Chemical Geology, 71 (1988) 1-10
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
1
MULTIPLE ORIGINS OF METHANE IN THE EARTH
MARTIN SCHOELL
Chevron Oil Field Research Company, La Habra, CA 90631 (U.S.A.)
(Accepted for publication September 21, 1988)
Abstract
Schoell,M., 1988.Multipleoriginsof methane in the Earth.In: M. Schoell(Guest-Editor),Originsof Methane in
the Earth.Chem. Geol.,71: 1-10.
Methane occurrences in the Earth's crust are predominantly of biogenic origin, i.e. their ultimate source is biologically formed organic matter. Methane can also form through inorganic reactions and is consequently termed
abiogenic. Biogenic methanes can either form through bacterial or thermogenic processes. Bacterial processes follow
a C02 reduction and/or fermentation pathway. The fermentation processes are quantitatively more important in
recent fresh sediments and swamps, Methane formed by C02 reduction, however, is most common in older sediments
and commercial gasfields. Temperature, organic substrate and age may be the major factors controlling the relative
importance of the two pathways. Stable-isotope concentrations in thermogenic methanes seem to be controlledby the
extent of conversion of organic matter, the timing of gas expulsion, and trapping. The different character of methane
in individual sedimentary basins may be a result of the geologic history. Geothermal methanes are most likely derived
from pyrolysis of organic matter. Abiogenic methane occurs in hydrothermal vents and ophiolite complexes. Inorganic
reactions, either surficial or deep-seated, are the likely source of such methanes. A uniform mantle origin of methane
is not supported by the observed isotope variations in naturally occurring methanes.
1. I n t r o d u c t i o n
M e t h a n e , a highly r e d u c e d f o r m o f carbon,
plays a n i m p o r t a n t role in m a n y geochemical
processes in t h e E a r t h ' s crust. In t h e E a r t h ' s
early a t m o s p h e r e m e t h a n e was m o s t likely a
crucial c o m p o n e n t ( C h a n g e t al., 1983 ). T o d a y ,
m e t h a n e is o n l y a m i n o r c o n s t i t u e n t o f t h e atm o s p h e r e b u t has r e c e n t l y received considerable a t t e n t i o n because of its role as a " g r e e n h o u s e
gas" ( W a n g et al., 1976). T h e s t u d y o f isotope
v a r i a t i o n s helps u n r a v e l t h e various sources o f
a t m o s p h e r i c m e t h a n e . F o r details t h e r e a d e r is
r e f e r r e d to t h e review o f S t e v e n s (1988 in this
special issue). In r e c e n t s e d i m e n t s m e t h a n e is
produced and consumed by bacterial processes
t h e r e b y a f f e c t i n g t h e i r early diagenesis. In
0009-2541/88/$03.50
deeper sections of the Earth's crust methane is
a product of the conversion of organic matter
under the influence of elevated temperatures
(Galimov 1973, 1988 in this special issue). In
even deeper sections of the earth's crust methane is found in fluid inclusions of metamorphic
rocks (Kreulen and Schuiling, 1982). Finally,
methane is emanating with geothermal waters
on continents (Des Marais et al.,1981, 1988 in
this special issue;Lyon and Hulston, 1984) and
hot water vents at oceanic spreading centers
(Welhan, 1981, 1988 in this special issue). The
question arises whether and how these various
sources can be differentiated.This brief review
summarizes how the carbon and hydrogen isotopic composition of the methane molecule can
aid in discriminating the various sources and
© 1988 Elsevier Science Publishers BN.
discusses the limitations of an isotopic characterization. In addition, areas where more research is needed are indicated.
2. S o m e d e f i n i t i o n s
Methane is predominantly a product of the
conversion of organic matter in different temperature regimes. Simple observations in nature
( methane -bubbling
swamps
and
sediments) and in the laboratory (pyrolysis experiments) as well as chemical and isotopic
analogies with natural methane occurrences
lead without any doubt to this conclusion.
