Fascinating. It tells here
http://www.citg.tudelft.nl/live/pagina.jsp?id=18b578ea-2cba-418f-8df9-0577c19373dd&lang=en of how at great depth the coal was degassed and the methane became trapped in a higher layer of permeable rock.
I always wondered about North Sea gas, since we seem to have coalfields extending offshore, and dipping. If the carboniferous is continuous across the North Sea, it will be very deep, way below the oil and gas wells, and presumably the coal will have been reduced to something resembling graphite.
I doubt that there would be any prospect of mining at that depth!