Gas chromatographic method for determination of gas composition from decomposed gas hydrate samples in Qilian Mountain permafrost

In this paper, gas chromatographic method for determination of C1~C5 gas composition from decomposed gas hydrate samples in Qilian Mountain permafrost was established, and the difference between single- and multi-point correction method, external standard and external standard normalization quantitative method, drainage and head space gas collection method was comparatively studied. The results indicate that the head space collection method, single correction method, and external standard normalization quantitative method are the best means for quantitatively measuring gas composition from the decomposed gas hydrate sample, with the relative standard deviations in the range of 0.10%-0.47%, and detection limits varying from 0.0003% to 0.0127%. Based on the determination of C1~C5 from ten decomposed gas hydrate samples of bohehole DK-2 in Qilian Mountain permafrost, it is found that the content of C1 ranges from 60.64% to 78.76%, C2 from 8.99% to 13.60%, and C3 from 6.58% to 21.24%, with C4~C5 being less abundant. Gas hydrate in Qilian Mountain permafrost is enriched with C2+ and has low C1/C2+ ratios（ranging from 1.5 to 3.7）, showing the characteristics of thermogenic gases obviously.