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 I sland, Hyogo prefecture, Japan. The chemical compositions and isotope ratio s of gases extracted from the drill core were investigated. Major component s were carbon dioxide (CO2) and methane (CH4). Microcracks in granodiorite outside the fracture zone were occupied mainly by CO2, and this CO2 is inte rpreted to have generated biogenically at shallow depths based on the measu red delta C-13 value of -17 to -22. The CO2 gas was probably transported wi th underground water to deeper portions to fill microcracks in the basement granodiorite with CO2. However, the pores in the fracture zone are occupie d predominantly by CH4. The ratio of CH4 to ethane (C2H6), 80 to100, and de lta C-13 of CH4, -40 to -52, suggest that CH4 and C2H6 formed by the therma l decomposition of organic materials at temperatures above 75 degreesC. We interpret that they originated at depths from organic materials and migrate d 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/o r 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 increa sed in quantity in the fracture zone. High concentration of H-2 in the frac ture zones is consistent with the idea that H-2 was generated by radical re actions 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 ra diogenic origin; that is, it is not from the mantle.