中扬子宜昌地区下寒武统水井沱组页岩气地球化学特征及其成因

Geochemical features and genesis of shale gas from the Lower Cambrian Shuijingtuo Formation shale in Yichang block,Middle Yangtze region

中扬子宜昌地区下寒武统水井沱组页岩钻遇良好的页岩气显示,区内宜页1井水井沱组页岩经水力压裂后获得了高产工业气流,是四川盆地外下寒武统页岩气新的勘探区。采集压裂产气段的9个页岩气样品,测试气体组分、碳和氢同位素组成以及He同位素组成,分析水井沱组页岩气地球化学特征并探讨了页岩气成因。研究表明,页岩气组成中甲烷含量为87.17%~92.75%,乙烷含量为0.83%~0.94%,含微量的丙烷,页岩气干燥系数为0.99,为典型干气;非烃气体中氮气含量稍高,平均为7.73%,二氧化碳平均含量低于1%,不含H2S。甲烷碳同位素值为-33.8‰^-33.1‰,乙烷碳同位素值为-39.2‰^-36.0‰,丙烷碳同位素值为-39.4‰^-38.5‰,二氧化碳的碳同位素值为-16.8‰^-14.6‰。甲烷氢同位素值为-133.8‰^-128.5‰,乙烷氢同位素值为-168.0‰^-146.1‰。气态烃稳定同位素分布具有δ^13C1>δ^13C2>δ^13C3和δDCH4>δDC2H6的倒转特征。He同位素R/Ra为0.04~0.08,表明He为典型的壳源成因。综合页岩气分子组成和比值、碳氢同位素倒转分布以及页岩现今处于过成熟热演化阶段等特征,认为宜昌地区下寒武统页岩气为油型气,具有二次裂解成因的特点,且与四川盆地筇竹寺组页岩气具有相似的特征。

Strong gas shows are discovered in drilling the Lower Cambrian Shuijingtuo Formation shale in Yichang area,Middle Yangtze,and high commercial gas flow was tested following hydraulic fracturing of the Shuijingtuo shale in Well Yiye1 in the study area,which is a prospective area for shale gas exploration in the Lower Cambrian shale outside the Sichuan Basin.Nine shale gas samples were collected after hydraulic fracturing to analyze the geochemistry and origin of shale gas in the Shuijingtuo Formation,following the study of shale gas composition,as well as carbon,hydrogen,and He isotopic compositions.The results show that methane is the main constituent of the shale gas and varies in concentration from 87.17%to 92.75%,the content of ethane ranges from 0.83%to 0.94%,and the propane is in trace amount;the drying coefficient(Cl/Cl-5)is about 0.99,indicating typical dry gas.The content of nitrogen in non-hydrocarbon gases is slightly higher,with an average of 7.73%,the carbon dioxide content is less than 1%,and there is no H2S.Theδ^13 C methane in the Cambrian shale gas varies between-33.8‰and-33.1‰in content,theδ^13 C ethane value ranges from-39.2‰to-36.0‰,δ^13 C propane value ranges from-39.4‰to-38.5‰,and theδ^13 C value for CO2 ranges from-16.8‰to-14.6‰.TheδD CH4 methane in the shale gas ranges from-133.8‰to-128.5‰in content,while theδDC2H6 ethane ranges from-168.0‰to-146.1‰.The stable carbon isotopic distribution of the shale gas shows a totally reversed distribution pattern withδ^13 C1>δ^13 C2>δ^13 C3,andδDCH4>δDC2H6.The R/R a ratio of helium in the Shuijingtuo shale gas ranges from 0.04 to 0.08,indicating the crustal origin of the helium.Considering the shale gas compositions and their ratios,as well as the reversed distribution of carbon and hydrogen isotopes at the high-over mature stage in thermal evolution,we believed that the shale gas of the Lower Cambrian Shuijingtuo Formation in Yichang area belongs to the oil-derived gas,featuring an origin of secondary cracking,which is similar to the Qiongzhusi shale gas in Sichuan Basin.