Vertical migration is the mechanism that is used to explain hydrocarbon seepage to the surface.
It is the process that causes the surface geochemical manifestation by which we can detect petroleum accumulations using various shallow methods. The seal rock above a petroleum reservoir has minor fractures or disconformities that allow minor amounts of hydrocarbons to escape and migrate to the soil above.
Vertical hydrocarbon movement via the seep mechanism for petroleum is:
- Mass-migrating through open faults and fractures
- Megaventing along migrational pathways
- Flowing from overpressurized and breached reservoirs via faults
- Expelling from currently generating hydrocarbon source rocks
Microseeps range in size from the oil-saturated sandstones of Utah's Tar Triangle and the methane-bubbling springs of California's San Joaquin Valley to hydrocarbons moving from pore to pore in a rock formation. Usually microseeps are invisible and so low-level that they can not be distinguished without modern analytical methods.