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Multiple origins of methane in the Earth
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-
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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
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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.
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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
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- 140
1 1 - 160
J - 40
~1
~ I a
- 30 ~13CCH 4
- 20
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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-
Abrajano, T.A., Sturchio, N.C., Bohlke, J.K., Lyon, G.L., Poreda, R.J. and Stevens, C.M., 1988. Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin? In: M. Schoell (Guest-Editor), Origins of Methane in the Earth. Chem. Geol., 71:211-222 (this special issue ). Barker, F.F. and Pollock, S.J., 1984. The geochemistry and origin of natural gases in southern Ontario. Bull. Can. Pet. Geol., 32: 313-326. Boigk, H., Hagemann, W., Stahl, W. and Wollanke, G., 1976. Isotopenphysikalische Untersuchungen zur Herkunft und Migration des Stickstoffs nordwestdeutscher Erd-
gase aus Oberkarbon und Rotliegend. Erd~l Kohle, Erdgas Petrochem., 29: 103-112. Burke, Jr., R.A., Martens, C.S. and Sackett, W.M., 1988. Seasonal variation of D/H and 13C/12C ratios of biogenic methane in surface sediments of Cape Lookout Bight, U.S.A. Nature, (London), 332: 829-831. Chang, S., Des Marais, D., Mac, R., Miller, S.L. and Strathearn, G.E., 1983. Prebiotic organic synthesis and origin of life. In: J.W. Schopf (Editor), Earth's Earliest Biosphere - Its Origin and Evolution. Princeton University Press, Princeton, N.J., pp. 53-92. Claypool, G.E. and Kaplan, I.R., 1974. The origin and distribution of methane in marine sediments. In: I.R. Kaplan (Editor), Natural Gases in Marine Sediments. Plenum, New York, N.Y., pp. 99-139. Coleman, D.D., Liu, C.-L. and Riley, K.M., 1988. Microbial methane in the shallow Paleozoic sediments and glacial drift deposits of Illinois, U.S.A. In: M. Schoell (GuestEditor), Originsof Methane in the Earth. Chem. Geol., 71:23-40 (this specialissue). Des Marais, D.J.,Donchin, J.H.,Nehring, N.L. and Truesdell,A.H., 1981. Molecular carbon isotopicevidence for the origin of geothermal hydrocarbons. Nature (London), 292: 826-828. Des Marais, D.J.,Stallard,M.L., Nehring, N.L. and Truesdell,A.H., 1988. Carbon isotopegeochemistry of hydrocarbons in the Cerro Prieto geothermal field,Baja California Norte, Mexico. In: M. Schoell (Guest-Editor),Originsof Methane in the Earth. Chem. Geol.,71: 159-167 (this specialissue). Faber, E., 1987. Zur Isotopengeochemie gasfdrrnigerKohlenwasserstoffe.ErdSl, Erdgas, Kohle, 103: 210-218. Galimov, E.M., 1967.Carbon isotopicvariationin methane flowingthrough wet rocks.Geokhimiya, 12: 1504-1505. Galimov, E.M., 1974. Carbon Isotopesin Oil and Gas Geology. N.A.S.A. (Natl. Aeron. Space Adm.) Washington, D.C., 1974 (Translationfrom: "Izotopy ugleroda v neftegazovoy geologii",Nedra, Moscow, 1973 ). Galimov, E.M., 1988. Sources and mechanisms of formation of gaseous hydrocarbons in sedimentary rocks.In: M. Schoell (Guest-Editor),Origins of Methane in the Earth. Chem. Geol.,71:77-95 (thisspecialissue). Gold, T. and Soter, S., 1982. Abiogenic methane and the originof petroleum. Energy Explor. Exploit.,1: I- 19. Jenden, P., 1985. Analysis of gases in the Earth's crust, Rep. Gas IRes. Inst., Chicago, Ill., 110 pp. Jenden, P.D. and Kaplan, I.R., 1986. Comparison of microbial gases from the Middle American Trench and Scripps Submarine Canyon: Implications for the origin of natural gas. Appl. Geochem., 1: 631-646. Jenden, P.D. and Kaplan, I.R., 1988. Origin of natural gas in the Sacramento Basin. Am. Assoc. Pet. Geol. Bull., 72 (in press). Kreulen, R. and Schuiling, R.D., 1982. N2-CH4-C02 fluids during formation of the DSme de l'Agout, France. Geochim. Cosmochim. Acta, 46: 193-203. Lyon, G.L. and Hulston, J.R., 1984. Carbon and hydrogen
