No single transport mechanism can explain the movement of hydrocarbons within source rock-type shales. Instead, it is likely that several different mechanisms are operating at different times and stages of burial. At relatively shallow intervals before the peak of hydrocarbon generation solution migration seems to be favored. The dominating and most effective form of primary migration of oil during peak generation seems to be a pressure-driven hydrocarbon phase migration. At greater depth and in very mature source rocks microfracturing due to internal pore pressures is initiated. For gases, in addition to a hydrocarbon phase movement, diffusion through the water-saturated pore space of shales is an important process of primary migration.