日本地球惑星科学連合2018年大会

講演情報

[JJ] Eveningポスター発表

セッション記号 A (大気水圏科学) » A-HW 水文・陸水・地下水学・水環境

[A-HW26] 都市域の水環境と地質

2018年5月23日(水) 17:15 〜 18:30 ポスター会場 (幕張メッセ国際展示場 7ホール)

コンビーナ:林 武司(秋田大学教育文化学部)、西田 継(山梨大学大学院総合研究部国際流域環境研究センター)、鈴木 弘明(日本工営株式会社 中央研究所、共同)

[AHW26-P06] 房総半島にみられる“上ガス”の分布と化学的特徴

*若林 泰葉1戸丸 仁1 (1.千葉大学)

キーワード:上ガス、天然ガス

The Boso Peninsula, south to east Chiba, is the largest field of natural gas dissolved in formation water in Japan with methane concentration of >99%. The gas seepage related to the subsurface gas accumulation have often been observed on land of the central part of Kujukuri plain to Otaki area and have been used as fuel by local residents, methane has been continuously released from the underground to atmosphere in this area. However, the detailed distribution, seep rate/volume etc. are not well known, in addition, the potential effect on the global warming or carbon cycle model has not been discussed. In this research, we mapped the seep location together with geological setting and collected gas samples to reveal the source, migration, and seep process of these gas by analyzing chemical and isotope composition. The gas seepage is usually located near the boundary between low permeable alluvial mudstone and sandy formations of the Kazusa Group, and is likely constricted by the change of permeability. The seep gas is composed of >75% methane and trace amounts of carbon dioxide, ethane, nitrogen derived from the atmosphere, methane is microbially produced in anaerobic environment. The δ13C value of methane is stable at around -70‰, indicating methane is generated mainly by carbon dioxide reduction and by acetate fermentation with some contribution of methane oxidation near the surface. These gases are mainly derived from deep gas dissolved in formation water and delivered through the permeable layers near the formation boundary to the surface.