# Quantification of bound water content, interstitial porosity and fracture porosity in the sediments entering the North Sumatra subduction zone from Cation Exchange Capacity and IODP Expedition 362 resistivity data

**Authors:** Jade Dutilleul, Sylvain Bourlange, Marianne Conin, Yves Géraud  
**arXiv:** [2006.09700](https://arxiv.org/abs/2006.09700)  
**Date:** 2020-06-17  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2006.09700v1/

## Key points

- Study focuses on porosity evolution in sedimentary input section of North Sumatra Subduction zone
- Total porosity measured during IODP Expedition 362 includes pore water and water bound to hydrous minerals
- Clay mineral composition varies across sedimentary section, with different compositions in Nicobar Fan units and prefan pelagic unit
- Anomalously high total porosity observed in prefan pelagic unit correlates with HANP seismic reflector, indicating potential for d{\'e}collement development
- Clay bound water content estimated using Cation Exchange Capacity data provides insights into composition and fluid flow properties
- No evidence of undercompaction in input section 2, suggesting normal consolidation driven by high sediment accumulation rate
- Porosity anomaly attributed to localized occurrences of water-bearing minerals rather than excess pore pressure
- Fracture porosity estimated at 4-6% in damaged parts of sedimentary section using resistivity model for shales based on wireline resistivity log data

## Summary

In this comprehensive study, Dutilleul et al. delve into the intricate porosity evolution within the sedimentary input section of the North Sumatra Subduction zone. By meticulously quantifying interstitial porosity, bound water content, and fracture porosity using data from IODP Expedition 362 and post-cruise chemical analyses, the researchers shed light on the complex geological processes at play in this region. During IODP Expedition 362, the team measured the total porosity of the sedimentary section entering the North Sumatra subduction zone. This total porosity encompasses both pore water and water bound to hydrous minerals like smectite, providing a holistic view of the subsurface composition. The clay mineral composition varies across the sedimentary section, with kaolinite/illite predominating in the Nicobar Fan units and smectite/illite in the prefan pelagic unit below. Of particular interest is the anomalously high total porosity observed in the prefan pelagic unit, which correlates stratigraphically with a high amplitude negative polarity (HANP) seismic reflector located landward. This reflector has been interpreted as a porous fluid-rich layer where d{\'e}collement may develop due to pore pressure buildup along parts of the margin. To further elucidate these findings, Dutilleul et al. estimate clay bound water content using Cation Exchange Capacity (CEC) data, which provides insights into smectite/illite composition and soluble chloride content. Interstitial porosity, corrected from clay bound water, offers valuable information on sediment compaction state and fluid flow properties. Notably, analysis of interstitial porosity versus vertical effective stress reveals no evidence of undercompaction in input section 2, suggesting normal consolidation driven by a high sediment accumulation rate. The researchers attribute the observed porosity anomaly in the prefan pelagic unit to localized occurrences of water-bearing minerals like smectite rather than excess pore pressure – although such pressures may accumulate more landward in the basin over time. Additionally, utilizing a resistivity model for shales based on wireline resistivity log data enables estimation of fracture porosity in damaged parts of the investigated sedimentary section, yielding values between 4-6%. By integrating various analytical techniques and geological interpretations, this study significantly advances our understanding of porosity dynamics within subduction zones like North Sumatra. The findings not only contribute to ongoing research efforts but also have implications for broader studies on tectonic processes and fluid migration in similar geologic settings.