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Abstract
Chapter from: M
66: Hydrocarbon Migration And Its Near-Surface Expression
Edited By
Dietmar Schumacher and 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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Foreword Utilization of Hydrocarbon Seep Information
Geochemical prospecting
for petroleum is the search for chemically identifiable surface or near-surface
occurrences of hydrocarbons as clues to the location of oil or gas accumulations.
It extends through a range from observation of clearly visible oil and
gas seepages at one extreme to the identification of minute traces of hydrocarbons
determinable only by highly sophisticated analytical methods at the other.
There is no question in principle about the value of the method for petroleum
exploration if properly applied. Historically, most of the world's major
petroleum-bearing areas and many of its largest oil and gas fields were
first called to attention because of visible oil and gas seepages. The
mere presence of higher hydrocarbons in a region is encouraging in that
it usually proves that conditions in that region have been suitable for
at least some petroleum generation. Often seepages are in close proximity
to commercial oil and gas pools, but the absence of seepage does not at
all negate prospects because it may only indicate that there has been little
escape from such pools due to good sealing rocks.
Oil and gas are mobile fluids
and rocks are generally permeable. Surface oil and gas seeps primarily
reflect avenues of migration (or escape) from deeper and sometimes laterally
distant locations. Moreover, because avenues of migration (or escape) from
deeper accumulations vary considerably in the degree to which they are
sealed, the quantitative size of a seep has little relationship to the
size of the accumulation. Some small accumulations are marked by strong
visible seepages, whereas some of the largest accumulations are so well
sealed that they show no visible seepages and only microscopic seepages
or none at all. The value of seepages, visible or microscopic, is thus
largely a matter of the accuracy with which they can be interpreted geologically.
In some case (e.g., Burgan field) a well drilled vertically at the site
of seepage would have discovered the field. In other cases where escape
of hydrocarbons has been along low dipping fault planes or low dipping
carrier beds, surface seepages may be many miles laterally from vertical
superposition over the oil or gas accumulation. Again, the value of the
information on the seepage, visible or microscopic, is always there, but
it is only the geologic interpretation that allows cashing in on its value.
On land, most visible seepages
have already been recorded and the nature of the relationship to subsurface
petroleum accumulations has been at least studied if not always successfully
determined. The main task now for geochemical prospecting is the identification
of the invisible or less clearly manifested "seepages" that can be determined
only by detailed chemical analysis of fluids in surface and near-surface
rocks. The problems are not whether there is any value to the data but
rather are (1) the techniques for identification, (2) the geologic interpretation,
and (3) the quality of the interpretation good enough to justify the cost.
Offshore, the situation is
slightly different. Visual observation of offshore seepages has been impeded
by the water cover, and reliance must be placed mainly on chemical analysis
of the water column and the interstitial waters filling the pores of the
blanket of young sediment covering the sea floor. Again, there seems to
me no question of the innate value of the geochemical information, positive
or negative. And again, the problems are with the techniques of identification
and geologic interpretation, and whether the interpretation is good enough
to justify costs. There is nothing wrong with the concept; it is only a
question of our ability to collect the data adequately and to interpret
the results correctly, at a reasonable cost.
A geochemical survey should
be thought of not as a black magic means of spotting the location of oil
and gas pools but only as a simple common sense method of gathering data
on hydrocarbon occurrences too dilute to make visible seeps or impregnations--data
which if collected reliably, interpreted wisely, and used intelligently
along with all other lines of evidence will always be helpful in petroleum
exploration of any area.
Hollis D. Hedberg |
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