Methane which ultimately is derived from organic matter should be termed biogenic as opposed to abiogenic methane derived from
processes that do not involve organic matter
(Welhan, 1988 in this special issue ). This definition is different from the common usage in
the literature (Schoell, 1980, 1983; Rice and
Claypool, 1981; Martens et al., 1986) where the
term biogenic has been used for methane of
bacterial origin. A much better term for methane which is derived from bacterial processes is
bacterial or microbial methane (Jenden and Kaplan, 1986; Coleman et al., 1988 in this special
issue). Other types of biogenic methanes may
be, consequently defined in analogy to terms
used for natural gases, i.e. thermogenic for
methane formed through thermochemical reactions, etc. (Schoell, 1983). The discrimination of bacterial and thermogenic methane is
sometimes ambiguous, specifically for methanes of ~ - 60%0 in their carbon isotopic composition (see pp. 3 and 4).
3. B a c t e r i a l m e t h a n e
Our understanding of bacterial methane formation has undergone substantial revision in
recent years and it is now accepted that bacterial methane formation follows two principal
pathways, i.e. via C02 reduction and fermentation (Schoell, 1980; Woltemate et al., 1984;
Jenden and Kaplan, 1986; Martens et al., 1986;
Whiticar et al. 1986; Burke et al., 1988). Fermentation-derived methane is characterized by
its depletion in deuterium (Fig. 1 ). Initially, it
was assumed that fermentation is the predominant pathway in freshwater environments
(Woltemate et al., 1984; Whiticar et al., 1986)
but later studies found similarly D-depleted
methane in marine environments (Jenden and
Kaplan, 1986; Burke et al., 1988). Also bacterial gases from Illinois (Fig. 1 ), a non-marine
freshwater setting, fit isotopically the group of
methanes formed through C02 reduction
(Schoell, 1980, 1984; Coleman et al., 1988 in this
special issue). The methanogenic pathway is
therefore independent of the depositional environment. The Cape Lookout Bight, North
Carolina, data of Martens et al. (1986) and in
particular of Burke et al. (1988) clearly demonstrate that the methanogenic processes are
seasonally controlled: In summertime and at
warmer sediment temperatures acetate fermentation is the predominant process whereas
in wintertime with lower sediment temperatures C02 reduction prevails, The characteristic negative correlation between the C- and Hisotopic composition of methane from Cape
Lookout Bight could be the result of a simple
mixing process between C02 reduction and acetate fermentation end-members. This in turn
would mean that both processes are operating
simultaneously but at different rates. It is interesting to note that a few methane data from
a swamp in NW Germany (Woltemate, 1982;
Whiticar et al., 1986) show the same inverse
isotope relationship.
The causes for the change of microbial pathways are not yet clear. Whiticar et al. (1986)
suggested that the acetate pool becomes depleted through methanogenesis. Martens et al.
(1986) found that pathways of acetate cycling
change during the seasons. Jenden and Kaplan
(1986) proposed that fermentation-derived
methane is primarily produced from fresh sediments of terrestrial origin and speculated that
fermentation could decrease with age {aging ef-
I
- 100
I
I
B1
f
- 200
II
-,/--/
~Z'~
~ '~ ~¢~"r~'~"~. "~' ~l ~
_~
-tO
~1~
- 300
2 *
3o
4A
,5o
6m
7~
,
8A
- 400
- 120
I
I
I__
I
- 100
- 60
-80
513CCH
I
"/
- 40
4 (%)
Fig. 1. Compilation of carbon and hydrogen isotope variations in bacterial methanes. BR denotes the field of the C02 reduction pathway and BE the field of fermentation pathway methanes. B1 is the field for methanes of bacterial origin but enriched
in ~3Cthrough secondary processes (Jenden and Kaplan, 1986). 1 = Wiirmsee, south Germany (Woltemate, 1982; Woltemate
et al., 1984); 2= Scripps Submarine Canyon, offshore California (Jenden and Kaplan, 1986); 3--- Cape Lookout Bight,
North Carolina (Burke et al., 1988); 4=swamp, NW Germany (Woltemate, 1982; Whiticar et al., 1986); 5=glacial drift
gases, (Schoell, 1984; Coleman et al., 1988 in this special issue); 6=marine sediments (Whiticar et al., 1986); 7=marine
sediments, Antarctic Peninsula (M.J. Whiticar and E. Suess, as referenced in Whiticar et al., 1986); 8=Baltic Sea (north
Europe) sediments (Whiticar et al., 1986).
