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Evidence of subsurface anaerobic biodegradation of hydrocarbons and potential secondary methanogenesis in terrestrial mud volcanoes
Author(s)
Language
English
Obiettivo Specifico
4.5. Studi sul degassamento naturale e sui gas petroliferi
Status
Published
JCR Journal
JCR Journal
Peer review journal
Yes
Title of the book
Issue/vol(year)
9/26 (2009)
Publisher
Elsevier Ltd.
Pages (printed)
1692-1703
Issued date
November 2009
Abstract
The assessment of gas origin in mud volcanoes and related petroleum systems must consider postgenetic processes which may alter the original molecular and isotopic composition of reservoir gas.
Beyond eventual molecular and isotopic fractionation due to gas migration and microbial oxidation, investigated in previous studies, we now demonstrate that mud volcanoes can show signals of anaerobic
biodegradation of natural gas and oil in the subsurface. A large set of gas geochemical data from more than 150 terrestrial mud volcanoes worldwide has been examined. Due to the very low amount of C2+ in
mud volcanoes, isotopic ratios of ethane, propane and butane (generally the best tracers of anaerobic biodegradation) are only available in a few cases. However, it is observed that 13C-enriched propane is
always associated with positive б13 CCO2 values, which are known indicators of secondary methanogenesis following anaerobic biodegradation of petroleum. Data from carbon isotopic ratio of CO2 are available for 134 onshore mud volcanoes from 9 countries (Azerbaijan, Georgia, Ukraine, Russia, Turkmenistan, Trinidad, Italy, Japan and Taiwan). Exactly 50% of mud volcanoes, all releasing thermogenic or mixed methane, show at least one sample with б13 CCO2>+5‰ (PDB). Thermogenic CH4 associated with positive carbon isotopic ratio of CO2 generally maintains its б13C-enriched signature, which is therefore not perturbed by the lighter secondary microbial gas. There is, however, high variability in the
б13 CCO2 values within the same mud volcanoes, so that positive б13 CCO2 values can be found in some vents and not in others, or not continuously in the same vent. This can be due to high sensitivity of
б13 CCO2 to gas–water–rock interactions or to the presence of differently biodegraded seepage systems in the same mud volcano. However, finding a positive б13 CCO2 value should be considered highly indicative of anaerobic biodegradation and further analyses should be made, especially if mud volcanoes are to be used as pathfinders of the conditions indicative of subsurface hydrocarbon accumulations in unexplored
areas.
Beyond eventual molecular and isotopic fractionation due to gas migration and microbial oxidation, investigated in previous studies, we now demonstrate that mud volcanoes can show signals of anaerobic
biodegradation of natural gas and oil in the subsurface. A large set of gas geochemical data from more than 150 terrestrial mud volcanoes worldwide has been examined. Due to the very low amount of C2+ in
mud volcanoes, isotopic ratios of ethane, propane and butane (generally the best tracers of anaerobic biodegradation) are only available in a few cases. However, it is observed that 13C-enriched propane is
always associated with positive б13 CCO2 values, which are known indicators of secondary methanogenesis following anaerobic biodegradation of petroleum. Data from carbon isotopic ratio of CO2 are available for 134 onshore mud volcanoes from 9 countries (Azerbaijan, Georgia, Ukraine, Russia, Turkmenistan, Trinidad, Italy, Japan and Taiwan). Exactly 50% of mud volcanoes, all releasing thermogenic or mixed methane, show at least one sample with б13 CCO2>+5‰ (PDB). Thermogenic CH4 associated with positive carbon isotopic ratio of CO2 generally maintains its б13C-enriched signature, which is therefore not perturbed by the lighter secondary microbial gas. There is, however, high variability in the
б13 CCO2 values within the same mud volcanoes, so that positive б13 CCO2 values can be found in some vents and not in others, or not continuously in the same vent. This can be due to high sensitivity of
б13 CCO2 to gas–water–rock interactions or to the presence of differently biodegraded seepage systems in the same mud volcano. However, finding a positive б13 CCO2 value should be considered highly indicative of anaerobic biodegradation and further analyses should be made, especially if mud volcanoes are to be used as pathfinders of the conditions indicative of subsurface hydrocarbon accumulations in unexplored
areas.
References
Abrams, M.A., 2005. Significance of hydrocarbon seepage relative to petroleum
generation and entrapment. Mar. Petrol. Geol. 22, 457–477.
