美国典型富氦无机成因气田中氦气地质特征与聚集机制

Geological characteristics and mechanism of helium accumulation in typical abiotic helium-rich gas fields in the United States

美国广泛发育具有经济效益的富氦无机成因天然气田,如其中富氦氮气田甚至可以含有高达10%的氦。原地和周边地区的基底提供充足的氦源而氮气可来自不同圈层,且通常N_(2)/He(He>0.1%)在5~50之间。但是富氦氮气田在美国独特地质环境之外是否也有发现还需要进一步的研究。富氦二氧化碳气田中的氦主要也来自壳源且产量可观。科罗拉多高原上的富氦二氧化碳气田均被认为是来源于新生代晚期的岩浆活动,且该地区岩浆岩具有较高的U、Th含量。地下水溶气脱气-再溶解(Groundwater Gas Stripping and Re-dissolution,GGS-R)模型被普遍认为可以合理解释CO_(2)气藏中氮气、氦等惰性气体的聚集成藏机制。具体来说,幔源CO_(2)载体气充注时将溶解在地下水中的大气源惰性气体与壳源惰性气体脱出成藏,并与地下水达到水/气溶解平衡。虽然不同气田的平衡值各有不同,但是科罗拉多高原上的各气田均显示出相似的范围值,即在相应的储层压力和温度下为0~100 cm^(3)水/cm^(3)气。本文系统分析美国无机成因富氦气藏的氦气生成、运移和聚集机制,讨论氦气在经历氦源岩内游离相扩散初次运移后通过水溶相、气容相集流或是多相渗流方式进行的二次运移及由无机成因载体气N_(2)和CO_(2)共同参与的富集成藏机制,既可为我国氦气勘查提供理论认识依据,也可为二氧化碳地质评价和开发利用及安全封存提供参考。

Economically viable abiotic helium-rich gas fields are widely developed in the United States.He-rich N_(2) gas fields,for example,contain as much as 10%He,as the basement rocks in situ and in the surrounding area provides sufficient crustal He and N_(2) to yield an N_(2)/He(He>0.1%)ratio typically between 550.However,He-rich N_(2) gas fields so far are only found in the United States,and further research is needed to determine whether they are due to unique geological conditions,or yet to be discovered elsewhere.He-rich CO_(2) gas fields also contain significant quantity of crustal helium.He-rich CO_(2) gas fields in the Colorado Plateau are all thought to have originated from late Cenozoic magmatism,and the magmatic rocks in the region have high U/Th contents.The groundwater gas stripping and re-dissolution(GGS-R)model is commonly used to explain the trapping mechanism of noble gases in CO_(2) gas reservoirs.According to this model,mantle-sourced CO_(2) carrier gas stripes air-sourced noble gases dissolved in the groundwater to charge reservoirs along with crustal-derived noble gases,and dissolution equilibrium subsequently reestablishes between the noble gases and groundwater.While the water/gas ratios at equilibrium differ between different gas fields,they are similar in all gas fields in the Colorado Plateau ranging between 0100(at reservoir pressure and temperature).By systematic analysis and summarization of the source,migration,and accumulation mechanisms of helium in helium-rich abiotic gas reservoirs in the United States,and discuss the enrichment of helium of primary migration by diffusion in helium source rocks,and of secondary migration by water phase mass flow or multiphase porous flow/seepage with inorganic carrier gases N_(2) and CO_(2),this paper provides a theoretical basis for helium exploration in China,and can be used as a reference for geological evaluation of CO_(2) reservoirs and development of CO_(2) safe storage.