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Abstract
Chapter from: M
66: Hydrocarbon Migration And Its Near-Surface Expression
Edited By
Dietmar Schumacher and Michael A. AbramsAuthors:
Stephan Holysh and József Tóth 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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Holysh,
S., and J. Tóth, 1996, Flow of formation waters: likely cause of
poor definition of soil gas anomalies over oil fields in east-central Alberta,
in
D. Schumacher and M. A. Abrams, eds., Hydrocarbon migration and its near-surface
expression: AAPG Memoir 66, p. 255-277. |
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Chapter
19
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Flow
of Formation Waters: Likely Cause for Poor Definition of Soil Gas Anomalies
Over Oil Fields in East-Central Alberta, Canada |
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Stephan Holysh
Regional Municipality
of Halton
Oakville, Ontario, Canada |
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József
Tóth
Department of Earth and
Atmospheric Sciences
University of Alberta
Edmonton, Alberta, Canada
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Abstract
Anomalous amounts
of hydrocarbon gases in soil pores have long been used in search of commercial
accumulations of petroleum by surface geochemical explorationists. Sufficient
new field discoveries are made on the basis of soil gas surveys to maintain
interest, but the results are too inconsistent to warrant routine use of
the method. We hypothesize that the main reason for the unconvincing track
record of soil gas surveys and of other surface geochemical exploration
techniques is a poor understanding of the mechanisms and pathways of hydrocarbon
migration from petroleum deposits to the surface. Thus, these techniques
are often used when their application is inappropriate.
The intensity and pathway
geometry of the hydrocarbon vertical leakage is strongly affected by groundwater
flow between a deposit and the surface. To test this hypothesis, we conducted
a comprehensive study of the dynamics, major ion chemistry, halogen ratios,
d18O
and d2H,
and 14C ages of groundwaters; the distribution of groundwater
recharge and discharge areas; and a survey and MS analysis of hydrocarbon
components in soil gas in an area with known oil fields in the Lower Cretaceous
Mannville Group in east-central Alberta. We found that the groundwater
flow pattern is regionally downward and into the underpressured Mannville
reservoirs. Shallow flow systems are associated with local topographic
highs and depressions. Soil gas anomalies, shown best by benzene and toluene,
were observed but are ill defined. Their geographic association seems to
be with local groundwater discharge areas rather than with subsurface petroleum
accumulations.
Our interpretation is that
descending groundwater flow (underpressuring of the reservoirs) prevents
the formation of well-defined soil gas anomalies, regardless of the mechanism
of migration from pool to surface. A slight increase in soil gas signatures
in local discharge areas is probably due to lateral transport of gas by
shallow groundwater flow. Our findings provide an explanation for the unsuccessful
attempts to apply surface geochemical exploration in Alberta, where most
of the near-surface petroleum fields are strongly underpressured, thus
inducing downward flow of groundwaters and obliterating near-surface geochemical
signatures. |
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