Behavior of gases in the Nojima Fault Zone revealed from the chemical composition and carbon isotope ratio of gases extracted from DPRI 1800 m drill core

An 1800 m borehole was drilled into the Nojima Fault Zone at Ogura, Awaji Island, Hyogo prefecture, Japan. The chemical compositions and isotope ratios of gases extracted from the drill core were investigated. Major components were carbon dioxide (CO2) and methane (CH4). Microcracks in granodiorite outside the fracture zone were occupied mainly by CO2, and this CO2 is interpreted to have generated biogenically at shallow depths based on the measured delta C-13 value of -17 to -22. The CO2 gas was probably transported with underground water to deeper portions to fill microcracks in the basement granodiorite with CO2. However, the pores in the fracture zone are occupied predominantly by CH4. The ratio of CH4 to ethane (C2H6), 80 to100, and delta C-13 of CH4, -40 to -52, suggest that CH4 and C2H6 formed by the thermal decomposition of organic materials at temperatures above 75 degreesC. We interpret that they originated at depths from organic materials and migrated upwards through the fault zone. It is interpreted that the concentration of CO2 in the fracture zone has decreased by the replacement with CH4 and/or by the consumption of CO2 in fault clay minerals. Although hydrogen (H-2) and helium (He) were minor components of the gases from cores, they increased in quantity in the fracture zone. High concentration of H-2 in the fracture zones is consistent with the idea that H-2 was generated by radical reactions on the fresh surface of fractured rocks during the earthquake. The He-3/He-4 ratio of 0.723 Ra in the fracture zones suggests that He is of radiogenic origin; that is, it is not from the mantle.