Although knowledge of the physical and chemical factors influencing petroleum migration is insufficient to permit the proposal of acceptable mechanisms for primary or secondary petroleum migration, geochemists have shown that petroleums undergo small but measurable changes in chemical composition during secondary migration. In most instances, these changes can be distinguished from those chemical transformations which stationary petroleums slowly experience in response to reservoir temperatures and pressures over geologic time intervals.

In contrast to the relatively minor chemical changes attributed to secondary migration, certain petroleums, produced from different reservoirs within a single field or limited geographic area, are markedly different in chemical composition. Other chemical characteristics of these oils, however, suggest that they were derived from a common source. The observed chemical differences can not be explained as transformations of the stationary maturation variety. Detailed studies of the compositional differences encountered in such oil sequences imply that these oils must have experienced physical separations of major petroleum fractions prior to or during the migration process. This variety of petroleum segregation, capable of producing major chemical changes, is designated as a "separation migration" mechanism to distinguish it from the typical secondary migration phenomenon, which results in relatively minor changes in chemical composition.

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