Identification of Pathways for Hydrogen Gas Migration in Fault Zones with a Discontinuous, Heterogeneous Permeability Structure and the Relationship to Particle Size Distribution of Fault Materials

• Published in: Pure and applied geophysics 2011-05, Vol.168 (5), p.887-900
• Main Authors: Niwa, Masakazu, Kurosawa, Hideki, Shimada, Koji, Ishimaru, Tsuneari, Kosaka, Hideki
• Format: Article
• Language: English
• Subjects: Breccia; Crystalline rocks; Damage; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Emission measurements; Exact sciences and technology; Faults; Geophysics; Geophysics/Geodesy; Granite; Hydrogen; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Particle size; Particle size distribution; Pathways; Permeability; Plate tectonics; Rocks; Tectonics. Structural geology. Plate tectonics; Tuff; Welding
• ISSN: 0033-4553; 1420-9136
• DOI: 10.1007/s00024-010-0167-0

Previous studies have reported that high concentrations of H 2 gas are released from active fault zones. Experimental studies suggest that the H 2 gas is derived from the reaction of water with free radicals formed when silicate minerals are fractured at hypocenter depths during fault activities. However, the pathways for migration of deep-seated fluids to surface are still unknown. In this study we performed quick, multipoint H 2 gas measurements across a fault zone using a portable gas monitor and a hand drill. The fault zone studied includes a smectite-rich fault core dividing two clearly distinguishable damage zones: granite cataclasite and welded tuff fault breccia. The measurements show that H 2 gas emissions collected in 2–3 h sampling periods from start of measurement range from 320.3 to 446.2 ppm/min in the granite cataclasite and 60.5 to 137.8 ppm/min in the welded tuff fault breccias. Negligible quantities of H 2 gas could be collected from the fault core. Particle size distribution analyses of fault rocks indicate that the granite cataclasite tends to be rich in particles that are finer, i.e., less cohesive and easy to disaggregate, which leads to the inference that the granite cataclasite has high permeability. Based on the H 2 gas measurements and the particle size distribution analyses, the H 2 gas is considered to have migrated in permeable damage zones mostly by advection with groundwater. Multipoint H 2 gas measurement will be effective in qualitative delineation of variations in permeability of regional structures.