DECC has published a study estimating the gas resources of the Bowland shale accessible at:
This estimates gas resources at depths of 5,000 ft (approx. 1.5 km) and greater. The report does not consider it likely that any gas resource which might be found at shallower depths could be commercially viable to extract with current technology.
No estimate has been made of shale gas reserves, that is, the proportion of the estimated resource that might be technically and economically producible.