fect). Woltemate (1982) described systematically-changing isotope values of methane with
increasing depth in a swamp in NW Germany
(4 in Fig. 1). As $13C-values changed from
-58.5%o in the top 5 cm to -62.2%o between
20 and 50 cm, the JD-values changed from - 374
to - 3 5 3 %0, respectively. A similar aging effect
was found for methane samples from Volo Bog,
Illinois (J.B. Risatti and D.D. Coleman, pets.
commun., 1987). It is noteworthy in this context that all locations so far described as producing fermentation methane are indeed only
young sediments with input of fresh organic debris. Sediment temperature may also influence
which of the metabolic pathways are at optimal
operating conditions. It may be interesting to
investigate whether fermentation is restricted
to sediments with elevated temperatures. The
Arctic Peninsula data of M.J. Whiticar and E.
Suess as referenced in Whiticar et al. (1986)
provide some circumstantial evidence for this
contention. With increasing age and continuing methanogenesis the organic substrate
changes and may not be suitable for fermenting
microbial communities. All these effects combined may control which pathway is predominant.
Fig. 2 tries to generalize the concept of fermentation- vs. CO2 reduction-derived microbial methanes. The two processes can operate
simultaneously but are quantitatively important at different stages of sediment deposition.
In very general terms, fermentation precedes
CO2 reduction. The combination of the
~DH20-OCDcH4and ~13CcH4-OeDcH4 diagrams allows one to roughly estimate the end-members
.
[
~o~
;,,,
~-1
...............
o
t~
i
"~\\%
Microbial Fermentation
1
6 DH20
b
(a)
I
~13CcH4
(b)
Fig. 2. A general model of bacterial processes during transformation of organic matter in sedimentary environments and
their control on the isotopic composition of methane.
a. The two bacterial pathways, fermentation and C02 reduction, are indicated by F and R, respectively. The left diagram
indicates the relationship between the deuterium concentration in the associated water and the methane formed through
the respective pathways.
b. Fermentation-derived methane is generallyenriched in 13C,compared to C02 reduction-derived methane, resulting in the
characteristic inverse correlation of the isotope concentrations. The possible controls on the predominant pathway are
indicated.
of the two microbial pathways. A large uncertainty still exists regarding the deuterium concentration of fermentation methanes (see
Jenden and Kaplan, 1986; Schoell et al., 1988
in this special issue, for discussion).
An important aspect of the aging effect is the
following: if indeed fermentation is the predominant process in very young, recently deposited sediments, one should expect this
methane to be predominantly lost to the atmosphere (Jenden and Kaplan, 1986; Coleman
et al. 1988 in this special issue). It would be very
unlikely for fermentation gases to become trapped in deeper strata. The aging hypothesis offers therefore a good explanation for the fact
that all bacterial gases in reservoirs and older
marine sediments are similar and have the isotopic character of methane derived by CO2 reduction (Claypool and Kaplan, 1974; Schoell,
1980; Mattavelli et al., 1983 ).
With respect to a genetic characterization of
methanes it is noteworthy that despite the isotopic similarity between oil-associated methane and fermentation methane (Fig. 3), both
types occur in such different geologic settings
that confusion of both methanes is very
unlikely.
4. T h e r m o g e n i c m e t h a n e
Rice and Claypool (1981) have estimated that
~ 80% of commercial natural gas is of thermogenic origin. Fig. 3 includes some new data on
thermogenic methanes from a wide variety of
geologic settings and sedimentary basins. It is
particularly interesting to compare the isotope
variations of methane from commercial thermogenic gases with methane occurrences from
geothermal areas, from the East Pacific Rise,
and from the Canadian Shield.
