Origin and in situ concentrations of hydrocarbons in the Kumano forearc basin from drilling mud gas monitoring during IODP NanTroSEIZE Exp. 319

• Published in: Applied geochemistry 2015-10, Vol.61, p.206-216
• Main Authors: Wiersberg, Thomas, Schleicher, Anja M., Horiguchi, Keika, Doan, Mai-Linh, Eguchi, Nobuhisa, Erzinger, Jörg
• Format: Article
• Language: English
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2015.06.002

•Exp. 319 of IODP was the first cruise in the history of scientific ocean drilling with drilling mud gas monitoring.•Hydrocarbons were the only formation-derived gases identified in drilling mud.•Chemical and isotopic compositions of hydrocarbons exhibit a microbial origin.•Absolute CH4 concentrations in the formation reaching up to 24Lgas/Lsediment. NanTroSEIZE Exp. 319 of the Integrated Ocean Drilling Program (IODP) was the first cruise in the history of scientific ocean drilling with drilling mud circulation through a riser. Drilling mud was pumped through the drill string and returned to the drill ship through the riser pipe during drilling of hole C0009A from 703 to 1604mbsf (meter below sea floor) and hole enlargement from 703 to 1569mbsf. During riser drilling, gas from returning drilling mud was continuously extracted, sampled and analyzed in real time to reveal information on the gas composition and gas concentrations at depth. Hydrocarbons were the only formation-derived gases identified in drilling mud and reached up to 14vol.% of methane and 48ppmv of ethane. The chemical and isotopic compositions of hydrocarbons exhibit a microbial origin. Hydrocarbons released from drilling mud and cuttings correlate with visible allochthonous material (wood, lignite) in drilling cuttings. At greater depth, addition of small but increasing amounts of hydrocarbons probably from low-temperature thermal degradation of organic matter is indicated. The methane content is also tightly correlated with several intervals of low Poisson’s ratio from Vp/Vs observed in sonic velocity logs, suggesting that the gas is situated in the pore space of the rock as free gas. The gas concentrations in the formation, determined from drilling mud gas monitoring, reaching up to 24Lgas/Lsediment for methane in hole C0009A, in line with gas concentrations from interpreted downhole sonic logs.