Term used for the possible routes or movement of pressurized gas along a preferential path. The movement of hydrocarbons from their source into reservoir rocks. Primary migration is the movement of newly generated hydrocarbons out of their source rock. The further movement of the hydrocarbons into reservoir rock in a hydrocarbon trap or other area of accumulation is secondary migration. Migration typically occurs from a structurally low area to a higher area because of the relative buoyancy of hydrocarbons in comparison to the surrounding rock. Migration can be local or can occur along distances of hundreds of kilometers in large sedimentary basins, and is critical to the formation of a viable petroleum system