The mechanisms and extent of oil and gas migration have long been controversial subjects among petroleum geologists. Acceptance of proposed "primary" migration mechanisms, which involve the initial transfer of oil or gas from source rock to reservoir, is further complicated because several of these hypotheses require that petroleum formation occurs during the primary migration stage. "Secondary" migration, which refers to the movement of oil and gas from one reservoir position to another, is better understood because geochemists have shown that petroleums undergo small but measurable changes in chemical composition during this type of migration. Fortunately, in many instances, these chemical 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 that can be attributed to secondary migration, certain petroleums, produced from distinct but narrowly separated horizons within a single field or limited geographic area, are markedly different in chemical composition. Other chemical characteristics of this group of oils, however, suggest that they were derived from a common source. The observed chemical differences cannot 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 undergone physical separations of major petroleum fractions prior to, or during, the migration process. This variety of petroleum segregation, capable of producing major chemical hanges, is herewith designated as a "separation-migration" mechanism to distinguish it from the typical secondary migration phenomenon, which results in relatively minor petroleum-composition changes.

Although the recognition of a new petroleum migration mechanism may appear to complicate further our already strained concepts of petroleum migration and segregation, the existence of a "separation-migration" mechanism is in accord with and a plausible consequence of some of the best-founded hypotheses of petroleum evolution.