A new method for the empirical conversion of logging data to clay mineral fraction in the Nankai accretionary prism

Abstract

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.

Multiple Types of Porosity – P‐Wave Velocity Relationships for the Nankai Trough

“…According to core-based measurements Guo, 2013, 2017), clay and smectite content are as much as 60% -80% and 40% -60%, respectively. Clay content empirically estimated from NGR log (Yabe et al, 2020) is consistent with core-based measurements. Here, we assume that smectite content is two thirds of clay content.…”

“…On the other hand, clay and smectite content increase from 30% -50% and 10% -20% in slope apron to 60% -70% and 20% -30% in the accretionary prism, respectively, at Site C0001. Clay content empirically estimated from the NGR log (Yabe et al, 2020) is consistent with core-based measurements. Here, we assume that smectite content is one Erickson and Jarrard (1998), whereas those for the accretionary prism, the fault-bounded package, and the underthrust slope apron are consistent with or have even higher V p than the high-V p model of Erickson and Jarrard (1998).…”

“…Above 55°C, the cementation collapses and the porosity steeply decreases (Spinelli et al, 2007). Such a rapid porosity decrease was also observed at the incoming sites (Sites C0011 and C0012) along the Kumano transect (Expedition 322 Scientists, 2010bYabe et al, 2020), although the porosity -V p relationships of the incoming USB obtained from Site C0012 are consistent with that of Hoffman and Tobin (2004) with the same porosity.…”

“…To deal with these problems, we also referred to the porosity estimated from logging data. Yabe et al (2020) estimated the in situ porosity using electrical resistivity logs for all available sites of the NanTroSEIZE project. They used the methodology of Yabe et al (2019) who simultaneously estimated the porosity and thermal structure from resistivity logs using empirical relationships among the porosity, thermal conductivity, and electrical resistivity based on laboratory measurements.…”

“…Depth profiles of the porosity estimated by Yabe et al (2020) used in this study are available at https://doi. org/10.5281/zenodo.4719173.…”