Carbon isotope geochemistry of natural gases from the Green Tuff basin, Japan
- Geological Survey of Japan, Ibaraki (Japan)
Carbon isotopic analyses of C{sub 1}-C{sub 4} hydrocarbons were carried out to investigate the origin and formation mechanism of natural gases commercially produced from volcanic rocks as well as sedimentary rocks in the Green Tuff basin of Northeast Japan, whose high {sup 3}He/{sup 4}He ratios have suggested a possible magmatic origin. {delta}{sup 13}C (PDB) values of methane, ethane, propane, and n-butane range from {minus}54.4 to {minus}33.3{per thousand}, from {minus}32.5 to {minus}20.9{per thousand}, from {minus}27.7 to{minus}20.6{per thousand}, and from {minus}25.3 to {minus}20.5{per thousand}, respectively, all of which fall within the ranges of published data of worldwide gases of biogenie origin. Most samples show the isotope distribution pattern wherein the {delta}{sup 13}C values become less negative with increasing molecular weight of the hydrocarbons, suggesting that the C{sub 2{sup +}} hydrocarbons were not derived by methane polymerization. The {delta}{sup 13}C values of C{sub 2}-C{sub 4} hydrocarbons are explained by thermogenic breakdown of organic matter, whereas the {delta}{sup 13}C values of methane are more or less lower than those expected from the same origin. These results indicate that the gases from the Green Tuff Basin consist of thermogenic hydrocarbons plus variable amounts of isotopically lighter microbial methane. Significant contributions of abiogenic methane, which would be isotopically heavier than the thermogenic methane, are not plausible for the gases.
- OSTI ID:
- 6082648
- Journal Information:
- Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 55:5; ISSN GCACA; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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58 GEOSCIENCES
580000 -- Geosciences
ALKANES
ASIA
BUTANE
CARBON ISOTOPES
CHEMICAL COMPOSITION
CHEMISTRY
DEVELOPED COUNTRIES
ENERGY SOURCES
ETHANE
FLUIDS
FOSSIL FUELS
FUEL GAS
FUELS
GAS FUELS
GASES
GEOCHEMISTRY
GEOLOGIC DEPOSITS
HYDROCARBONS
IGNEOUS ROCKS
ISOTOPES
JAPAN
METHANE
MINERAL RESOURCES
NATURAL GAS
NATURAL GAS DEPOSITS
ORGANIC COMPOUNDS
ORIGIN
PROPANE
RESOURCES
ROCKS
VOLCANIC ROCKS