of waste or flowback water once it has been used in the fracking process.

Philip Mitchell, a member of the Blackpool and Fylde Green Party, said he also has serious concerns about the dangers of air pollution.

"Benzene is present in shale gas and it is dangerous. I certainly would not want the public exposed to it.

"If it it gets in the air it can cause cancers."

Leon Jennings, Cuadrilla's health and safety director, insisted fracking is safe.

He said "testing is all contained" and "strictly regulated".

Cuadrilla have applied for a permit to frack at their sites in St Annes and Banks

"Yes it is safe and it's not going to cause any harm to immediate staff on our site or the environment around us."

Tina Rotherby, of the Residents Action Fylde Fracking, has concerns about the water returning after extraction.

Mr Egan said: "The treatment of flowback water is governed by the Environment Agency and [it] has confirmed the waste water is non-hazardous.

"It does contain low levels of radioactivity as does bananas and brazil nuts but it is not a danger to human health."

He added: "It is naturally occurring radioactivity not radioactive waste."

Fracking was temporarily banned in the UK after it was blamed for two earth tremors in Blackpool in 2011.

A government review has now concluded that fracking is safe if adequately monitored.