Migration is the least understood step in the sequence of processes leading to the formation of a subsurface hydrocarbon accumulation. Hydrocarbons are redistributed by primary and secondary migration, which lead under ideal circumstances, to the formation of a reservoir accumulation. During geologic time, the reservoir accumulation may be destroyed by dissipation, that is, the leakage of hydrocarbons through the caprock (tertiary migration or dis-migration).

On the basis of case histories from geochemical analysis of exploration wells and of several shallow core holes, it is demonstrated that these hydrocarbon migration processes can be recognized by routine geochemical techniques. Certain changes with depth in light and heavy hydrocarbon composition reveal the extent and effectiveness of hydrocarbon migration. In particular, migration patterns become evident from a comparison of geochemical data of two adjacent exploration wells which penetrate the same stratigraphic sequence of interbedded source and reservoir rocks, but at different depth and maturation levels. Patterns of primary migration are evident also from regular compositional trends on either side of the contact between a source rock and a reservoir bed in a Jurassic-age sequen e from Svalbard, Norway. Finally, geochemical evidence shows light hydrocarbons escaping from a gas reservoir by diffusion through the overlying seal. Based on recently determined diffusion rates for certain light hydrocarbons, one can estimate the hydrocarbon mass transport as a function of geologic time.

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