中扬子宜昌地区下寒武统水井沱组页岩现场解吸气特征及地质意义

Characteristics and geological significance of canister desorption gas from the Lower Cambrian Shuijingtuo Formation shale in Yichang area,Middle Yangtze region

中扬子宜昌地区下寒武统水井沱组页岩具有良好的气体显示,是四川盆地外页岩气新的勘探区。通过对2口页岩气探井共64块水井沱组页岩的含气性现场解吸,测定了解吸气含量、气体组分、解吸气碳和氢稳定同位素组成,分析了解吸气组分变化、解吸过程中气态烃和二氧化碳同位素的变化,同时探讨了页岩气赋存状态、气体稳定同位素倒转特点及其地质意义。研究结果表明:水井沱组页岩现场解吸气含量为0.32~5.48 m3/t,连续含气量大于2 m3/t的地层厚44.05 m,含气性与TOC有很强的正相关性;解吸气甲烷含量为81.90%~95.48%,乙烷含量为0.78%~3.95%,含微量丙烷,为典型的干气;非烃气体中氮气含量稍高,平均约为6.7%,二氧化碳含量低于1%,不含H2S;吸附气占50%~60%,游离气占40%~50%。解吸早期吸附性弱的CH4和N2先脱附出来,吸附性强的C2H6和CO2后脱附出来,至解吸结束仍有相当量的C2H6和CO2未脱附出来。解吸过程中碳、氢同位素均发生变化,δ13CCH4变化范围为-39.92‰^-25.86‰,δ13CC2H6为-41.57‰^-39.34‰,δ13CC3H8为-40.89‰^-35.46‰,δ13CCO2为-23.42‰^-19.23‰;δDCH4为-136.90‰^-128.00‰,δDC2H6为-160.45‰^-155.30‰;由于同位素的质量分馏效应,解吸过程中残留的甲烷碳同位素增大了5.15‰~13.33‰,甲烷氢同位素增大1.64‰~8.90‰,乙烷碳、氢同位素和二氧化碳的碳同位素基本不变。气体碳同位素分馏还受页岩物性的影响,大的孔隙体积引起更显著的甲烷碳同位素分馏效果,同时还引起乙烷体积含量的差异。利用解吸半量体积所取气样的同位素值代表全部气体的平均值,δ13CCH4平均值为-33.19‰,δ13CC2H6平均值为-40.04‰,δ13CC3H8平均值为-39.07‰,页岩气表现出δ13CCH4>δ13CC2H6且δ13CC3H8>δ13CC2H6、δDCH4>δDC2H6的同位素'倒序'特征。与威远地区下寒武统筇竹寺组类似,宜昌地区水井沱组页岩气同样处于气态烃同位素反转阶段的早期,具有多源复合热成因气的特点。

As displaying good gas reserves,the Lower Cambrian Shuijingtuo Formation shale in Yichang area,Middle Yangtze region is a new exploration area of shale gas in Sichuan Basin.Based on the onsite analysis of gas-bearing property for 64 pieces of Shuijingtuo Formation gas shale from two shale gas exploration wells,this paper determines the analytical gas content,gas composition,carbon and hydrogen stable isotopic composition of analytical gas,analyzes the changes in the analytical gas composition and the gaseous hydrocarbon and carbon dioxide isotope during analytical process,and explores the occurrence state of shale gas,inversion characteristics and geological significance of gas stable isotope.The results show that Shuijingtuo Formation shale contains the analytical gas of 0.32-5.48 m3/t,and the formation with continuous gas content greater than 2 m3/t is 44.05 m thick,demonstrating a strong positive correlation between gas content and TOC.The analytical gas contains methane and ethane of81.90%-95.48%and 0.78%-3.95%,respectively and also a trace of propane,classified as a typical dry gas;the content of nitrogen is a bit high in non-hydrocarbon gases,with an average of about 6.7%,and the carbon dioxide content is less than1%,free of H2 S.The adsorption gas and free gas accounts for 50%-60%and 40%-50%respectively.In the early stage of adsorption,CH4 and N2,with weak adsorbability,are first desorbed,followed by C2H6 and CO2,with strong adsorbability;until the end of gas analysis,a considerable amount of C2H6 and CO2 still has not been desorbed.During the analytical process,the carbon and hydrogen isotopes have changed,the variation range ofδ13 CH4,δ13 C2H6,δ13 C3H8,δ13 CO2,δDCH4 andδDC2H6 is-39.92‰--25.86‰,-41.57‰--39.34‰,-40.89‰--35.46‰,-23.42‰--19.23‰,-136.90‰--128.00‰and-160.45‰--155.30‰,respectively.Due to the mass fractionation effect of isotope,the methane carbon isotope and methane hydrogen isotope remained in the analytical process is increased by 5.15‰-13.33‰and 1.64‰-8.90‰respectively;the ethane carbon,hydrogen isotope and carbon isotope of carbon dioxide is basically unchanged.The gaseous carbon isotope fractionation is also affected by the physical properties of shale,and the larger pore volume will lead to a more significant methane carbon isotope fractionation effect,as well as a difference in the volume content of ethane.The isotope value of the gas sample taken by desorbing the half volume represents the average value of all gases;the average value ofδ13 CH4,δ13 C2H6 andδ13 C3H8 is-33.19‰,-40.04‰and-39.07‰,respectively;the shale gas shows the'reverse order'feature of isotope:δ13 CH4>δ13 C2H6,δ13 C2H6<δ13 C3H8,δDCH4>δDC2H6.Similar to the Lower Cambrian Qizhusi Formation in Weiyuan,Shuijingtuo Formation shale gas in Yichang area is also in the early stage of reversal of gaseous hydrocarbon isotope,and has the characteristics of multi-source composite thermogenic gas.