  In this study, we investigate porosity evolution through the sedimentary
input section of the North Sumatra Subduction zone by quantifying interstitial
porosity, bound water content and fracture porosity based on IODP Expedition
362 data and post-cruise chemical analyses. During IODP Expedition 362, total
porosity of the sedimentary section entering the North Sumatra subduction zone
was measured. This total porosity is derived from the total water content of
core samples thus including pore water and water bound to hydrous minerals like
smectite. Clay mineral composition varies over the sedimentary section and is
mainly kaolinite/illite in the Nicobar Fan units and smectite/illite in the
prefan pelagic unit below. The prefan pelagic unit shows anomalously high total
porosity values and is stratigraphically correlated to a high amplitude
negative polarity (HANP) seismic reflector located landward. This HANP
reflector has been previously interpreted as a porous fluid-rich layer where
the d{\'e}collement may develop along parts of the margin as a consequence of
pore pressure buildup. We estimate clay bound water content from Cation
Exchange Capacity (CEC) which gives information about the smectite/illite
composition and soluble chloride content data. Interstitial porosity
corresponds to onboard total porosity corrected from clay bound water and is
more relevant in terms of sediment compaction state and fluid flow properties.
Interstitial porosity versus vertical effective stress curve shows no evidence
of undercompaction and suggests that the input section 2 has been experiencing
normal consolidation due to high sediment accumulation rate. The porosity
anomaly observed in the prefan pelagic unit results from the local occurrence
of water-bearing minerals like smectite rather than excess pore pressure, which
might, however, buildup more landward in the basin. We also estimate fracture
porosity using a resistivity model for shales used in previous works based on
wireline resistivity log and show that fracture porosity yields 4-6% in damaged
parts of the sedimentary section investigated.
