Sediment lithology, especially the clay mineral fraction, is basic geologic information, which is important for understanding faulting and evolutionary process of the accretionary prism. During scientific drilling, the clay mineral fraction is measured using X-ray diffraction analyses of samples from the acquired cores. However, coring in an accretionary prism is sometimes difficult, which hinders the acquisition of clay mineral fraction data. This study details the development of a new method used to empirically estimate the clay mineral fraction from the electrical resistivity and natural gamma ray logs obtained from the Kumano section of the Nankai Tough, where multiple scientific drilling expeditions have been conducted. This method is composed of two steps. First, porosity is estimated using electrical resistivity logs. In order to compensate for the effect of porosity, the natural gamma ray logs are then normalized using (1 − porosity). Second, the normalized natural gamma ray logs are converted into clay mineral fractions using an empirical conversion function. This empirical function was determined by comparing all available normalized natural gamma ray logs and clay mineral fractions data collected from the Kumano section of the Nankai Trough. As a byproduct of the porosity estimations, thermal conductivity and temperature depth profiles were also estimated for all of the logging sites. As electrical resistivity logs and natural gamma ray logs are essential measurements made during scientific drilling, this new method can be applied to future drilling expeditions in the Nankai Trough. Although the empirical conversion function established in this study is valid only at the Kumano section of the Nankai trough, the same procedure can be applicable to other subduction zones to establish local empirical conversion functions if enough data are available. The clay mineral fraction depth profile estimated using this method can provide useful information for various geological studies, including sedimentology and structural geology of the shallow accretionary prism.

中文翻译：

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南海增生棱柱测井资料经验转化为粘土矿物分数的新方法

沉积岩性，尤其是粘土矿物组分，是基本的地质信息，对于理解增生棱柱的断层和演化过程具有重要意义。在科学钻探过程中，粘土矿物分数是通过对采集的岩心样品进行 X 射线衍射分析来测量的。然而，在增生棱柱中取心有时很困难，这阻碍了粘土矿物分数数据的获取。本研究详细介绍了一种新方法的开发，该方法用于根据从 Nankai Tough 的熊野段获得的电阻率和天然伽马射线测井来经验性地估计粘土矿物比例，在那里进行了多次科学钻探。该方法由两个步骤组成。首先，使用电阻率测井估算孔隙度。为了补偿孔隙度的影响，自然伽马射线测井然后使用（1 - 孔隙度）归一化。其次，使用经验转换函数将归一化的自然伽马射线测井转换成粘土矿物部分。该经验函数是通过比较所有可用的归一化天然伽马射线测井数据和从南海海槽熊野剖面收集的粘土矿物组分数据确定的。作为孔隙度估算的副产品，还估算了所有测井点的热导率和温度深度剖面。由于电阻率测井和自然伽马射线测井是科学钻探过程中必不可少的测量数据，这种新方法可用于未来南海海槽的钻探。尽管本研究建立的经验转换函数仅在南海海槽熊野段有效，但如果有足够的数据，同样的程序可以适用于其他俯冲带建立局部经验转换函数。使用这种方法估计的粘土矿物部分深度剖面可以为各种地质研究提供有用的信息，包括浅层增生棱柱的沉积学和构造地质学。