Shallowly buried coal is usually highly permeable, because as the organic material matures fractures
form called cleats, which cause mined coal to appear blocky. Cleats increase coal permeability dramatically. This
is why fresh water can frequently be found in shallow coal beds, and why coal bed methane extraction
frequently involves what is called “dewatering the coal.” This is also why methane can flow easily within
the coal, especially at shallow depths. When coal is more deeply buried, however, the pressure of the overlying
rocks can close the cleats and lower the permeability of the coal. Natural gas companies prefer to extract gas
from shallower coal units because of this. Because coal bed methane is sought from a wide range of depths,
and because coals have a higher permeability, stimulated fractures are usually intended to connect between
multiple coal layers.