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.
Diffusion is a viable mechanism for primary migration.
Water transport of hydrocarbons could be as dissolved constituents, or as a separate gaseous or liquid phase. In this mechanism, the transport is related to the hydrodynamics of a basin and hydrocarbon migration follows the water phase to a suitable reservoir. Lateral offset of anomalies would be the general rule. This mechanism is also inconsistent with the vertical projection of the reservoir and the rapid change in the anomaly with production. Water transport is frequently considered a major factor in the secondary migration process.