Aitken, C.M., Jones, D.M., Larter, S.L., 2004. Anaerobic hydrocarbon degradation in
deep subsurface reservoirs. Nature 431, 291–294.
Alain, K., Holler, T., Musat, F., Elvert, M., Treude, T., Kruger, M., 2006. Microbiological
investigation of methane- and hydrocarbon-discharging mud volcanoes in the
Carpathian Mountains, Romania. Environ. Microbiol. 8, 574–590.
Ansted, D.T., 1866. On the mud volcanoes of the Crimea, and on the relation of these
and similar phenomena to deposits of petroleum. Proc. R. Inst. G.B. IV, 628–640.
Bernard, B.B., Brooks, J.M., Sackett, W.M., 1978. Light hydrocarbons in recent Texas
continental shelf and slope sediments. J. Geophys. Res. 83, 4053–4061.
Berner, U., Faber, E., 1996. Empirical carbon isotope/maturity relationships for gases
from algal kerogens and terrigenous organic matter, based on dry, open-system
pyrolysis. Org. Geochem. 24, 947–955.
Capozzi, R., Picotti, V., 2002. Fluid migration and origin of a mud volcano in the
Northern Apennines (Italy): the role of deeply rooted normal faults. Terranova
14, 363–370.
Chu, P.H., Wu, J.J., Cheng, T.W., Lin, L.H., Chen, K.C., Wang, P.L., 2007. Evaluating the
potential of anaerobic methane oxidation in terrestrial mud volcanoes of
southern Taiwan. In: Proceedings of the 9th International Conference on Gas
Geochemistry. National Taiwan University, p. 154.
Ciocardel, R., 1949. Regiunea petrolifera Berca – Beciu – Arbanasi. Inst. Geol., St.
Tehn. Ec., A 4, 1–32.
Clayton, C.J., Hay, S.J., Baylis, S.A., Dipper, B., 1997. Alteration of natural gas during
leakage from a North Sea salt diapir field. Mar. Geol. 137, 69–80.
Coleman, D.D., Risatti, J.B., Schoell, M., 1981. Fractionation of carbon and hydrogen
isotopes by methane-oxidizing bacteria. Geochim. Cosmochim. Acta 45 (7),
1033–1037.
Deville, E., Battani, A., Griboulard, R., Guerlais, S.H., Herbin, J.P., Houzay, J.P.,
Muller, C., Prinzhofer, A., 2003. Mud volcanism origin and processes. New
insights from Trinidad and the Barbados Prism. In: Van Rensbergen, P.,
Hillis, R.R., Maltman, A.J., Morley, C. (Eds.), Surface Sediment Mobilization.
Special Publication of the Geological Society (London), vol. 216, pp. 475–490.
Dimitrakopoulos, R., Muehlenbachs, K., 1987. Biodegradation of petroleum as
a source of 13C enriched carbon dioxide in the formation of carbonate cements.
Chem. Geol. 65, 283–291.
Etiope, G., Feyzullayev, A., Baciu, C.L., 2009. Terrestrial methane seeps and
mud volcanoes: a global perspective of gas origin. Mar. Petrol. Geol. 26,
333–344.
Etiope, G., Feyzullayev, A., Baciu, C.L., Milkov, A.V., 2004. Methane emission from
mud volcanoes in eastern Azerbaijan. Geology 32, 465–468.
Etiope, G., Martinelli, G., Caracausi, A., Italiano, F., 2007. Methane seeps and mud
volcanoes in Italy: gas origin, fractionation and emission to the atmosphere.
Geophys. Res. Lett. 34, doi:10.1029/2007GL030341.
Favara, R., Gioia, C., Grassa, F., Inguaggiato, S., Proietto, F., Valenza, M., 2001. Studio
geochimico delle manifestazioni fluide della riserva naturale integrale ‘‘Macalube
di Aragona’’. Nat. Sicil. 25, 137–154 (in Italian).
Feyzullayev, A., Movsumova, U., 2001. About the origin of isotopically heavy CO2 in
gases of Azerbaijan mud volcanoes. Azerb. Geol. 6, 96–105 (in Russian).
GIA–BP–STATOIL, 1994. Mud Volcanoes of Azerbaijan. GIA–BP–STATOIL. Report of
the GIA, BP and Statoil Joint study. EXT 67370.
