アーケアによるメタンの生成と間隙水への濃集機構

抄録

Generation of "bacterial" methane by Archaea and its accumulation mechanism into interstitial water in early diagenesis were investigated by a mass balance calculation for observed isotopic compositions and geological evolution through aggradation from bathyal sediments for several hydropressured (so-called "dissolved-in-water type") natural gas fields in Japan. Hydrogen isotopic ratios of methane from the Minami-Kanto and Niigata gas fields reveal that CO₂-reduction was the main reaction of the methane formation, and its carbon isotopic ratios show a concentration between -67 and -66%_0 relative to PDB. Model calculation in a semi-closed system based on coexisting carbon isotopic compositions of methane and dissolved inorganic carbon for the Minami-Kanto gas field reveals the considerable contribution of carbonate minerals to both compositions. For example, if CO₂ had been supplied only from organic matter, the efficiency of methane generation would have been as low as 40%. Whereas, if methane generation rate had been equal to the input rate of CO₂ from organic matter, the dissolution and precipitation of carbonate minerals and/or isotope exchange reaction between dissolved inorganic carbon and carbonate minerals should have occurred about nine times faster than methane generation. From a geological standpoint, it is inferred that the existence of thick marine argillaceous sediments rich in terrigenous organic matter and carbonate minerals was important for the generation of archaeal methane and adsorption of iodine. Accumulation of methane and iodine into interstitial water must have been promoted when interacting solid/liquid ratio of sediments was increased by the burial suppression of interstitial water caused by basin subsidence, sedimentation of sandstone, recovery of abnormal compaction and/or dehydration of clay minerals and other causes. Interstitial water dissolving methane and iodine in thick marine argillaceous rocks was removed to the upper sand/silt or mud interbeds, which had higher sedimentation rates, with the compaction and burial of solid sediments.