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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
Author(s) -
Arai Takashi,
Okusawa Tamotsu,
Tsukahara Hiroaki
Publication year - 2001
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1111/j.1440-1738.2001.00341.x
Subject(s) - geology , mineralogy , methane , fracture (geology) , carbon dioxide , fault (geology) , fracture zone , isotopes of carbon , chemical composition , borehole , geochemistry , total organic carbon , chemistry , environmental chemistry , seismology , geotechnical engineering , organic chemistry
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 (CO 2 ) and methane (CH 4 ). Microcracks in granodiorite outside the fracture zone were occupied mainly by CO 2 , and this CO 2 is interpreted to have generated biogenically at shallow depths based on the measured δ 13 C value of –17 to –22. The CO 2 gas was probably transported with underground water to deeper portions to fill microcracks in the basement granodiorite with CO 2 . However, the pores in the fracture zone are occupied predominantly by CH 4 . The ratio of CH 4 to ethane (C 2 H 6 ), 80 to100, and δ 13 C of CH 4 , –40 to –52, suggest that CH 4 and C 2 H 6 formed by the thermal decomposition of organic materials at temperatures above 75°C. We interpret that they originated at depths from organic materials and migrated upwards through the fault zone. It is interpreted that the concentration of CO 2 in the fracture zone has decreased by the replacement with CH 4 and/or by the consumption of CO 2 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 3 He/ 4 He ratio of 0.723 Ra in the fracture zones suggests that He is of radiogenic origin; that is, it is not from the mantle.