Figure 3. Possible relationships between reservoir gas pressure and hydrostatic pressure above the seal where the gas must exsolve. (1) If the reservoir gas pressure is less than the hydrostatic pressure above the seal, diffusion upwards through the seal is impossible.  (2) If reservoir pressure is higher than the hydrostatic pressure at the top of the seal, but the overpressure is negative formation of a gas phase requires that diffusivity of the seal exceed that of the rock over the seal, which is inconsistent with experimental data and theory.  (3) If the overpressure is positive, then formation of a gas phase is possible where the ratio of the seal to sandstone diffusivity is less than the ratio of the seal to sandstone concentration gradients.