The essential characteristic of shale gas is that the resource volume is often huge and the magnitude and effort required to extract it is correspondingly enormous. What attracts the fossil fuel companies is the same thing that alarms people living near the shale gas resource. It worries those of us who are concerned about dangerous climate change, as well.
In 2013, the British Geological Survey (BGS) published an assessment on the gas resources of the Bowland Shale in northern England. They concluded that the median gas-in-place resource was 1329 trillion cubic feet. To put this in perspective, this is about 16 times the amount of gas produced from the UK North Sea over 50 years. The BGS did not estimate the recoverable gas resource, because they considered that the recovery factors are too uncertain to quantify.

From the BGS Bowland Shale Study 2013 https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/226874/BGS_DECC_BowlandShaleGasReport_MAIN_REPORT.pdf
How much drilling would it take to exploit this resource?