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Abstract
Chapter from: M
66: Hydrocarbon Migration And Its Near-Surface Expression
Edited By
Dietmar Schumacher and Michael A. AbramsAuthor:
Michael A. Abrams Geochemistry, Generation, Migration
Published 1996 as
part of Memoir 66
Copyright © 1996 The American Association of Petroleum
Geologists. All Rights Reserved. |
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Abrams,
M. A., 1996, Distribution of subsurface hydrocarbon seepage in near-surface
marine sediments, in D. Schumacher and M. A. Abrams, eds., Hydrocarbon
migration and its near-surface expression: AAPG Memoir 66, p. 1-14. |
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Chapter
1
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Distribution
of Subsurface Hydrocarbon Seepage in Near-Surface Marine Sediments |
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Michael A.
Abrams
Exxon Ventures (CIS),
Inc.
Houston, Texas, U.S.A. |
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Abstract
Hydrocarbon
seeps in surficial marine sediments are of two types : active and passive.
Active seeps occur where gas bubbles, pockmarks, or bright spots are visible
on seismic profiles and where chemosynthetic communities are present in
conjunction with large concentrations of migrated hydrocarbons (macroseeps).
These generally occur where generation and migration of hydrocarbons from
source rocks are ongoing today (at maximum burial) or where significant
migration pathways have developed from recent tectonic activity. Passive
seeps occur where concentrations of migrated hydrocarbons are usually low
(microseeps) with few or no geophysical anomalies. These occur typically
in areas where generation and expulsion is relict (no longer at maximum
burial) or regional seals prevent significant vertical migration.
The type of seepage controls
the distribution of migrated hydrocarbons in the near-surface sediments
and should dictate the sampling equipment and approach used to detect seeps.
Active seeps are usually detected near the water-sediment interface, in
the water column or at the sea surface, and at relatively large distances
from major leak points. Most conventional sediment and water samplers can
capture active seeps. The Gulf of Mexico, Santa Barbara Channel, and parts
of the North Sea have active hydrocarbon seeps.
Passive seeps can only be
detected relatively far below the water-sediment interface and require
samples to be collected near leak points. Sampling equipment must penetrate
the zone of maximum disturbance or any shallow migration barriers. In areas
where surficial sediments are coarse grained or compacted, conventional
gravity corers will not work. Other options for subsurface sampling include
vibracores, jet cores, and rotary cores. Precise location of samples (site-specific)
using seismic profiles to locate leak points is critical to detect passive
hydrocarbon seeps. The Beaufort and Bering seas, offshore Alaska, and parts
of the North Sea contain passive seeps. |
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