CARBON ISOTOPE GEOCHEMISTRY OF NATURAL GASES FROM THE GREEN TUFF BASIN, JAPAN

Carbon isotopic analyses of C1-C4 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 He-3/He-4 ratios have suggested a possible magmatic origin. Delta-C-13 (PDB) values of methane, ethane, propane, and n-butane range from -54.4 to -33.3 parts per thousand, from -32.5 to -20.9 parts per thousand, from -27.7 to -20.6 parts per thousand, and from -25.3 to -20.5 parts per thousand, respectively, all of which fall within the ranges of published data of worldwide gases of biogenie orgin. Most samples show the isotope distribution pattern wherein the delta-C-13 values become less negative with increasing molecular weight of the hydrocarbons, suggesting that the C2+ hydrocarbons were not derived by methane polymerization. The delta-C-13 values of C2-C4 hydrocarbons are explained by thermogenic breakdown of organic matter, whereas the delta-C-13 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.