Abstract Conventionally, high gamma ray (GR) values usually indicate high argillaceous contents in sedimentary rocks, while high-GR sections in marine shales have more siliceous minerals and fewer clay minerals. However, the origin of the anomalously high-GR peaks in marine shales has been less well studied. In addition, high-GR marine shale intervals are usually important sections for horizontal drilling and fracturing. The Wufeng-Longmaxi shales in the Sichuan Basin and its periphery were investigated to study the sedimentary characteristics and genesis of anomalously high-GR peaks based on elemental and natural GR spectral logging data, sedimentary observations and geochemical analysis. Uranium enrichment is concluded to control the GR peak occurrences in the Wufeng-Longmaxi shales, and uranium enrichment is affected by a hypoxic reducing environment, high organic matter content, well-developed tuff beds, and the adsorption of phosphorus-bearing minerals and iron-bearing minerals. The anomalously high-GR peaks are manifestations of volcanic activity and rapid transgression. Rutile, uranium-bearing minerals, apatite, barium feldspar and tuff beds are petrological evidence of volcanic activity, and Fe, K and U/Th ratio anomalies also show geochemical evidence of volcanic activity. The degree of water stagnation, disorderly mineral distribution, and reducing environment are evidences of rapid transgression. The uranium enrichment indicates that volcanic eruptions provided large numbers of uranium ions for the seawater uranium pool, forming a material basis for uranium shale deposition. De-glaciation transgression provided a sedimentary environment and pathways for uranium enrichment. Two uranium enrichment events are identified in the Wufeng-Longmaxi shales. The high-GR shale section controlled by notable uranium enrichment is characterized by an anoxic reducing environment, high TOC content, good pore structure and high brittleness. High-GR segment identification contributes to shale gas sweet spot detection. This study demonstrates that high-GR anomalies in marine shales are related to geological events and also provide certain time references for marine shales.

中文翻译：

---

海相页岩伽马射线异常的地质意义——以四川盆地及其周边奥陶系-志留系五峰-龙马溪系列为例

摘要 传统上，高伽马射线（GR）值通常表明沉积岩中泥质含量高，而海相页岩中高GR值的部分硅质矿物较多，粘土矿物较少。然而，对海相页岩中异常高 GR 峰的起源研究较少。此外，高GR海相页岩层段通常是水平钻井和压裂的重要地段。以四川盆地及周缘五峰—龙马溪页岩为研究对象，基于元素和自然GR谱测井资料、沉积观测和地球化学分析，研究了异常高GR峰的沉积特征和成因。五峰—龙马溪页岩铀浓缩控制GR峰出现 铀浓缩受低氧还原环境、有机质含量高、凝灰岩层发育以及对含磷矿物和含铁矿物的吸附等影响。异常高的 GR 峰是火山活动和快速海侵的表现。金红石、含铀矿物、磷灰石、钡长石和凝灰岩层是火山活动的岩石学证据，Fe、K 和 U/Th 比值异常也显示了火山活动的地球化学证据。水停滞的程度、矿物质分布的无序和环境还原是快速海侵的证据。铀浓缩表明，火山喷发为海水铀池提供了大量的铀离子，形成了铀页岩沉积的物质基础。去冰期海侵为铀浓缩提供了沉积环境和途径。在五峰-龙马溪页岩中发现了两个铀浓缩事件。受显着铀浓缩控制的高GR页岩段具有缺氧还原环境、高TOC含量、良好的孔隙结构和高脆性的特点。高 GR 段识别有助于页岩气甜点检测。本研究表明，海相页岩的高GR异常与地质事件有关，也为海相页岩提供了一定的时间参考。孔结构好，脆性高。高 GR 段识别有助于页岩气甜点检测。本研究表明，海相页岩的高GR异常与地质事件有关，也为海相页岩提供了一定的时间参考。孔结构好，脆性高。高 GR 段识别有助于页岩气甜点检测。本研究表明，海相页岩的高GR异常与地质事件有关，也为海相页岩提供了一定的时间参考。