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Abstract
Chapter from: M
66: Hydrocarbon Migration And Its Near-Surface Expression
Edited By
Dietmar Schumacher and Michael A. AbramsAuthor:
Dirk Kettel 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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Kettel,
D., 1996, A method for processing adsorbed methane stable isotope data
from the near surface based on fractionation, in D. Schumacher and
M. A. Abrams, eds., Hydrocarbon migration and its near-surface expression:
AAPG Memoir 66, p. 319-336. |
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Chapter
23
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A
Method for Processing Adsorbed Methane Stable Isotope Data from the Near
Surface Based on Fractionation* |
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| Dirk Kettel
Oil and Gas Consultant
Hannover, Germany
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Abstract
Molecular and isotopic fractionation
of gases that are controlled by flow processes are observed in nature.
Because gas flow in sedimentary basins is highly dynamic, reasonable fractionation
occurs on all geochemical parameters that depend on flow. Isotopic ratios
of methane adsorbed in near-surface sediments are often measured in geochemical
surface exploration. It is apparent, however, that the rough data cannot
be used directly to identify hydrocarbon sources in the subsurface because
the isotopic composition of methane fractionates during the transition
from the dissolved  phase in which methane is transported into the adsorbed
phase in which it is measured. This implies a knowledge of the physical
processes involved in gas migration through near-surface sediments, which
is derived here from one of the exceptional cases of prestationary fluid
flow found in nature. This physical background is then applied to the more
common stationary flow processes, in which isotopic fractionation reaches
equilibrium conditions. d13C1
fractionates toward the negative and dDC1
toward the positive. The degree of fractionation depends on the methane
flux in the subsurface and on the petrophysical properties of the sampled
near-surface host sediment. Determination of the fractionation and the
properties of the host sediment allows the methane flux from the subsurface
to be calculated. The flux is observed to be stronger over gas accumulations
and weaker over oil accumulations. The isotopic data processing method
derives hydrocarbon fluxes and identifies source rocks from near-surface
measurements and in this way allows the precise identification of hydrocarbon
accumulations in the subsurface. |
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