自然伽马能谱测井在富有机质页岩评价和地质导向中的应用

Application of Natural Gamma Spectroscopy Logging in Evaluation and Guidance of Black Shale

自然伽马能谱测井反映了地层天然放射性强度和元素类型,是识别、评价页岩气储层以及为完井提供参数的重要测井资料之一。鄂西及周缘地区下寒武统水井沱组富有机质页岩底部的高自然伽马层段为主要产气段,综合利用自然伽马能谱测井、综合录井、有机地化等资料,将页岩段划分为5个小层,在富有机质页岩识别的基础上,开展了页岩总有机碳含量(TOC)预测、黏土矿物类型识别、页岩沉积环境分析,并结合水平井钻探,分析了随钻自然伽马测井在水平井地质导向上的应用。研究表明:①5个小层的铀、钍、钾与地层自然放射性总强度(GRSL)的变化特征及相关性显示,宜页1井下寒武统水井沱组富有机质页岩具有高TOC、高GRSL、高铀的特征,利用铀含量、铀钾比值预测总有机碳含量精度较高。②不同小层铀、钾变化揭示黏土矿物类型存在差异,水井沱组主要以伊/蒙混层和伊利石为主,还含有少量绿泥石,第Ⅴ小层伊利石含量最大。③铀、钍测井揭示,Ⅳ~Ⅴ小层页岩形成于碳酸盐岩台地前缘缓坡-深水陆棚相带缺氧环境。④水井沱组页岩底部具有“异常高铀、异常高自然伽马”特征,可作为水平井钻进的标志层,对高效中靶具有良好的指示作用。

Natural gamma spectroscopy logging reflects the natural radioactive intensity and classification of the formation,which is one of the most important logging parameters for identifying and evaluating shale gas reservoirs,as well as providing parameters for later completion.The Lower Cambrian Shuijingtuo formation is characterized by organic-rich shale with high nature gamma interval at the bottom of the gas-producing interval in Exi and peripheral areas.Base on the characteristics of natural gamma spectroscopy logging,comprehensive logging and organic geochemistry,five individual layers are identified.Using natural gamma ray spectral logging,the organic-rich shale identification,shale total organic content(TOC)prediction,clay mineral type identification,and shale sedimentary environment analysis are analyzed.The application of natural gamma logging while drilling in horizontal well geo-steering is also analyzed.The research shows that:①The logging parameters of U,Th,K and GRSL in five layers and correlation between those parameters show that the organic-rich shale is characterized by high TOC,GRSL,and U content.There is a high accuracy in TOC content prediction using parameters of U content and U and K ratio.②U and K elements in different layers revealed differences in types of clay minerals.The Shuijingtuo shale is mainly dominated by illite/smectite and illite,and a small amount of chlorite.TheⅤlayer of organic shales have the maxima content of illite.③The IV~Ⅴlayers of organic-rich shales are deposited in the gentle slope to deep water continental shelf at the front edge of the carbonate platform.The organic matter is formed in an anoxic environment.④The characteristics of abnormally high U and GR at the bottom of the Shuijingtuo shale can be used as a marker layer for horizontal well drilling,and it’s useful to improve shale targets during drilling.