Grassa, F., Capasso, G., Favara, R., Ingaggiato, S., Faber, E., Valenza, M., 2004.
Molecular and isotopic composition of free hydrocarbon gases from Sicily, Italy.
Geophys. Res. Lett. 31, L06607, doi:10.1029/2003GL019362.
Guliyev, I.S., Feyzullayev, A., 1997. All About Mud Volcanoes. Baku Publishing House,
Nafta-Press, 120 pp.
Guliyev, I.S., Feyzullayev, A., Aliyev, A.A., Movsumova, U., 2005. Composition of
gases and organic matter of rock ejecta of mud volcanoes in Azerbaijan. Geologiya
nefti i gaza 3, 27–30 (in Russian).
1702 G. Etiope et al. / Marine and Petroleum Geology 26 (2009) 1692–1703
Head, I.M., Jones, D.M., Larter, S.R., 2003. Biological activity in the deep subsurface
and the origin of heavy oil. Nature 426, 344–352.
Hong, W.L., Yang, T.F., 2007. Methane flux from accretionary prism through mud
volcano area in Taiwan – from present to the past. In: Proceedings of the 9th
International Conference on Gas Geochemistry. National Taiwan University, pp.
80–81.
James, A.T., Burns, B.J., 1984. Microbial alteration of subsurface natural gas accumulations.
Am. Assoc. Pet. Geol. Bull. 68, 957–960.
Jeffrey, A.W.A., Alimi, H.M., Jenden, P.D., 1991. Geochemistry of the Los Angeles
Basin oil and gas systems. In: Biddle, K.T. (Ed.), Active Margin Basins. American
Association of Petroleum Geologists Memoir, vol. 52, pp. 197–219.
Jenden, P.D., Hilton, D.R., Kaplan, I.R., Craig, H., 1993. Abiogenic hydrocarbons and
mantle helium in oil and gas fields. In: Howell, D. (Ed.), Future of Energy Gases,
vol. 1570. USGS, pp. 31–35. Professional Paper.
Katz, B., Narimanov, A., Huseinzadeh, R., 2002. Significance of microbial processes in
gases of the South Caspian basin. Mar. Petrol. Geol. 19, 783–796.
Larter, S., di Primio, R., 2005. Effects of biodegradation on oil and gas field PVT
properties and the origin of oil rimmed gas accumulations. Org. Geochem. 36,
299–310.
Lavrushin, V.Y., Polyak, B.G., Prasolov, E.M., Kamenskii, I.L., 1996. Sources of material
in mud volcano products (based on isotope, hydrochemical, and geological
data). Litol. Polezn. Iskop. 6, 625–647.
Link, W.K., 1952. Significance of oil and gas seeps in world oil exploration. Am.
Assoc. Pet. Geol. Bull. 36, 1505–1540.
Martinis, B., 1962. Manifestazioni petrolifere. In: Colombo, C. (Ed.), Enciclopedia del
petrolio e del gas naturale. Roma, pp. 1251–1265.
Milkov, A.V., 2005. Global distribution of mud volcanoes and their significance in
petroleum exploration, as a source of methane in the atmosphere and hydrosphere,
and as geohazard. In: Martinelli, G., Panahi, B. (Eds.), Mud Volcanoes,
Geodynamics and Seismicity. NATO Science Series, IV Earth and Environmental
Sciences, vol. 51, Springer, pp. 29–34.
Milkov, A.V., Dzou, L., 2007. Geochemical evidence of secondary microbial methane
from very slight biodegradation of undersaturated oils in a deep hot reservoir.
Geology 35, 455–458.
Mizobe, K., Waseda, A., Tanaka, K., 2007. Geochemical characteristics of natural
gases from mud volcanoes in Tokamachi City, Niigata Prefecture. In: Japan
Geoscience Union Meeting (abstract Y239-P011).
Niemann, H., Duarte, J., Hensen, C., Omoregie, E., Magalhaes, V.H., Elvert, M.,
Pinheiro, L.M., Kopf, A., Boetius, A., 2006. Microbial methane turnover at mud
volcanoes of the Gulf of Cadiz. Geochim. Cosmochim. Acta 70, 5336–5355.
Pallasser, R.J., 2000. Recognising biodegradation in gas/oil accumulations through
the d13C compositions of gas components. Org. Geochem. 31, 1363–1373.
