The principle of differential entrapment is outlined as an explanation of the so-called anomalous occurrences of oil and gas in contiguous structures. Selective trapping or "gas flushing" is responsible for the fact that some traps are gas fields and not oil fields, and explains why many apparently good structures are dry. The more important factors which modify accumulations of hydrocarbons are listed and discussed.

The Bonnie Glen-Wizard Lake reef trend of Alberta, Canada, is described as an example in which the accumulation of oil and gas is largely controlled by the principle of differential entrapment. Many other examples of actual geological occurrences that have been described in the literature are illustrated and reinterpreted. Many of the accumulations in the Rocky Mountain province of the United States are classic examples. Relationships in the Persian fields are mentioned briefly, and a new explanation of the synclinal occurrence of oil is presented.

New concepts introduced are: (1) hydrocarbons move along definite migration paths; (2) they migrate as streams or rivulets; (3) traps located along these migration paths will be filled with oil and/or gas; those not on the path of migration will remain loaded with salt water; (4) traps filled with oil are still effective gas traps but a trap filled with gas is not an effective trap for oil; (5) regional or distant migration is believed essential for commercial accumulations of hydrocarbons.

The greatest argument against the theory of a distant source and migration for long distances has been refuted, and a clear understanding of this important principle and its application should lead to many new-pool and new-field discoveries.