元素俘获测井在页岩储层古沉积环境辨识中的应用

Application of GEM elemental logging in shale reservoir sedimentary environment identification

元素俘获测井可以获取地层中的岩石元素含量,为了查明页岩气储层发育和展布规律,以中扬子地区保靖区块寒武统牛蹄塘组首口页岩气参数井—BY2井为研究对象,通过对伽马元素俘获测井仪(Gamma Elemental Mineralogical Analysis Logging Tool,GEM)资料的解释和处理,获取精准的页岩储层元素组成及含量,建立牛蹄塘组地球化学指标剖面,利用元素地球化学指标开展古沉积环境的辨识和反演。研究结果表明:牛蹄塘组页岩中元素以Si、Al、Fe为主,少量的K、Ca、Mg、S等,页岩中Si、Fe、S等元素含量较高,页岩可压性较好;牛蹄塘组页岩属活动大陆边缘沉积,沉积物源岩的原始物质应来自西北端康滇古陆,基本为活动大陆构造带、有热液参与的正常沉积;牛蹄塘组上段沉积期气候干燥,为缺氧的滞留盆地边缘缓坡沉积,水体盐度高,陆源供给充分,古生产力低下;牛蹄塘组下段沉积期气候湿润,为深水滞留型盆地沉积,陆源供给不足,水体盐度较高,具有较高的古生产力,有利于有机质的形成,水体环境以贫氧相为主,有利于有机质保存,为页岩气形成和富集的有利沉积环境。

[Background]Element capture logging can be used to determine the elemental contents of rocks in formations.[Purpose]This study aims to obtain accurate elemental composition,the content of shale reservoirs and the inaugural parameter well for shale gas in the Cambrian Niutitang Formation of the Baojing Block,located in the Middle Yangtze region of China,with emphasis on the developmental and distributional characteristics of shale gas reservoirs in this formation.[Methods]The shale gas parameter well BY2 was taken as the research object,by interpreting and processing elemental capture logging data,precise elemental compositions of the shale reservoirs were determined.This analysis led to the creation of a comprehensive geochemical index profile for the Niutitang Formation.Additionally,elemental geochemical indicators were used to identify and reconstruct the paleosedimentary environments.[Results]The analysis results reveal that the predominant elements in the Niutitang Formation's shale are Si,Al,and Fe,accompanied by lower amounts of K,Ca,Mg,and S.The shale features relatively high concentrations of Si,Fe,and S,which contributed to its enhanced fracturing ability.The sedimentation process of this shale can be categorized as active continental margin sedimentation.The source material for the sedimentary rocks is originated from the Kangdian ancient land,located in the northwest.The sedimentation is primarily normal but was influenced by the presence of hydrothermal fluids in the region's active tectonic zone.[Conclusions]The upper section of the Niutitang formation is subject to a dry climate during its depositional period,featuring gentle slope sedimentation at the periphery of a stagnant basin and a lack of oxygen,characterizing with high water salinity,ample land supply,and low water body paleoproductivity this region.Conversely,the lower section is experienced a humid climate and served as a deep-water retention basin,with limited the land supply,but high water salinity and paleoproductivity,leading to the accumulation of organic matter.The aquatic setting is primarily anaerobic,conditions that are conducive to the preservation of organic matter,and provides an optimal sedimentary environment for the generation and concentration of shale gas.