HD
Atmospheric
Methane
~ , ~
EPR
...~lti~, ""
:
ZOM
(HC)
- 100
,,,~-s'i~7!~7~-77 i r l
.ct
- 200
"r
O
'~
i/
,,BR ~
...." " i
-
~:~"
Zl
"
2
300
i
i
- 400
i
i--3
'\"
LD/'~), i
-
500
-
120
/ .i
'
I
I
I
I
I
- 100
- 80
- 60
- 40
- 20
0
~13CCH 4 (%0)
Fig. 3. 13C and deuterium concentrations in naturallyoccurring methanes. FieldsBR and BF are the areas which encompass
bacterialmethanes that form by C02 reduction and fermentation, respectively(seeFig. 1 ).The heavy outlinedarea encompasses methane of thermogenic origin,wherein the shaded part depictsmethane associatedwith oilsand the unshaded part
the non-associatedmethane. I = Sacramento Basin, California (Jenden and Kaplan, 1988); 2 = Cooper Basin, Western Australia (Rigby and Smith, 1981 );3 = Canadian Shield gases (Sherwood et al.,1988); 4 = geothermal methane (Des Marais et
al., 1981; Lyon and Hulston, 1984; Welhan, 1988 in this special issue); E P R = E a s t Pacific Rise (Welhan, 1981);
Z O M = Zambales Ophiolite methane Philippines (Abrajano et al.,(1988 in this specialissue);Migr. = migrated Rotliegend
gases, G.D.R. (Runge, 1980); L C and H C and L D and H D are highest and lowest concentrations for ]3C and deuterium,
respectively,found so far in natural methanes (seetext);Atmospheric methane (Wahlen et al.,1987 ).
4.1. Methane in commercial thermogenic gases
Methanes in reservoirs which are associated
with oil show a large isotopic variation as indicated by the shaded area in Fig. 3. Several authors report oil-associated methane with C- and
H-isotope values of ~ - 60 and ~ - 200%o, respectively, and suggest that mixing with bacterial methane occurred in the reservoir
(Jenden and Kaplan, 1986; Faber, 1987) (note,
however, that this bacterial methane is C02-reduction methane which may have been in the
reservoir before thermogenic methane entered).
Methane in thermogenic gases in the Cooper
Basin, Western Australia which are often associated with oil or condensate (Rigby and
Smith, 1981) are enriched in 13C compared to
other oil-associated occurrences such as in the
North Sea, NW Europe (Schoell, 1980; Faber,
1987), the Ventura Basin, California (Jenden,
1985) and southern Ontario, Canada (Barker
and Pollock, 1984). The Cooper Basin is special inasmuch as the liquid and the gaseous hydrocarbons are supposedly derived from coals.
The variability of C- and H-isotope concentrations in methane associated with oils and the
processes which control them are poorly understood. Mixing can additionally obscure primary
concentrations. It seems that individual basins
have characteristic patterns (North Sea, Ventura Basin, Cooper Basin etc.) which may be
the result of the specific geologic and thermal
evolution of a basin.
4.2. Methane in subduction zone related basins
The Sacramento Basin, California provides
a well-studied example of geologic controls on
natural gas geochemistry in a subduction-zone
setting (Jenden and Kaplan, 1988). Due to an
active tectonic history, commercial fields in the
basin contain complex mixtures of high-maturity thermogenic methane, bacterial methane
and oil-and condensate-associated methane
( Jenden and Kaplan, 1988). Locally present is
a nitrogen-rich component inferred to be derived from subducted metasedimentary rocks
and methane derived from magmatic heating of
intruded sediments. The interaction of these
sources is reflected in the broad range of C- and
H-isotope values in this basin. The Sacramento
Basin may be considered as a new type of gasprovince (subduction-related basins) which
could be important in circum-Pacific settings
( Poreda et al., 1988 in this special issue).
4.3. Genetic characterization of thermogenic
methane
Similar isotope concentrations in methane do
not necessarily mean that similar settings of
methane formation can be inferred. A case in
point is three occurrences of methane with similar isotopic composition: (1) Sacramento
Basin; (2) coal gases in NW Germany; and (3)
geothermal methane in New Zealand {Fig. 3).
Methane in NW Germany is a late-stage degassing product of mature to overmature coals
which differs from the complex deep-source
mixtures in the Sacramento Basin, and the hydrothermal processes in New Zealand. Although these gases are all of thermogenic origin
we have to be cautious with more detailed implications to the .geologic environment of gas
formation.
