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A technique has been developed to determine C12/C13 and D/H isotopic ratios on small quantities of methane through pentane hydrocarbons and has been applied to natural gases from various genetic sources (i.e., early-diagenetic, oil-associated, late-catagenic and mixed-gas sources).
Carbon isotopes measured from 27 natural gases have ^dgr13C range of 23, 15, and 20^pmil for the C1 to C3 alkanes and maximum ^dgr13C values of -35.1, -26.8, and -20.8 ^pmil respectively. With a smaller sample base, butane and pentane vary within 16 and 4 ^pmil, respectively, for those samples with the most positive ^dgr13C = -26.5 and -26.9 ^pmil.
Deuterium isotopes exhibit greater isotopic variation than the corresponding carbon values. D/H variations clearly decrease toward the higher homologues with ^dgrD ranges of 182, 110, 75, 43, and 29 ^pmil for C1 through C5. Most negative D/H measurements also decrease with carbon number from ^dgrD = -311 ^pmil for methane to -128 ^pmil for pentane.
These relative changes in carbon and hydrogen isotopic contents for the higher homologues are useful in the classification of natural gases, particularly those of mixed origin.
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