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.

中文翻译：

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实时钻井泥浆气监测，用于定性评估南海槽熊野盆地深海钻井过程中的烃气成分。

背景技术综合海洋钻探计划探险队338是D / V Chikyu进行的第二次科学探险，其间进行了立管钻探，作为南海海槽地震发生带实验的一部分。竖井钻探通过车载科学钻探泥浆气监测系统（“ SciGas”）实现了对钻探泥浆气的采样和实时监测。第二套独立的系统由商业录井服务Geoservices提供。两种系统都可以测定（非）碳氢化合物气体，而SciGas系统还可以监测甲烷碳同位素比（δ（13）CCH4）。碳氢化合物气体成分主要由甲烷（> 1％）组成，而乙烷和丙烷则降低了两个数量级。δ（13）CCH4值表明在不超过海床以下1600米处出现了产热气体。这项研究旨在通过对钻井泥浆气样品进行岸基分析来评估机载数据和后续的地质解释。结果在船上对泥浆气的监测中，SciGas和Geoservices系统分别记录了高达8.64％和16.4％的甲烷。在SciGas系统中，乙烷和丙烷的浓度分别达到0.03％和0.013％，但在Geoservices数据中分别达到0.09％和0.23％。通过气相色谱法对离散样品进行岸基分析表明，气体成分中含约0.01至1.04％的甲烷，2-18 ppmv的乙烷和2-4 ppmv的丙烷。四重质谱对甲烷的分析结果相似（0.04至4.98％）。δD值在-171‰至-164‰之间，甲烷的稳定氢同位素组成几乎没有井下变化。结论尽管两个独立的泥浆气监测系统和基于海岸的离散气体样本分析产生了不同的绝对浓度，但它们在烃类气体的井下变化方面都非常吻合。数据指向主要是生物甲烷源，但暗示可能是由于混合作用，而热源在深度上有一定贡献。根据在81°C至85°C之间的原位温度估计值和0.23的累积时间-温度指数，不太可能在2000 mbsf的较浅深度进行原位产热气体的生产。总之，机载SciGas数据采集有助于对气体成分，原位温度和气体迁移的可能性进行初步的定性评估。