祁连山木里冻土区天然气水合物矿区稀有气体氦、氖地球化学特征及其指示意义
Geochemical Characteristics and Implications of Helium and Neon in Natural Gas Hydrates Deposits in the Muli Permafrost, Qilian Mountains

摘要： 稀有气体氦(He)、氖(Ne)是天然气水合物中的痕量组分，其化学性质稳定，在地质作用过程中其丰度变化几乎不受复杂化学反应和近地表微生物的影响。顶空气He、Ne地球化学勘查方法可以排除沼泽区微生物的强烈干扰，提高包括天然气水合物在内的油气近地表地球化学勘查的效用及精度。选择祁连山木里冻土区天然气水合物矿区进行试验研究，获得近地表土壤顶空气He平均含量为799×10-6，Ne平均含量208×10-6，均高于其在大气中的丰度。稀有气体He、Ne具有很强的穿透能力，平面上，在已知水合物矿藏和水合物远景区上方具有明显的地球化学顶部异常特征；钻井地球化学垂向剖面上，水合物富集层位上方具有明显的上置气异常特征。顶空气He、Ne近地表平面异常和钻井岩屑剖面异常特征证实，祁连山水合物形成过程中烃类气体发生了分异和垂向微渗漏，其携带笼中的稀有气体He、Ne以“类气相”地气流形式垂向迁移。试验证明，顶空气He、Ne异常对木里天然气水合物矿藏具有良好的指示作用。顶空气He、Ne勘查方法是冻土区水合物地球化学勘查技术的有效补充。
Abstract: The noblegas helium(He)and neon(Ne)are rare components of natural gas hydrates, and their concentration changes are almost unaffected by complex chemical reactions and nearsurface microorganism activities. Discussion of raregas geochemical exploration can not only eliminate the strong interference of microorganisms, but can also improve the intensity of the nearsurface gas geochemical anomalies including natural gas hydrates. In this paper, natural gas hydrates mining area in the Muli permafrost region of Qilian mountains was investigated, where the average headspace He and Ne contents is 799×10-6 and 208×10-6, respectively, both higher than the average atmospheric concentrations. It is found that He and Ne have strong penetrative power, and show clear geochemical anomaly halos above the known hydrate deposits/prospects. Drillinghole profiles show that there is obvious upper gas anomaly above the hydrate accumulation horizon, and the noblegas differentiation and vertical percolation occur during the rare gas hydrate formation in Qilian, while the noblegas He and Ne carry migrates vertically in the form of “gaslike” geogas current. Our study demonstrates that HeNe anomaly has a good indication of the gas hydrate deposits, and HeNe exploration technology is an effective supplement for hydrate exploration in permafrost regions