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Near-Surface Hydrocarbon Migration
: Mechanisms and Seepage Rates
By
Michael A. Abrams, EGI University of Utah, Salt Lake City, Utah
Abstract
In April 1994
the AAPG Hedberg Research Conference “Near-surface expression of hydrocarbon
migration
” was held to critically examine the process of hydrocarbon
migration
and its varied near-surface expression. The conference resulted in the
publication of AAPG Memoir 66 (Schumacher and Abrams, 1996). Key conclusions
resulting from the research conference and publication include:
• hydrocarbon accumulations are dynamic.
• all petroleum basins have some type of near-surface expression.
• leakage is not always detectable by conventional methods.
• near-surface seepage rates and concentrations vary.
• migration
does not always occur vertically.
• relationship between subsurface and surface expression is often complex.
Continued
studies over the years since the conference have further demonstrated the wide
variety of near-surface expressions of hydrocarbon migration
:
• seepage activity: qualitative expression of comparative rates at which hydrocarbons leak to the near-surface, active versus passive.
• leakage rates: episodic or non-continuous versus continuous leakage.
• seepage type: magnitude of leakage to the near-surface, macro-visible versus micro-only chemically detectable.
•
migration
focus: surface signal nearly vertical versus very large
lateral displacement.
• surface seep distribution: near-surface expression focused from point source, lateral displacement, or dispersed.
The rate and
volume of seepage to the surface greatly controls the near-surface geological
and biological responses. The variety of near-surface responses to hydrocarbon
leakage include seabed fluid or gas escape features (pockmarks and mud
volcanoes), chemosynthetic organisms, ocean column geochemical anomalies, wave
dampening features (oil slicks), and acoustic anomalies (wipe out zones,
chimneys, pull downs, bright spots, and BSRs). Factors which control subsurface
to surface migration
pathways and flow rates are determined by the
interrelationship of sediment fill, sedimentation rates, tectonics, and fluid
flow.
One key issue
not fully addressed in either the conference or book was a migration
mechanism
capable of moving hydrocarbons in a relatively short time (weeks to years)
vertically through thousands of meters of strata without observable faults or
fractures. Many mechanisms have been proposed over the years for microseepage:
diffusion, effusion, advection with moving waters, colloidal bubble ascent via
microfractures, and continuous gas-phase flow (Price, 1986; Brown, 2000).
Diffusion most likely contributes to the near-surface movement of hydrocarbons,
but can not account for the relatively rapid movement observed in many studies.
Effusion requires the hydrocarbons to move as an immiscible fluid through water
saturated pore spaces. This would require a relatively porous
migration
pathway
to overcome capillary pressures. Advection with moving waters was discounted by
Price (1986) because it requires significant vertical water movement. The gas
bubble ascent concept assumes small colloidal bubbles (microbubbles) move
vertically via a micro-fracture network. Lastly, continuous gasphase flow in
fractures where small colloidal size gas bubbles migrate as a continuous,
non-wetting phase in water wet rock through microfractures.
The 2001 AAPG
Hedberg Research Conference “Near-surface Hydrocarbon Migration
: Mechansims and
Seepage Rates” has been designed to evaluate mechanisms responsible for
geochemical signals observed in near-surface soils and sediments as well as why
these signals may not always be easily observed. The first session, seepage
rates and fluxes, includes papers looking at both ancient and present day
seepage measurements. The second session includes papers relating the role of
hydrocarbon seepage in the analysis of subsurface fluids. Session three is
designed to examine interpretation and analytical methods to help enhance the
evaluation of near-surface hydrocarbon
migration
. The last session will address
the
migration
mechanism issue.
References
Brown, A, 2000, Evaluation of possible gas microseepage mechanisms, American Association Petroleum Geology Bulletin, v. 84, no. 11, p. 1775-1789.
Price. L. C., 1986, A critical overview and proposed working model of surface geochemical exploration. in M. J. Davidson, ed., Unconventional Methods Exploration for Petroleum and Natural Gas - IV: Southern Methodist University Press, Dallas, p. 245-304.
Schumacher, D. and
M. A. Abrams, 1996, Hydrocarbon migration
and its near surface effects: American
Association Petroleum Geology Memoir 66, 450p.