Primary hydrocarbon migration, which is defined as movement of hydrocarbons from nonreservoir to reservoir rocks, is believed to be one of the principal mechanisms in the formation of significant petroleum accumulations. Unlike the other important factors involved in petroleum accumulations, such as reservoir, trap, seal, and source rock, which are rock or a form of rock, primary migration is an action most of which occurred in the geologic past. Therefore, it is relatively difficult to prove.

On the basis of the relative concentrations and types of organic matters in shales and reservoirs (sandstones and limestones), and of the availability of water that would have facilitated hydrocarbon migration, primary migration is a necessity in forming significant petroleum accumulations. Statistical and indirect evidence of primary migration is based on the 7,241 sandstone reservoirs in the United States, and geochemical evidence is based on source-rock analyses of shales from different parts of the world. The principal direction of primary migration can be either vertical or horizontal.