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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, Interpretation of methane carbon isotopes extracted from surficial
marine sediments for detection of subsurface hydrocarbons, in D.
Schumacher and M. A. Abrams, eds., Hydrocarbon migration and its near-surface
expression: AAPG Memoir 66, p. 309-318.
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Chapter
22
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Interpretation
of Methane Carbon Isotopes Extracted from Surficial Marine Sediments for
Detection of Subsurface Hydrocarbons* |
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| Michael A.
Abrams
Exxon Ventures (CIS),
Inc.
Houston, Texas |
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*This chapter
is a modified version of a paper previously published in the Association
of Petroleum Geochemical Explorationists Bulletin, v. 5, p. 139-166 (1989). |
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Abstract
In surface prospecting
for hydrocarbons, it is often assumed that the carbon isotopic composition
of methane can provide information on the origin of the gas. This assumption
may not be valid for methane carbon isotopes extracted from surficial marine
sediments because of (1) mixing of gases with multiple origins and (2)
alteration resulting from secondary processes. Examination of a relatively
large data set reveals the correlation of methane concentrations with carbon
isotopic compositions. Three groups have been defined based on a comparison
of molecular concentrations to methane carbon isotopes:
type I--small
concentrations of methane with heavy isotopic compositions;
type II--large
concentrations of methane with very light isotopic compositions; and type
A--average to above average concentrations of methane with intermediate
isotopic compositions.
A large data set from deep
core samples must be collected to assess accurately the reliability of
methane carbon isotope values to deduce the origin of gas contained in
surface sediments. A plot of methane concentration versus d13C
is used in combination with the hydrocarbon composition data to help determine
the significance of the isotopic composition and subsequent interpretation
of origin. Samples that contain both type A isotopic compositions and anomalous
higher molecular weight hydrocarbons provide the most reliable information
on the origin of migrated gas. |
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