Shale is only one type of sedimentary rock in the Hamilton Group. Important properties of all sedmentary rocks are their porosity and permeability. Porosity is the amount of space available between grains in a sedimentary rock, and permeability
is the ability of anything trapped between grains in a sedimentary rock to flow between pores. When natural gas is trapped
in a sedimentary rock with high porosity and permeability, like a sandstone, it has the ability to flow through the rock
to pockets of relatively low pressure. When natural gas is trapped in a sedimentary rock with low porosity and permeability, like the Marcellus