Pan, C., Yu, L., Liu, J., Fu, J., 2006. Chemical and carbon isotopic fractionations of
gaseous hydrocarbons during abiogenic oxidation. Earth Planet Sci. Lett. 246,
70–89.
Prinzhofer, A., Battani, A., 2003. Gas isotopes tracing: an important tool for
hydrocarbon exploration. Oil Gas Sci. Technol., Rev. IFP 58 (2), 299–311.
Rhakmanov, R.R., 1987. Mud Volcanoes and Their Importance in Forecasting of
Subsurface Petroleum Potential. Nedra, Moscow (in Russian).
Schoell, M., 1983. Genetic characterization of natural gases. Am. Assoc. Pet. Geol.
Bull. 67, 2225–2238.
Seewald, J.S., 2003. Organic–inorganic interactions in petroleum producing sedimentary
basins. Nature 426, 327–333.
Stadnitskaia, A., Ivanov, M.K., Poludetkina, E.N., Kreulen, R., van Weering, T.C.E.,
2008. Sources of hydrocarbon gases in mud volcanoes from the Sorokin Trough,
NE Black Sea, based on molecular and carbon isotopic compositions. Mar. Petrol.
Geol. 25, 1040–1057, doi:10.1016/j.marpetgeo.2007.08.001.
Valyaev, B.M., Grinchenko, Y.I., Erokhin, V.E., Prokhorov, V.S., Titkov, G.A., 1985.
Isotopic composition of gases from mud volcanoes. Lithol. Miner. Resour. 20,
62–75.
Wang, W.C., Zhang, L.Y., Liu, W.H., Kang, Y., Ren, J.H., 2005. Effects of biodegradation
on the carbon isotopic composition of natural gas – a case study in the
Bamianhe oil field of the Jiyang Depression, Eastern China. Geochem. J. 39, 301–
309.
Waseda, A., Iwano, H., 2008. Characterization of natural gases in Japan based on
molecular and carbon isotope compositions. Geofluids 8, 286–292.
Whiticar, M.J., 1999. Carbon and hydrogen isotope systematics of bacterial formation
and oxidation of methane. Chem. Geol. 161, 291–314.
generation and entrapment. Mar. Petrol. Geol. 22, 457–477.
Aitken, C.M., Jones, D.M., Larter, S.L., 2004. Anaerobic hydrocarbon degradation in
deep subsurface reservoirs. Nature 431, 291–294.
Alain, K., Holler, T., Musat, F., Elvert, M., Treude, T., Kruger, M., 2006. Microbiological
investigation of methane- and hydrocarbon-discharging mud volcanoes in the
Carpathian Mountains, Romania. Environ. Microbiol. 8, 574–590.
Ansted, D.T., 1866. On the mud volcanoes of the Crimea, and on the relation of these
and similar phenomena to deposits of petroleum. Proc. R. Inst. G.B. IV, 628–640.
Bernard, B.B., Brooks, J.M., Sackett, W.M., 1978. Light hydrocarbons in recent Texas
continental shelf and slope sediments. J. Geophys. Res. 83, 4053–4061.
Berner, U., Faber, E., 1996. Empirical carbon isotope/maturity relationships for gases
from algal kerogens and terrigenous organic matter, based on dry, open-system
pyrolysis. Org. Geochem. 24, 947–955.
Capozzi, R., Picotti, V., 2002. Fluid migration and origin of a mud volcano in the
Northern Apennines (Italy): the role of deeply rooted normal faults. Terranova
14, 363–370.
Chu, P.H., Wu, J.J., Cheng, T.W., Lin, L.H., Chen, K.C., Wang, P.L., 2007. Evaluating the
potential of anaerobic methane oxidation in terrestrial mud volcanoes of
southern Taiwan. In: Proceedings of the 9th International Conference on Gas
Geochemistry. National Taiwan University, p. 154.
Ciocardel, R., 1949. Regiunea petrolifera Berca – Beciu – Arbanasi. Inst. Geol., St.
Tehn. Ec., A 4, 1–32.
Clayton, C.J., Hay, S.J., Baylis, S.A., Dipper, B., 1997. Alteration of natural gas during
leakage from a North Sea salt diapir field. Mar. Geol. 137, 69–80.
