Monitoring of Deep Fluids in the Nankai Subduction Complex, SE offshore Japan

Abstract

Fluids are of great importance for various geological processes. To improve our understanding of fluid generation and migration in the Nankai Trough subduction complex, SE offshore Japan, this PhD thesis uses different borehole monitoring techniques that were applied in the course of the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment. Long-term monitoring with a SmartPlug borehole observatory was conducted at a splay fault system cutting through the accretionary prism. Further landward, beneath the Kumano forearc basin, information about in situ composition of fluids was obtained by drilling mud gas monitoring and sampling during riser drilling with drilling vessel Chikyu. Long-term pressure data revealed that the fluid pressure data is affected by a wide range of natural signals. Tidal noise was used to estimate hydrogeological fault zone properties. Beneath the Kumano forearc basin, fluid migration and mixing in the accretionary prism is supported by analyses of drilling mud gas focusing on noble gas isotopes and hydrocarbon gas composition. Fluid pathways and mechanisms of fluid migration remain unclear.