Study On Field Analysis And Testing Technology Of Gas Hydrate And Application In Permafrost Zone

In recent years, China has made important progress in gas hydrate investigation and research in the Qinghai-Tibet Plateau, the invironment conditions of which is extremely bad, with an average altitude of 4000?5000 m and inconvenient traffic. Sending the collected samples back to the laboratory takes quite a long time. However, the test analysis of the samples is the basis of geochemical studies. Taking into account the decomposition and volatilization of gas hydrate and limited sampling techniques and other factors, it is necessary to establish and develop the test method and apparatus, in-situ, fast and high sensitivity, and to carry out the field analysis and testing of the samples.The experiments on the indoor and field test methods were carried out, relying on the Shimadzu GC-2014 gas chromatograph purchased for the on-site laboratory. Gas characteristics and causes and possible sources were analysised and discussed, using the gas chromatography test results and carbon isotopic values of the headspace samples from the borehole DK-8 in the Qilian Mountain permafrost, and QK-1, QK-2, QK-3 in the Qiangtang Basin. The testing of the samples from DK-8 and QK-1 was done in-house, while the testing of the samples from QK-2 and QK-3 was done on-site. Finally, according to the contents and carbon isotope values of hydrocarbon gases, the characteristics, genetic types and the possible sources of gases were analyzed and discussed.Gas chromatography was first established in the field of Qiangtang Basin, determining the gas content of natural gas hydrate, including CH4, C2H6, C2H4, C3H8, C3H6,i-C4H10, n-C4H10,i-C5H12, n-C5Hi2 and H2, CO2,02, N2. Its detection limits for gaseous components are 10"7?10-6 (mol/mol) with precision of less than 3% RSD (n=6). Practice has proved that the method is of higher sensitivity and precision, which can meet the requirements of field measurement for the samples.The hydrocarbon gases in Qilian Mountain permafrost are mainly pyrolysis gas, and oil-type gas, followed by coal-type gas. Source rocks, oil shale and coal bed are likely to form a large number of hydrocanbon gases, providing adequate material basis for the formation of gas hydrate. Local faults and tectonic fracture zone are the good migration pathways for deep pyrolysis, at the same time, provide a favorable reservoir space for the formation of gas hydrate. The hydrocarbon gas content from the borehole QK-1 in the Qiangtang Basin is lower, dominated by methane, containing relatively high ethane, propane and other heavy hydracarbon components, which are pyrolysis gases. It is estimated that the hydrocarbon gases come from the deeper pyrolysis gas which has migrated upward from the depth, and may be mixed with biogenic gas in the shallow. The low gas content from the borehole QK-2 and QK-3 results in failing to detect methane carbon isotope, inferring the hydrocarbon gases are pyrolysis only on the basis of C1/(C2+C3)<1000.