Tight gas sands are defined as sandstone formations with less than 0.1 millidarcy permeability. The millidarcy is a unit of measurement related to the ability of a fluid to pass through a porous medium. The degree of permeability depends upon the size and shape of the pores, the size and shape of their interconnections, and the extent of the latter. It has been estimated that total-gas-in-place in the United States may exceed 15,000 trillion cubic feet.