Empiric data compilations such as in Fig. 3
are useful but have their limitations. W h a t is
missing are controlled laboratory experiments
and sound theories of isotope distributions in
methane as a function of maturation of organic
matter. The most recent attempt in this direction is a new theory of Galimov {1988 in this
special issue) which applies the concept of different activation-energy distributions in coals
and oilprone kerogens and their effect on the
carbon isotopics of methane. This model predicts that the ~3C concentration of CH4 varies
considerably during the course of maturation,
i.e. during subsidence of a source rock in a basin.
This would imply that the carbon isotopic composition of methane is highly dependent on the
accumulation history, i.e. early-trapped vs. latetrapped methane or cumulative vs. instantaneous methane. It is conceivable that much of
the basin's specific character of isotope variations in methane is due to these specific timing
relationships of expulsion and trapping.
4.4. Canadian Shield methane
An extraordinary isotopic variability has been
found for methanes flowing in mining operations in the Canadian Shield (Sherwood et al.,
1988 in this special issue). The carbon isotope
variations in these methanes are well within the
range of thermogenic methane but the hydrogen isotope variations by far exceed the range
so far known. The methane from one of these
gases is with gD = -470%~ the most depleted
in deuterium reported in the literature. The
processes of the formation of these methanes is
not entirely clear. Sherwood et al. propose either
a biogenic or abiogenic origin. In my own opinion, the Ce+ content specifically argues for a
thermogenic origin.
4.5. Geothermal methane
The range of carbon isotope values of geothermal methanes between - 3 0 and -20%~
makes them similar to coal gases in N W Europe
or methane from the Sacramento Basin; however, their deuterium concentrations are consistently lower. Welhan (1988 in this special
issue) rules out bacterial processes and suggests cracking processes of high-molecularweight organic matter as a likely source.
Des Marais et al. {1988 in this special issue)
present experimental evidence that pyrolysis
processes are the source of methane in conti-
nental geothermal wells. This is corroborated
by the C- and H-isotope variations in geothermal methane from various areas (Fig. 3 ) which
resemble pyrolysis gases (see discussion of fig.
4 in Schoell et al. 1988 in this special issue).
The association of geothermal gases with 3He
is in most cases coincidental and reflects the
magmatic heat source. The occurrence of 3He
cannot be accepted as evidence that the associated methane is also of a mantle origin.
5. C- and H-isotope fractionation during
migration
Little is known about the effects of migration
on the C-and H-isotopic composition of methane. One case history so far overlooked in the
literature has been reported on methane from
Permian Rotliegend sandstone gases in the
German Democratic Republic (Runge, 1980).
The Rotliegend sandstones are important reservoirs in NW Europe for gases sourced from
- 60
I
I
t .-.wE
8
Carboniferous coals (Boigk et al., 1976). A
comparison of gases from Rotliegend reservoirs
in NW Germany (3 and 4 in Fig. 4) with those
in Carboniferous reservoirs in NW Germany
reveals small but systematic enrichment of 13C
and D in the latter. Runge {1980) found very
large enrichments in reservoired gases in Rotliegend sandstones (5 and 6 in Fig. 4). It must
be suspected that the heavy-isotope enrichment in the Rotliegend gas methane is a result
of fractionation caused by gas migration. A
similar enrichment in the heavy isotope was reported by Galimov (1967) from an experiment
in which gas was flowed through a reservoir.
These instances of isotope fractionations presumably induced by migration are so far the only
ones known to the author that considerably
change the isotopic signature of methane. These
observations are in contrast to the assumption
that migration has little effect on the isotope
concentration of naturally occurring methanes
[for discussion see review in Ricchiuto and
Schoell (1988) and references therein ]. More
research is needed in this area, possibly applying natural experiments using subsurface gasstorage facilities.
