四川盆地焦石坝地区五峰-龙马溪组页岩元素地球化学特征及对页岩气开发的意义

选择四川盆地焦石坝地区JYA、JYD两口井的85块页岩岩心样品，进行了有机碳和主、微量元素测试分析。通过分析主、微量元素在剖面上的纵向变化特征，对焦石坝地区五峰组-龙马溪组含气页岩的氧化还原条件和古生产力状况进行了研究，讨论了五峰组-龙马溪组有机质富集的主控因素；探讨了地球化学元素对页岩可压性的影响和含气性的指示。研究区五峰组-龙马溪组下部优质气层段有机质含量高，平均值为3.07%。与含气页岩段相比，优质气层段页岩中的SiO₂和CaO含量较高，Mo、Cr、V、Ni、Th和U等氧化还原敏感元素富集。V/Cr、V/Sc、U/Th和Ni/Co等氧化还原条件判别指标表明，五峰组沉积期以缺氧-贫氧环境为主，龙马溪组沉积期下部以缺氧环境为主，往上则主要为含氧环境。古生产力指标Ba_（xs）指示五峰组-龙马溪组页岩沉积期具有高等生产力背景。优质气层段有机碳含量与Mo/Al、U/Th、Ni/Co、V/Sc值之间存在明显的正相关，说明有机质富集主要受氧化还原条件控制。另外，优质气层段页岩生物成因的硅质含量高，且有机碳含量与SiO₂含量呈正相关，有利于形成天然裂缝和后期人工压裂改造。优质气层段的地化元素比值明显高于含气页岩段，说明化学元素比值与页岩含气性之间存在一定的相关性。 

Element geochemical characteristics of the Wufeng-Longmaxi shale in Jiaoshiba area,Sichuan Basin and their significance to shale gas development

A total of 85 shale core samples from wells JYA and JYD in Jiaoshiba area of Sichuan Basin were selected to test their organic carbon contents, major and trace elements. The redox conditions and paleoproductivity of the gas-bearing shale in the Wufeng-Longmaxi formations in the Jiaoshiba area were studied by analyzing the vertical variation characteristics of the major and trace elements in the profile. Meanwhile, the controls for organic matter enrichment in the Wufeng-Longmaxi formations were explored. Moreover, the impact of geochemical elements on shale compressibi-lity and gas-bearing property were studied. The organic carbon content in the high gas-bearing interval from the Wufeng Formation to the lower part of Longmaxi Formation is high with an average value of 3.07%. Compared with the common gas-bearing shale, SiO₂ and CaO contents are higher in the high gas-bearing shale, and redox-sensitive elements such as Mo, Cr, V, Ni, Th and U are enriched. The redox condition indicators such as V/Cr, V/Sc, U/Th and Ni/Co indicated that the Wufeng Formation was mostly deposited in suboxic to anoxic conditions, whereas the anoxic environment dominated during the early sedimentary stage of Longmaxi shale and evolved into aerobic conditions later on. The contents of biological productivity-related element Ba_(xs) reflected a high paleoproductivity, whereas TOC content has an obvious positive correlation with redox proxies (Mo/Al, U/Th, Ni/Co and V/Sc) in the high gas-bearing interval, suggesting that the enrichment of organic matter was predominantly controlled by redox environment. In addition, the biogenic siliceous content of high-quality gas-bearing shale is high, and the organic carbon content is positively correlated with the SiO₂ content, which is conducive to the formation of natural fractures and the later artificial fracturing. The ratio of geochemical elements in the high gas-bearing interval is significantly higher than that of common interval, indicating that there is a certain correlation between chemical element ratios and shale gas.