[M-IS32_30AM2] 地球掘削科学

[MIS32-09] IODP 第348 次航海の船上分析に基づいた南海トラフの炭化水素ガスの分布と起源

*淵田 茂司¹、Hammerschmidt Sebastian²、IODP 第348次航海乗船 研究者一同 ³ (1.大阪市立大学、2.University of Bremen、3.ウィスコンシン大学　他)

キーワード：IODP, Expedition 348, Nantrosize, hydrocarbon

The recent International Ocean Discovery Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Expedition 348 at Site C0002 drilled and cored successfully up to 3058.5 mbsf. During drilling and coring, hydrocarbon and other inorganic gas concentrations were monitored on board. Here, we will report the distribution and origin of the hydrocarbon gas in Holes C0002N (838 to 2330 mbsf) and C0002P (1954 to 3058 mbsf). Methane, ethane, and propane concentrations in the headspace gas were measured by Geoservices and by using the scientific drilling mud gas monitoring system onboard D/V Chikyu. Total gas concentrations were dominated by methane, with the highest concentrations of up to 8% at around 1305 mbsf. Downhole gas concentrations steadily decreased to values <0.2 %. Ethane and propane were only present in minor concentrations, and higher homologues (i.e. n-butane, i-butane, n-pentane, i-pentane) stayed typically below 0.01 %. Below 2200 mbsf, ethane and propane increase steadily with depth. Bernard diagram (i.e. Bernard parameter vs. d¹³C_CH4, Bernard et al., 1978) indicates that the gas in Hole C0002NP was gradually changed from biogenic to thermogenic with increasing depth. Headspace gas samples from cores in Hole C0002P (2160-2220 mbsf) were all dominated by methane, with up to 23455 ppm. Methane concentration in the headspace gas samples was higher than the drilling mud gas samples at the same interval. This underestimation of methane in the drilling mud is due to the influence of drilling parameter (e.g. rate of penetration), mud properties (e.g. mud weight) and degassing efficiency.