6. Abiogenic m e t h a n e
- 100
'W
3;
0
,,.,,
- 140
1
1
- 160
J
- 40
~1
~
I a
- 30
- 20
Rotllegend
Sandstone
Carb°niler°us
- 10
~ 1 3 C C H 4 (%)
Fig. 4. ~3C and deuterium concentrations in methane from
natural gases in Rotliegend and Carboniferous reservoirs
in Germany 1 =Carboniferous Emsland, F.R.G.;2 =WeserE m s area, F.R.G.; 3=Rotliegend Emsland, F.R.G.;
4 = Rotliegend East Hannover area between S~lingen and
Wustrow, F.R.G.; 5, 6=Rotliegend gases, unidentified,
G.D.R. Data for 1-4: Schoell ( 1984 ), and for 5 and 6: Runge
(1980).
Methane emanating in mid-ocean-ridge
(MOR) hydrothermal systems is one of the few
occurrences for which an abiogenic formation
is an unescapable conclusion simply because of
the setting of the MOR's in a sediment-free environment (Welhan, 1988 in this special issue ).
The 13C concentration between - 1 8 and
-15%o puts this methane indeed out of the
range of most biogenic methanes (Fig. 3). It
must be stated, however, that isotope values of
this range as such do not unambiguously argue
for an abiogenic origin. Welhan (1988 in this
special issue) suggests that high-temperature
equilibration processes between CO2 and CHa
account for the high '3C concentration in the
methane.
The most 13C-enriched methane so far re-
ported in the literature is derived from seeps in
the Zambales Ophiolite in the Philippines
(Abrajano et al., 1988 in this special issue).
With J~C-value of the methane of -7%c it is
strikingly similar to primordial carbon or mantle CO2. With this relatively strong enrichment
in 13C a biologic precursor is very unlikely. Both
bacterial and thermochemical processes tend to
generate methane which is more depleted in the
heavy isotope than its precursor. It would require very special closed-system conditions, for
example bacterial oxidation, to account for this
isotopic composition involving a biologic precursor. An abiogenic process is therefore very
likely responsible for these gas seeps (Abrajano
et al., 1988 in this special issue).
7. C o n c l u s i o n s
Methane in the Earth's crust is formed by
various processes. Basic evidence is provided by
observations of methane formation in highly
different environments and correspondingly
large variations in isotope signatures. Isotope
concentrations in methane are predominantly
controlled by the processes of methane formation. The observed variations of stable carbon
and hydrogen isotope variations of 110 and
400~i~, respectively, appear to be completely
incompatible with an origin of methane only
from the Earth's mantle as one source (Gold
and Soter, 1982). On the contrary, evidence so
far suggests that abiogenic methane is a rather
exotic form of methane and seems quantitatively insignificant in the Earth.
prises the most ~C and deuterium-enriched or
depleted methanes. These methanes are of very
different origins and attest to the multiple
origins of methane in the Earth.
- Most D-enriched (HD in Fig. 4: -71%~ vs.
SMOW. Origin: Thermogenic methane in a
Rotliegend sandstone (Runge, 1980).
- Most D-depleted (LD in Fig. 4): -470%~ vs.
SMOW. Sample N256-1985 from Norita mine
in Canada (Sherwood et al., 1988 in this special
issue). Origin: Not yet well known, possibly
abiogenic.
- Most l:~C-enriched methane (HC in Fig. 4):
Jr~D = -7(i~ vs. PDB. Seeping methane in the
Zambales Ophiolite on the Philippines reported by Abrajano et al. (1988 in this special
issue). Origin: Most likely abiogenic related to
serpentinization.
- Most ~3C-depleted methane (LC in Fig. 4):
J 1:3C= -109%o vs. PDB. A methane occurring
in marine sediments of the Antarctic Peninsula
(M.J. Whiticar and E. Suess, reported by Whiticar et al., 1986). Origin: Bacterial methane of
the CO2 reduction pathway.
The author is glad to continue this unofficial
record of records and accepts further entries.
After finishing this manuscript new methane
isotope data have been reported by Oremland
et al. (1988). A methane degassed from a sediment of Big Soda Lake, Nevada, had a 6D-value
o f -531J~),,
C" making this the most deuteriumdepleted methane so far reported in the literature. The methane is of microbial origin.
References
Acknowledgement
I thank Peter Jenden for review of this
manuscript.
Post scriptum
Isotope compositions in methane for a
geochemical "Guinness' Book of Records ''®
The compilations of C- and H-isotope data
of naturally occurring methanes (Fig. 3 )com-
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