Search

Using logging data and samples collected by the four expeditions of the International Ocean Discovery Program NanTroSEIZE drilling campaign that occupied Site C0002 in the Nankai margin located southwest of Japan, we determined effective porosity and sonic velocity profiles down to 3 km below the seafloor in the accretionary prism, above the seismogenic plate interface. We measured cation exchange capacity to differentiate clay‐bound water content from effective porosity, which is representative of compaction. The decrease in effective porosity with depth is fitted with a single exponential decay, which suggests hydrostatic conditions for most of the borehole. The Erickson‐Jarrard template was adapted to account for the actual clay‐bearing nature of the sediments. We used this improved P‐wave velocity–effective porosity relationship to fill shipboard data gaps and predict P‐ and S‐wave velocity profiles at Site C0002. Plain Language Summary: Effective porosity reflects the network of pores that affects the transport properties and the history of sediment compaction. However, evaluating this property is not straightforward given that standard methods provide total porosity, which also includes clay‐bound water. We can estimate the effective porosity using cation exchange capacity of the rocks. This method was used at Site C0002, the deepest hole in history of the International Ocean Discovery Program, drilled down to the heart of the Nankai accretionary prism, located southwest of Japan. We show that the effective porosity follows a simple exponential decay law with depth. We also demonstrate that there is a single relationship between effective porosity and P‐wave velocity. We use this relationship to fill the data gaps in the P‐ and S‐wave velocity profiles at Site C0002. Conversely, this provides a direct method to estimate effective porosity, and hence, the compaction state, from seismic profiles acquired across the Nankai accretionary prism. Key Points: Effective porosity was derived from cation exchange capacity analysis on samples from Site C0002, the deepest International Ocean Discovery Program borehole ever drilledTotal and effective porosities decay exponentially all along Site C0002, down to the heart of the Nankai accretionary prism (3,000 mbsf)Modifying the parameters of the Erickson‐Jarrard model using effective porosity and clay content improves the P‐wave velocity prediction [ABSTRACT FROM AUTHOR]

Several mechanisms involving excess pore pressure related to gas hydrate have been proposed to explain active creeping at landslides such as the north Hikurangi margin Tuaheni Landslide Complex (TLC). Cores and logging data were retrieved by the International Ocean Discovery Program (IODP) Expeditions 372 and 375 from the South TLC at Site U1517. Here, the evolutions of porosity, pore structure and permeability are determined to assess the compaction state in the landslide and compare it with that of the undeformed sequence at Site U1519. Although no evidence of gas hydrate in the landslide at Site U1517 was identified by the cruise, zones suspected to host gas hydrates below the landslide or at Site U1519 are characterised by higher porosity, pore diameter and permeability. We show that most of the sedimentary section is in hydrostatic conditions, except the base of the TLC at Site U1517 and a zone below the base of the gas hydrate stability zone at Site U1519. These zones might be candidates for excess pore pressure build-up. There is no obvious evidence of the involvement of gas hydrate in active creeping at the TLC, which is more likely induced by hydrogeomechanical processes. [ABSTRACT FROM AUTHOR]

Hosting both tsunami earthquakes and slow slip events at shallow depth, the northern Hikurangi margin has motivated strong research efforts over the last two decades to better understand the relation between fluid pressure and fault mechanical behavior. Recently, IODP Expeditions 372 and 375 drilled, cored, and logged the basin entering this subduction zone providing a unique opportunity to characterize initial hydrogeological and petrophysical properties that drive seismic hazards along the margin. We inventory bound and interstitial fluid contents and assess the compaction state of the heterogeneous incoming sequence by correcting shipboard total connected porosity from the bound water content that typically characterizes clay‐rich sediments. By combining mercury injection capillary pressure, nuclear magnetic resonance, and nitrogen gas adsorption, we further document the evolution of pore structure with increasing depth. We also compare porosity data at the logging and sample scales. Both logging and laboratory data evidence contrasted porosity, pore structure, and fluid distribution across the entering section. Shallow macroporous siliciclastic units show very low bound water content and undergo compaction‐induced dewatering as they are normally consolidated during burial. In contrast, mesoporous volcaniclastic Hikurangi Plateau facies are characterized by high potential for bound water release from smectite, zeolite, and opal dehydration at greater depths. They mostly exhibit very low permeability except potentially in uncemented highly porous intervals where the décollement may initiate. Macroporous to mesoporous pelagic carbonates sandwiched in between exhibit mixed behavior and may also host the future décollement, similarly to the southern part of the margin. Key Points: The Northern Hikurangi margin input section is characterized by contrasted fluid distribution, pore structure, and potential for excess pore pressure buildupIt is essential to use corrected porosity instead of apparent porosity in clay‐rich sediments and altered oceanic crustThe future décollement may develop in the lower part of pelagic carbonates or at the top of volcaniclastic facies of the Hikurangi Plateau [ABSTRACT FROM AUTHOR]

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 décollement 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 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. • Cation Exchange Capacity shows variation in clay mineralogy through the input section. • Corrected porosity shows that the Nicobar basin experienced normal consolidation. • High total porosity in prefan Unit III is an artifact due to high smectite content. • Smectite-rich Unit III exhibits potential for fluid production by clay dehydration. • Damaged parts of the input sedimentary section show fracture porosity. [ABSTRACT FROM AUTHOR]