Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.  All Rights Reserved.
 

Martin D. Matthews

Texaco International Exploration Division
Bellaire, Texas, U.S.A.

Many mechanisms have been proposed for hydrocarbon migration, and many processes have been described that modify the composition of migrating hydrocarbons. Examination of subsurface and surface data indicates that all the proposed mechanisms and processes are active. However, many play minor roles only recognizable in special situations. The dominant migration mechanism is as a free phase, rising under the forces of buoyancy within carrier and reservoir rocks, and capillary imbibition in the transition from sources and seal into carrier rocks. The migration pathway is determined by three-dimensional heterogeneity at all scales, from the individual pore systems to the interrelationships of facies. The dominant process modifying the composition of migrating hydrocarbons is phase partitioning, as evidenced by subsurface and surface data on hydrocarbon expulsion, migration, and accumulation.

In the near surface, many processes act to modify this seepage, particularly biogenic activity and diffusion (both chemical and mechanical). Free-gas surface hydrocarbon survey measurements, however, are dominated by this seepage mechanism. It explains (1) the spatial variability of the data; (2) the relationship of high-magnitude sites to high-permeability geologic features such as faults, fractures, unconformities, and outcropping reservoir units; (3) the compositional relationship of subsurface reservoired hydrocarbons to source rocks and the lack of relationship to ineffective source rocks; the variation of magnitude with time, both long and short; and current estimates of the rate of transport in the near surface.