The primary cause of the movement of oil from a shale source bed to reservoir beds is compaction within the shale beds. Reservoir beds such as limestones and sandstones show little compaction in comparison with shale. Compaction in shale always occurs when there is an increase in the depth of burial and causes an outward movement of the fluid content of the shale toward adjacent non-compressible horizons of lower pressure or with easier outlet to the surface.

Compaction in shale may be expressed as an exponential function of the depth of burial, and continues to be operative to an appreciable extent at depths ranging from 3,000 to 4,000 feet.

Zonal migration of fluid as caused by compaction may help explain the variation in composition of waters at different depths.

Temperature changes, buoyancy, and capillarity are not effective in causing oil migration if the oil is in a disseminated state, or adsorbed on the mineral grains, or associated with free gas in bubbles.