| As a new kind of clean alternative unconventional resource, gas hydrate has been considered to be a solution to the shortage of energy supply. The gas hydrate has been discovered and obtained from the scientific wells in the Qilian Mountain Permafrost, Qinghai Province, China. Compared with polar permafrost regions abroad that discovered the gas hydrate, the accumulation laws and control factors of the gas hydrate in the Qilian Mountain Peramafrost are relatively more complicated that lead to the difficulties in the field geophysical prospecting. The numerical simulation based on the rock physics is an effective way to study the geophysical response laws of the gas hydrate reservoirs. In accordance with the geological conditions of the gas hydrate in the Qilian Mountain Permafrost, the geophysical response laws of the gas hydrate were studied by the numerical simulation methods with different geophysical models in this study.By the numerical simulation on audio magnetotelluric sounding(AMT) response characteristics of different gas hydrate reservoirs with finite element method, it can be seen that the porosity is less than 5%, the saturation of the gas hydrate is higher than 70%, the occurrence scale is less than 50 m, or the bury depth is greater than 500 m, and the AMT could not identify and delineate the favorable gas hydrate reservoir. With the nonlinear conjugate gradient(NLCG) algorithm, the influences of the acquisition parameters on the AMT inversion results were studied, and the results showed that the survey line should be triple the length of the occurrence scale, the stations in the possible occurrence region should be 11, and the frequency points setting at high frequency section(100~1000Hz) should be 4.Basing on the numerical simulation study of the response characteristics of seismic reflection wave amplitude variation with offset(AVO) of the gas hydrate reservoir with different properties by the forward modeling velocity model that is established by the effective medium theory basing on the rock physics and the ray tracing theory, it revealed that the AVO attribute partition changing with the porosity was quite well, the AVO attribute partition changing with the saturation presented continuous transition and could not be distinguished easily, and the AVO attribute partition changing with the thickness of the gas hydrate reservoir was linear. Through the numerical simulation on the sensitivity of the different elastic parameters changing with the saturation and the lithology of the gas hydrate reservoir, we could see that “μ” and “E” were the most sensitive parameters to the gas hydrate reservoir properties, the crossplot of “μ” and “E” could effectively identify the gas hydrate reservoir, and the crossplot of “μ” and “λ/μ” could identify the lithology of the gas hydrate reservoir.By the numerical simulation on the acoustic velocity and the saturation response characteristics with the forward modeling velocity model that is established by the effective medium theory and the two component laminated media model in pore-and fracture-filling gas hydrate reservoirs, it can be concluded that the hole DKXX-13 was the typical pore-filling gas hydrate reservoir, tha average value of the gas hydrate saturation was 61.9%, and the range of the gas hydrate saturation was 13.0-85.0%, which was basically in accordance with the standard Archie equation. The hole DKXX-19 was the typical fracture-filling gas hydrate reservoir, and the range of the gas hydrate saturation was 14.1-89.9%.With the gas hydrate reservoir geophysical response laws in the Qilian Mountain Permafrost, it is greatly significant to the field geophysical exploration and the data processing and interpretation. What’s more, it can provide feasible technical support for the exploration and development of the gas hydrate resources in China. |
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