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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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The gases of Candela, south Italy, have been investigated earlier and have been considered as an example of natural gas migration. In a new study, we have performed hydrocarbon and carbon isotope analyses on gases as well as GC-analyses up to the pentanes.
The gases range from -62 to -42^pmil in the 13C/12C ratios and from -200 to -170% in their D/H ratios of the methane. The ^dgr13C and ^dgrD values are linearly correlated when cross plotted in a ^dgr13C/^dgrD diagram. This relationship is strong evidence for mixing of two gases: (1) bacterial gas and (2) thermogenic gas. The compositional changes in the gases also follow mixing relationships.
These data are evidence that variations in the isotopic composition of gases are not necessarily the result of gas migration. Simple mixing of two sources in various proportions are likely to account for much of the observed variation in natural gases.
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