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0.8<br />
15<br />
0.6<br />
10<br />
Ar(%) O2(%)<br />
0.4<br />
0.2<br />
y = 0.0192x - 0.0558<br />
R 2 = 0.9561<br />
5<br />
CO2(%)<br />
0.0<br />
0 5 10 15 20<br />
N 2(%)<br />
0<br />
Ar O2 Air CO2 Ar-N2<br />
Figure 3. Gas composition at Akabira<br />
Hydrocarbon Analyses<br />
Methane and ethane contents in total hydrocarbon in various districts are shown in Figure 4.<br />
Other data in Figure 4, which were referred to in previous papers or derived from personal<br />
communications, suggest that Akabira CBM is mixed with more deeply derived <strong>gas</strong> such as<br />
that from oil and <strong>gas</strong>-field components. Biogenic methane <strong>gas</strong> such as that in the Chiba and<br />
Miyazaki <strong>gas</strong>-fields also typically has low contents of C 2 +, but carbon isotope data do not<br />
support a biogenic origin in this case.<br />
Carbon Isotope Analyses<br />
The 13 C isotope ratio of methane is very good for discriminating between thermogenic and<br />
biogenic origin. Analytical results for the carbon isotopes of Akabira CBM are shown in<br />
Figure 5 together with some previously reported data. It is clear that the Akabira CBM was<br />
derived from mature source rocks and is of thermogenic origin. This result is in concordance<br />
with hydrocarbon analyses (above).<br />
Figure 4. Hydrocarbon composition<br />
Figure 5. Result of 13 C isotope analysis and<br />
orgin of <strong>gas</strong><br />
New Energy Resources in the <strong>CCOP</strong> Region - Gas Hydrates and Coalbed Methane 161
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