Coleman, D.D., Risatti, J.B., Schoell, M., 1981. Fractionation of carbon and hydrogen
isotopes by methane-oxidizing bacteria. Geochim. Cosmochim. Acta 45 (7),
1033–1037.
Deville, E., Battani, A., Griboulard, R., Guerlais, S.H., Herbin, J.P., Houzay, J.P.,
Muller, C., Prinzhofer, A., 2003. Mud volcanism origin and processes. New
insights from Trinidad and the Barbados Prism. In: Van Rensbergen, P.,
Hillis, R.R., Maltman, A.J., Morley, C. (Eds.), Surface Sediment Mobilization.
Special Publication of the Geological Society (London), vol. 216, pp. 475–490.
Dimitrakopoulos, R., Muehlenbachs, K., 1987. Biodegradation of petroleum as
a source of 13C enriched carbon dioxide in the formation of carbonate cements.
Chem. Geol. 65, 283–291.
Etiope, G., Feyzullayev, A., Baciu, C.L., 2009. Terrestrial methane seeps and
mud volcanoes: a global perspective of gas origin. Mar. Petrol. Geol. 26,
333–344.
Etiope, G., Feyzullayev, A., Baciu, C.L., Milkov, A.V., 2004. Methane emission from
mud volcanoes in eastern Azerbaijan. Geology 32, 465–468.
Etiope, G., Martinelli, G., Caracausi, A., Italiano, F., 2007. Methane seeps and mud
volcanoes in Italy: gas origin, fractionation and emission to the atmosphere.
Geophys. Res. Lett. 34, doi:10.1029/2007GL030341.
Favara, R., Gioia, C., Grassa, F., Inguaggiato, S., Proietto, F., Valenza, M., 2001. Studio
geochimico delle manifestazioni fluide della riserva naturale integrale ‘‘Macalube
di Aragona’’. Nat. Sicil. 25, 137–154 (in Italian).
Feyzullayev, A., Movsumova, U., 2001. About the origin of isotopically heavy CO2 in
gases of Azerbaijan mud volcanoes. Azerb. Geol. 6, 96–105 (in Russian).
GIA–BP–STATOIL, 1994. Mud Volcanoes of Azerbaijan. GIA–BP–STATOIL. Report of
the GIA, BP and Statoil Joint study. EXT 67370.
Grassa, F., Capasso, G., Favara, R., Ingaggiato, S., Faber, E., Valenza, M., 2004.
Molecular and isotopic composition of free hydrocarbon gases from Sicily, Italy.
Geophys. Res. Lett. 31, L06607, doi:10.1029/2003GL019362.
Guliyev, I.S., Feyzullayev, A., 1997. All About Mud Volcanoes. Baku Publishing House,
Nafta-Press, 120 pp.
Guliyev, I.S., Feyzullayev, A., Aliyev, A.A., Movsumova, U., 2005. Composition of
gases and organic matter of rock ejecta of mud volcanoes in Azerbaijan. Geologiya
nefti i gaza 3, 27–30 (in Russian).
1702 G. Etiope et al. / Marine and Petroleum Geology 26 (2009) 1692–1703
Head, I.M., Jones, D.M., Larter, S.R., 2003. Biological activity in the deep subsurface
and the origin of heavy oil. Nature 426, 344–352.
Hong, W.L., Yang, T.F., 2007. Methane flux from accretionary prism through mud
volcano area in Taiwan – from present to the past. In: Proceedings of the 9th
International Conference on Gas Geochemistry. National Taiwan University, pp.
80–81.
James, A.T., Burns, B.J., 1984. Microbial alteration of subsurface natural gas accumulations.
Am. Assoc. Pet. Geol. Bull. 68, 957–960.
Jeffrey, A.W.A., Alimi, H.M., Jenden, P.D., 1991. Geochemistry of the Los Angeles
Basin oil and gas systems. In: Biddle, K.T. (Ed.), Active Margin Basins. American
Association of Petroleum Geologists Memoir, vol. 52, pp. 197–219.
Jenden, P.D., Hilton, D.R., Kaplan, I.R., Craig, H., 1993. Abiogenic hydrocarbons and
mantle helium in oil and gas fields. In: Howell, D. (Ed.), Future of Energy Gases,
vol. 1570. USGS, pp. 31–35. Professional Paper.
Katz, B., Narimanov, A., Huseinzadeh, R., 2002. Significance of microbial processes in
gases of the South Caspian basin. Mar. Petrol. Geol. 19, 783–796.