isotopic compositions of New Zealand geothermal gases. Geochim. Cosmochim. Acta, 48: 1161-1171. Martens, C.S., Blair, N.E., Green, C.D. and Des Marais, D.J., 1986. Seasonal variations in the stable carbon isotopic signature of biogenic methane in a coastal sediment. Science, 233: 1300-1303. Mattavelli, L., Ricchiuto, T., Grignani, D. and Schoell, M., 1983. Geochemistry and habitat of natural gases in the Po Basin, Northern Italy. Am. Assoc. Pet. Geol. Bull., 67: 2239-2254. Oremland, R.S., Whiticar, M.J., Strohmaier, F.E. and Kiene, R.P., 1988. Bacterial ethane formation from reduced, ethylated sulfur compounds in anoxic sediments. Geochim. Cosmochim. Acta, 52: 1895-1904. Poreda, R.J., Jeffrey, A.W.A., Kaplan, I.R. and Craig, H., 1988. Magmatic helium in subduction-zone natural gases. In: M. Schoell (Guest-Editor), Origins of Methane in the Earth. Chem. Geol., 71:199-210 (this special issue). Ricchiuto, T. and Schoell, M., 1988. Origin of natural gases in the Apulian Basin in South Italy: A case history of mixing of gases of deep and shallow origin. In: L. Novelli and L. Mattavelli (Editors), Advances in Organic Geochemistry 1987. Org. Geochem. (in press). Rice, D.D. and Claypool, G.E., 1981. Generation, accumulation and resource potential of biogenic gas. Am. Assoc. Pet. Geol. Bull., 65: 5-25. Rigby, D. and Smith, J.W., 1981. An isotopic study of gases and hydrocarbons in the Cooper Basin. APEA (Aust. Pet. Explor. Assoc.) J., 21: 222-229. Runge, A., 1980. Kohlenstoff- und Wasserstoffisotopenvariationen in organischen Sedimenten und in Gasen. Chem. Erde, 39: 52-62. Schoell, M., 1980. The hydrogen and carbon isotopic composition of methane from natural gases of various origins. Geochim. Cosmochim. Acta, 44: 649-661. Schoell, M., 1983. Genetic characterization of natural gases. Am. Assoc. Pet. Geol. Bull., 67: 2225-2238. Schoell, M., 1984. Wasserstoff und Kohlenstoffisotope in organischen Substanzen, ErdSlen und Erdgasen. Geol. Jahrb., D67, 161 pp. Schoell, M., Tietze, K. and Schoberth, S.M., 1988. Origin of methane in Lake Kivu (East-Central Africa). In: M. Schoell (Guest-Editor), Origins of Methane in the Earth. Chem. Geol., 71:257-265 (this special issue ). Sherwood, B., Fritz, P., Frape, S.K., Macko, S.A., Weise, S.M. and Welhan, J.A., 1988. Methane occurrences in the Canadian Shield. In: M. Schoell (Guest-Editor), Origins of Methane in the Earth. Chem. Geol., 71: 223236 (this special issue ). Stevens, G.M., 1988. Atmospheric methane. In: M. Schoell (Guest-Editor), Origins of Methane in the Earth. Chem. Geol., 71:11-21 (this special issue). Wahlen, M., Tanaka, N., Henry, R., Yoshimari, T., Fairbanks, R.G., Sheruesh, A. and Broecker, W.S., 1987. 13C, D, and 14C in methane. Eos (Trans. Am. Geophys. Union), 68:1220 (abstract). Wang, W.C., Yung, Y.L., Lacis, A.A., Mo, T. and Hansen,
10 J.E., 1976. Greenhouse effects due to anthropogenic perturbations. Science, 194: 685-690. Welhan, J.A., 1981. Carbon and hydrogen gases in hydrothermal systems: the search for a mantle source. Ph.D. Thesis, University of California, San Diego, Calif., 194 pp. Welhan, J.A., 1988. Origins of methane in hydrothermal systems. In: M. Schoell (Guest-Editor), Origins of Methane in the Earth. Chem. Geol., 71:183-198 (this special issue). Whiticar, M.J., Faber, E. and Schoell, M., 1986. Biogenic
methane formation in marine and freshwater environments: C02 reduction vs. acetate fermentation - Isotope evidence. Geochim. Cosmochim. Acta, 50: 693-709. Woltemate, I., 1982. Isotopische Untersuchungen zur bakteriellen Gasbildung in einem Siisswassersee. Diplomarbeit, Technical University Clausthal, ClausthalZellerfeld, 90 pp. Woltemate, I., Whiticar, M.J. and Schoell, M., 1984. Carbon and hydrogen isotopic composition of bacterial methane in a shallow freshwater lake. Limnol. Oceanogr., 29: 985-992.
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-
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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
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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
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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
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- 30 ~13CCH 4
- 20
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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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