Lithology-related hydraulic properties of subduction zone sediments at Japan Trench, IODP Exp.405 Site C0026

Making a link between hydraulic properties and lithology is important for understanding fluid flow and pore pressure evolution in subduction zone sediments. However, it's a difficult challenge to get the in-situ downhole hydraulic responses due to the complex lithological variations and strong vertical heterogeneity.  IODP Expedition 405 - JTRACK, drilled Site C0026 in the Japan Trench, penetrating a thick sedimentary sequence by hemipelagic mud, pelagic clay, chert and basalt intervals. Continuous logging-while-drilling (LWD) and coring operations provide a unique opportunity to examine lithology-related hydraulic properties in the sedimentary structures.

In this study, we integrate the interpretated downhole annular pressure (DHAP) from LWD time series dataset and other physical properties, including gamma ray, resistivity, sonic velocity, and caliper logs, together with interpretated lithogical logging units. Comparing forward-modelled DHAP with measured DHAP data, the results indicates that the in-situ fluid pressure evolution is highly correlated with lithological variability. In the hemipelagic mud at shallow depth, apparent inflow is largely influenced by borehole enlargement, where caliper increases strongly affect flow modelling and make it difficult to identify the real formation inflow. In the pelagic clay interval, little to no fluid inflow or loss is observed, indicating the absence of significant overpressure or under pressure. Localized fluid loss is identified within chert layers, consistent with their brittle behaviour and increased caliper values, suggesting fracture-controlled fluid escape. In contrast, a clear and active inflow is observed in the basalt interval, where fractures identified from borehole image logs provide efficient pathways for fluid flow to the borehole.

These observations indicate that hydraulic properties at Site C0026 are strongly controlled by lithological contrasts, leading to vertically variations fluid flow behavior. This study highlights the importance of integrating with borehole logs lithological information to constrain fluid transport processes in subduction zone sedimentary sequences.