壳源富氦天然气藏成藏模式及关键条件

Accumulation models and key conditions of crustal-derived helium-rich gas reservoirs

近年来,氦气资源勘探与研究工作在中国得到了高度重视,但有关壳源富氦天然气藏的成藏条件与成藏机理的研究仍较薄弱。氦气特殊的物理化学性质以及氦源条件等决定了其从生成、释放、运移、聚集到保存均具有显著的独特性。壳源氦气主要是由岩石中铀、钍元素的放射性衰变而生成的,从生氦矿物中释放后,大部分溶解到地下流体中,进而以水溶态或气溶态进行运移。根据氦气的运聚过程以及载体的特征,氦气的聚集模式主要可归纳为3种:①地下水脱氦聚集模式;②独立气相“抽吸”聚集模式;③混合流体脱气聚集模式。通过分析和总结国内外代表性富氦天然气藏的地质特征,提出了连续成藏和幕式成藏2种壳源氦气的成藏模式。其中,美国潘汉德—胡果顿气田中的氦气具有连续成藏的特征,氦气充注时间长,主要通过地下水脱氦聚集到早先形成的烃类气藏中;坦桑尼亚Rukwa盆地、中国四川盆地威远气田以及塔里木盆地和田河气田则表现出幕式成藏特征,富氦天然气藏的形成主要受控于构造活动,且氦气与载气可能为同时成藏。基于系统总结提出了壳源富氦天然气藏的3个成藏关键条件:①稳定古老的基底,充足的氦源;②促进释放的热作用与构造活动,推动运聚的流体介质与构造活动;③早期存在的或与氦气同时成藏的载体气藏。

Recently,the exploration and research of helium resources have received much attention in China,but few researches about the accumulation conditions and mechanisms of crustal-derived helium-rich gas reservoirs have been reported.The physical and chemical properties and source conditions of helium have determined its strong uniqueness in generation,release,migration,accumulation and preservation stages.Helium is generated by radioactive decay of uranium and thorium in ancient rocks.Most of helium dissolves in subsurface fluids after releasing from the helium generating minerals,and then migrates as water-soluble or gas-soluble phase.Based on the migration and accumulation processes of helium and the characteristics of carrier fluids,three main accumulation models of helium were classified:(1)helium degases from groundwater,(2)independent gas phase extract helium,(3)gases degas from mixed fluids.Analyses and summaries of the accumulation characteristics of representative helium-rich gas reservoirs worldwide showed that two types of helium accumulation processes were developed,i.e.,episodic and continuous accumulations.The Panhandle-Huguton Gas Field in the United States has the characteristics of continuous accumulation.Helium was mainly dissolved and accumulated into the early hydrocarbon gas reservoirs over a long period of time.Whereas,the Rukwa Basin in Tanzania,the Weiyuan Gas Field in the Sichuan Basin and the Hetianhe Gas Field in the Tarim Basin show the characteristics of episodic accumulation.The formation of these helium-rich gas reservoirs were mainly controlled by tectonic activities.Helium and carrier gases simultaneously charged into these reservoir.Moreover,three key accumulation conditions for crustal-derived helium-rich natural gas reservoirs were proposed:(1)stable ancient basements and sufficient helium sources;(2)thermal events and tectonic activities that promote helium releasing,carrier fluids and tectonic activities that facilitate helium migration and accumulation;(3)carrier gas reservoirs that exist previously or formed together with helium.