Figure 2.  Concentration (as partial pressure) vs. depth for steady diffusion of methane through a seal where a gas phase forms above the seal.  Methane concentration gradient in the seal is controlled by the pressure in the underlying gas reservoir, the requirement to form a gas phase above the seal, and the thickness of the seal. The minimum possible methane partial pressure gradient in the overlying sandstone is the hydrostatic gradient G if water is gas-saturated at the seal-sandstone interface.