Larter, S., di Primio, R., 2005. Effects of biodegradation on oil and gas field PVT
properties and the origin of oil rimmed gas accumulations. Org. Geochem. 36,
299–310.
Lavrushin, V.Y., Polyak, B.G., Prasolov, E.M., Kamenskii, I.L., 1996. Sources of material
in mud volcano products (based on isotope, hydrochemical, and geological
data). Litol. Polezn. Iskop. 6, 625–647.
Link, W.K., 1952. Significance of oil and gas seeps in world oil exploration. Am.
Assoc. Pet. Geol. Bull. 36, 1505–1540.
Martinis, B., 1962. Manifestazioni petrolifere. In: Colombo, C. (Ed.), Enciclopedia del
petrolio e del gas naturale. Roma, pp. 1251–1265.
Milkov, A.V., 2005. Global distribution of mud volcanoes and their significance in
petroleum exploration, as a source of methane in the atmosphere and hydrosphere,
and as geohazard. In: Martinelli, G., Panahi, B. (Eds.), Mud Volcanoes,
Geodynamics and Seismicity. NATO Science Series, IV Earth and Environmental
Sciences, vol. 51, Springer, pp. 29–34.
Milkov, A.V., Dzou, L., 2007. Geochemical evidence of secondary microbial methane
from very slight biodegradation of undersaturated oils in a deep hot reservoir.
Geology 35, 455–458.
Mizobe, K., Waseda, A., Tanaka, K., 2007. Geochemical characteristics of natural
gases from mud volcanoes in Tokamachi City, Niigata Prefecture. In: Japan
Geoscience Union Meeting (abstract Y239-P011).
Niemann, H., Duarte, J., Hensen, C., Omoregie, E., Magalhaes, V.H., Elvert, M.,
Pinheiro, L.M., Kopf, A., Boetius, A., 2006. Microbial methane turnover at mud
volcanoes of the Gulf of Cadiz. Geochim. Cosmochim. Acta 70, 5336–5355.
Pallasser, R.J., 2000. Recognising biodegradation in gas/oil accumulations through
the d13C compositions of gas components. Org. Geochem. 31, 1363–1373.
Pan, C., Yu, L., Liu, J., Fu, J., 2006. Chemical and carbon isotopic fractionations of
gaseous hydrocarbons during abiogenic oxidation. Earth Planet Sci. Lett. 246,
70–89.
Prinzhofer, A., Battani, A., 2003. Gas isotopes tracing: an important tool for
hydrocarbon exploration. Oil Gas Sci. Technol., Rev. IFP 58 (2), 299–311.
Rhakmanov, R.R., 1987. Mud Volcanoes and Their Importance in Forecasting of
Subsurface Petroleum Potential. Nedra, Moscow (in Russian).
Schoell, M., 1983. Genetic characterization of natural gases. Am. Assoc. Pet. Geol.
Bull. 67, 2225–2238.
Seewald, J.S., 2003. Organic–inorganic interactions in petroleum producing sedimentary
basins. Nature 426, 327–333.
Stadnitskaia, A., Ivanov, M.K., Poludetkina, E.N., Kreulen, R., van Weering, T.C.E.,
2008. Sources of hydrocarbon gases in mud volcanoes from the Sorokin Trough,
NE Black Sea, based on molecular and carbon isotopic compositions. Mar. Petrol.
Geol. 25, 1040–1057, doi:10.1016/j.marpetgeo.2007.08.001.
Valyaev, B.M., Grinchenko, Y.I., Erokhin, V.E., Prokhorov, V.S., Titkov, G.A., 1985.
Isotopic composition of gases from mud volcanoes. Lithol. Miner. Resour. 20,
62–75.
Wang, W.C., Zhang, L.Y., Liu, W.H., Kang, Y., Ren, J.H., 2005. Effects of biodegradation
on the carbon isotopic composition of natural gas – a case study in the
Bamianhe oil field of the Jiyang Depression, Eastern China. Geochem. J. 39, 301–
309.
Waseda, A., Iwano, H., 2008. Characterization of natural gases in Japan based on
molecular and carbon isotope compositions. Geofluids 8, 286–292.
Whiticar, M.J., 1999. Carbon and hydrogen isotope systematics of bacterial formation
and oxidation of methane. Chem. Geol. 161, 291–314.
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