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| Matthews,
M. D., 1996, Migration--a view from the top, in D. Schumacher and
M. A. Abrams, eds., Hydrocarbon migration and its near-surface expression:
AAPG Memoir 66, p. 139-155.
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Abstract
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. |
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