Japan Geoscience Union Meeting 2014

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Symbol M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS32_30AM2] Drilling Earth Science

Wed. Apr 30, 2014 11:00 AM - 12:45 PM 416 (4F)

Convener:*Saneatsu Saito(Japan Agency for Marine-Earth Science and Technology), Ken Ikehara(Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology), Tetsuro Hirono(Department of Earth and Space Science, Graduate School of Science, Osaka University), Keita Umetsu(Japan Agency for Marine-Earth Science and Technology), Chair:Keita Umetsu(Japan Agency for Marine-Earth Science and Technology), Yusuke Yokoyama(Atmosphere and Ocean Research Institute, University of Tokyo)

11:15 AM - 11:30 AM

[MIS32-09] Determination of hydrocarbon gas in drilling mud and cores during Expedition 348 at the Nankai Trough, Japan

*Shigeshi FUCHIDA1, Sebastian HAMMERSCHMIDT2, EXPIDITION 348, Shipboard scientists 3 (1.Osaka City University, 2.University of Bremen, 3.University of Wisconsin and others)

Keywords: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. d13CCH4, 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.