title: Real-time drilling mud gas monitoring for qualitative evaluation of hydrocarbon gas composition during deep sea drilling in the Nankai Trough Kumano Basin.
abstract: BACKGROUND
  Integrated Ocean Drilling Program Expedition 338 was the second scientific expedition with D/V Chikyu during which riser drilling was conducted as part of the Nankai Trough Seismogenic Zone Experiment. Riser drilling enabled sampling and real-time monitoring of drilling mud gas with an onboard scientific drilling mud gas monitoring system ("SciGas"). A second, independent system was provided by Geoservices, a commercial mud logging service. Both systems allowed the determination of (non-) hydrocarbon gas, while the SciGas system also monitored the methane carbon isotope ratio (δ(13)CCH4). The hydrocarbon gas composition was predominated by methane (> 1%), while ethane and propane were up to two orders of magnitude lower. δ(13)CCH4 values suggested an onset of thermogenic gas not earlier than 1600 meter below seafloor. This study aims on evaluating the onboard data and subsequent geological interpretations by conducting shorebased analyses of drilling mud gas samples.
  
  
  RESULTS
  During shipboard monitoring of drilling mud gas the SciGas and Geoservices systems recorded up to 8.64% and 16.4% methane, respectively. Ethane and propane concentrations reached up to 0.03 and 0.013%, respectively, in the SciGas system, but 0.09% and 0.23% in the Geoservices data. Shorebased analyses of discrete samples by gas chromatography showed a gas composition with ~0.01 to 1.04% methane, 2 - 18 ppmv ethane, and 2 - 4 ppmv propane. Quadruple mass spectrometry yielded similar results for methane (0.04 to 4.98%). With δD values between -171‰ and -164‰, the stable hydrogen isotopic composition of methane showed little downhole variability.
  
  
  CONCLUSIONS
  Although the two independent mud gas monitoring systems and shorebased analysis of discrete gas sample yielded different absolute concentrations they all agree well with respect to downhole variations of hydrocarbon gases. The data point to predominantly biogenic methane sources but suggest some contribution from thermogenic sources at depth, probably due to mixing. In situ thermogenic gas production at depths shallower 2000 mbsf is unlikely based on in situ temperature estimations between 81°C and 85°C and a cumulative time-temperature index of 0.23. In conclusion, the onboard SciGas data acquisition helps to provide a preliminary, qualitative evaluation of the gas composition, the in situ temperature and the possibility of gas migration.
authors: MARUM, University of Bremen, Leobener Str., Bremen, 28359 Germany.
authors: Sebastian B
authors: SB
authors: Hammerschmidt
authors: GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Germany.
authors: Thomas
authors: T
authors: Wiersberg
authors: MARUM, University of Bremen, Leobener Str., Bremen, 28359 Germany.
authors: Verena B
authors: VB
authors: Heuer
authors: MARUM, University of Bremen, Leobener Str., Bremen, 28359 Germany.
authors: Jenny
authors: J
authors: Wendt
authors: GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Germany.
authors: Jörg
authors: J
authors: Erzinger
authors: MARUM, University of Bremen, Leobener Str., Bremen, 28359 Germany.
authors: Achim
authors: A
authors: Kopf
keywords: Accretionary prism; Drilling mud gas monitoring; Hydrocarbon gas; IODP; Kumano basin; Mud logging; NanTroSEIZE; Nankai Trough; Riser drilling
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