柴达木盆地东坪气田氦源岩成因及其对天然气藏中氦气的影响

Genesis of helium source rock in Dongping Gas Field,Qaidam Basin and its influence on helium in natural gas reservoirs

柴达木盆地西北缘东坪气田花岗岩在区域地质演化研究及资源开发方面均具有重要价值,因而对东坪花岗岩进行了地球化学特征及锆石U—Pb年代学分析。结果表明,东坪花岗岩可分为2类:一类为高Sr、低Yb的埃达克质花岗岩,具有埃达克质岩的地球化学特征,源区存在石榴石、金红石、角闪石的残留,无或仅有少量斜长石残留,推测可能为加厚下地壳的含金红石榴辉岩部分熔融形成,熔融深度大于50 km;另一类为低Sr、低Yb的喜马拉雅型花岗岩,源区残留相为石榴石、角闪石及斜长石,可能是由加厚下地壳的麻粒岩部分熔融形成,熔融深度大致为40~50 km。LAICP-MS锆石U—Pb定年结果表明,两类花岗岩的结晶年龄在418 Ma左右,是同一次岩浆活动侵位的结果,而且构造判别图解显示,两类花岗岩均属于同碰撞花岗岩。结合前人研究结果,认为418 Ma左右南阿尔金洋已经闭合,转而进入陆内俯冲碰撞造山阶段,造成地壳相互叠置加厚,加厚下地壳由于受到地幔物质的底侵加热,在不同深度发生部分熔融,形成了东坪两类不同类型的花岗岩。东坪及其邻区尖北、牛东气田中氦气的成因及含量与基底花岗岩密切相关,具体表现为:氦气主要是由基底花岗岩及花岗片麻岩中的U、Th等放射性元素衰变产生;基底花岗岩类岩体形成时代越久远,气田中的氦气含量越高;在碰撞造山构造背景下,由纯地壳物质部分熔融形成的花岗岩具有更高的U、Th含量,生氦潜力更强。

Dongping granite in the northwest margin of Qaidam Basin is of great value in regional geological evolution research and resource development.Therefore,the geochemical characteristics and zircon U-Pb chronology of Dongping granite are analyzed.Dongping granite can be divided into two types.The first type is Adakitic granite with high Sr and low Yb,which has the geochemical characteristics of Adakitic rock.There are garnet,rutile and amphibolite residues in the source area,and no or only a small amount of plagioclase residue.It is speculated that the rock may be formed by partial melting of rutile-bearing eclogite in the thickened lower crust,with melting depth greater than 50 km.The other type is Himalayan type granite with low Sr and low Yb,and the residual facies in the source area are garnet,amphibolite and plagioclase,which may be formed by partial melting of granulite in the thickened lower crust,and the melting depth is about 40-50 km.LA-ICP-MS zircon U-Pb dating results show that the crystallization age of the two types of granites is about 418 Ma,which is the result of the same magmatic activity,and the tectonic discrimination diagram shows that both types of granites belong to the same collision granite.Based on previous research results,this paper believes that at about 418 Ma,the“Southern Altyn Ocean”has been closed,and turned to the stage of intracontinental subduction collision orogeny,resulting in the mutual overlap and thickening of the crust.As the thickened lower crust is heated by the underplating of mantle materials,partial melting occurs at different depths,forming two different types of granites in Dongping.The origin and content of helium in Dongping and its adjacent Jianbei and Niudong gas fields are closely related to the basement granite.Specifically,helium is mainly produced by the decay of U,Th and other radioactive elements in the basement granite and granite gneiss.The longer the basement granitoid body is formed,the higher the helium content in the gas field.Under the background of collision orogenic structure,the granite formed by partial melting of pure crustal materials has higher U and Th contents and stronger helium generation potential.