国内外典型富氦气藏稀有气体地球化学特征及对氦气成藏过程的示踪意义

Geochemical characteristics of noble gases in typical helium-rich gas reservoirs and the significance for tracing helium enrichment process

氦气是重要的战略性资源,在航空航天、医疗成像和高新制造业发挥着不可替代的作用。稀有气体具有壳源、幔源和大气源三大来源,且在不同储集库中同位素特征差别大,可有效揭示壳—幔演化以及各圈层相互作用,也为示踪弱源氦气成藏过程提供重要手段。研究统计了中国各大盆地的含氦特征,并进一步对美国Hugoton-Panhandle气田、国内柴北缘地区油气田、坦桑尼亚裂谷带温泉区和国内渭河盆地地热田等典型富氦气藏中稀有气体同位素特征进行了归纳性研究。结果表明,国内多个含油气盆地均发现有富氦气藏,国内外目前具有工业开采价值的氦气藏中3He/4He值基本显示壳源特征,即由铀、钍等氦源元素通过α放射性衰变生成。不同气藏中20Ne/36Ar值及20Ne和36Ar等同位素含量特征差异较大,代表不同富氦气藏形成时所经历的油—气—水等多相平衡过程不尽相同,氦气的富集与地下水及气藏主要气体组分的运移密切相关。壳源氦气藏中4He与20Ne之间常呈正相关关系,表明这些气藏中4He富集可能与地下水密切相关,氦气应先溶于地下水再脱溶进入油气藏。幔源CO_(2)富氦气藏通常规模较小,氦气富集过程可能是因为CO_(2)易溶解于地下水或矿化形成碳酸盐矿物,使得气藏中CO_(2)减少,氦气相对丰度增加。进一步提出氦气成藏的四大有利条件:古老克拉通及富铀钍花岗岩提供充足的氦气来源、发育新构造活动、存在地下水系统和游离气相以及气藏中主要气体组分相对于氦气补给量适中。

Helium is an important strategic resource that plays an irreplaceable role in aerospace,medical imaging and high-tech manufacturing.The noble gases have three major sources:crust,mantle and atmosphere,and their isotopic characteristics vary greatly among reservoirs,which can effectively reveal the crust-mantle evolution and the interactions between the various layers,and provide an important tool for tracing the enrichment process of"weak source"helium.Here it summarized the studies on the noble gas isotopic characteristics in typical helium-rich areas such as Hugoton-Panhandle in the U.S.and the oil and gas fields in the northern margin of Qaidam Basin,China,as well as the hot springs in the Tanzanian rift zone and the geothermal gas in the Weihe Basin in China.The results show that the helium-rich reservoirs were discovered in petroliferous basins in China and most of the 3He/4He ratios that are currently valuable for industrial exploitation in the world show a crustal source,i.e.,they are generated through the alpha radioactive decay of helium source elements such as uranium and thorium.In addition,the 20Ne/36Ar ratios and isotopic concentrations of 20Ne and 36Ar in different reservoirs are quite different,suggesting that the heliumrich reservoirs have undergone different oil-gas-water equilibrium processes,and helium enrichment is closely related to the transportation of the groundwater and main gas components.The positive correlations between 4He and 20Ne in hydrocarbons and N2 gas reservoirs rich in crustderivedhelium indicate that the enrichment of 4He in gas reservoirs may be closely related togroundwater, and that helium should be dissolved in groundwater before being exsolved into thegas reservoirs. Besides, mantle-derived CO_(2) reservoirs are usually small in size and the process ofhelium enrichment may be due to the nature that CO_(2) can be easily dissolved in groundwater or bemineralized into carbonate minerals, resulting in the decrease of CO_(2) concentration and the increaseof the relative abundance of helium. In the end, four favorable conditions for helium reservoirformation were further put forward: sufficient helium sources from ancient cratons and U-T-richgranites, development of new tectonic activities, existence of groundwater systems and free gasphases, and moderate recharge of the main gas components in the reservoir relative to helium gas.