Figure 4.  Calculated relative diffusivity necessary to form a gas phase above a seal with and without gas compositional effects. Under the conditions assumed here, a gas column of at least 415.3 m is required for Dsh/Dss <1 where compositional differences affect diffusion. Assumed conditions are hydrostatic pressure gradient, reservoir depth of 3 km, gas density of 0.15 g/cc, and methane fraction of 0.85 in the reservoir gas and 0.95 in the gas exsolved above the seal.  Gas columns of 10, 100, and 1000 m create capillary pressures of 0.085, 0.85, and 8.5 MPa, respectively.