Surface techniques are dependent upon some degree of leakage from a subsurface oil or gas reservoir. All viable methods of surface geochemistry depend on mechanisms of loss from the reservoir and vertical migration, without frequent lateral offset or dispersion.
Diffusion represents a minimal loss rate from a reservoir.
Rates are slow and occur in all directions in response to a concentration gradient. Diffusion, operating in all directions, results in significant dispersion of the migrating hydrocarbons. Many surface geochemical surveys have demonstrated anomalies closely related to the vertical projection of the reservoir to the surface. Repeated geochemical surveys also show a decline in the intensity of the anomaly with production. These two critical observations are inconsistent with diffusion as a mechanism for vertical migration.