Time of oil migration is a controversial matter in petroleum geology; modern geochemical data suggest that a relatively long geologic time is necessary to generate oil and therefore to migrate it, whereas many petroleum geologist still believe in relatively early migration. A new concept of rate of oil generation is proposed on the basis of geochemical source rock analysis. This new concept suggests that the time of generation and migration of oil must be earlier than the time which the geochemical data of accumulated oil indicate.

Most oil must migrate in its own phase because of its low solubility in water; however, most gas may migrate in solution in water. To ensure the migration of oil in the oil phase, the importance of semi-solid or structured water was proposed. In compacted shales, a larger proportion of water may be in semi-solid state. Concentration of oil in the remaining liquid water may be relatively high. If the shales compact further, the oil may migrate along with water, provided the oil saturation is more than that for critical oil migration in the oil-phase.

Four possible agents of primary migration are considered--compaction, aquathermal, clay mineral conversion, and osmosis. These agents govern the movement and/or generation of liquid water. It must be noted that there is a large quantity of water and a relatively small amount of hydrocarbons at the time of primary migration. In other words, movement of some kind of water would be very important in understanding the movement of hydrocarbons at the primary migration stage.