Many facts of distribution of oil indicate that it is capable of migrating for considerable distances. The great decrease in the viscosity and surface tension of oil when gases are dissolved in it and at the higher temperatures found at considerable depth render oil much more mobile at depth than it is at the surface. The same is true of water.

It is not necessary to think of oil as migrating mainly in the beds which now form its reservoirs. The reservoir beds should be considered, rather, as beds into which the oil once entered and became trapped so that it has since been unable to escape.

Migration may be mainly through especially porous beds or "carrier beds" such as coarse sheet sands or cavernous limestones. From these the oil works its way upward at every opportunity into higher beds where it may be trapped, or in which it may continue its migration.

Anticlines and other structural disturbances, by causing fissures through which fluids can escape upward from the carrier beds, become centers toward which rock fluids, including oil, move from all directions and thereby become localizers of oil accumulation from wide areas. The fissures cause fluid movement toward the anticlines and their structure provides effective traps for the oil.

Oil may enter lenticular sands from carrier beds below wherever fissuring has occurred, and especially where they cross fissured anticlines, but, having entered them, it finds escape more difficult.

In the later physiographic history of a region the carrier beds may be flushed by circulating water while the beds above, and especially lenticular sands, may be unaffected.