东海冲绳海槽泥火山发育区甲烷气体来源研究

ORIGIN OF PORE WATER METHANE RECOVERED FROM MUD VOLCANOS IN THE OKINAWA TROUGH

高沉积速率、构造活动发育和高热流值3个重要地质条件促使了冲绳海槽广泛发育泥火山、气烟囱等烃类流体渗漏构造,前人对该类泥火山及气烟囱的地球物理特征做过较多的研究,但是鲜有研究从地球化学角度揭示渗漏流体来源及形成机制。本研究通过对东海冲绳海槽中部泥火山发育区2个泥火山站位开展海底钻探取样,获取浅表层60 m沉积物并开展孔隙水烃类浓度、甲烷稳定碳、氢同位素研究。通过分析发现,18-01孔孔隙水顶空烃类比值C1/C2为960.53~1120.75,甲烷稳定碳同位素(δ^13CCH4)为-36.07‰^-56.60‰V-PDB,甲烷稳定氢同位素(δDCH4)为-163.94‰^-237.81‰V-SMOW;在18-05孔,孔隙水顶空烃类比值C1/C2为1064.66~1546.74,δ^13CC H4为-36.10‰^-62.92‰V-PDB,δDCH4为-122.86‰^-282.09‰V-SMOW。系统分析2个站位甲烷气源均为热解成因或以热解成因为主的混合成因。综合分析2个站位泥火山及气烟囱发育的地质背景以及高通量甲烷渗漏的特征认为,深部地层中有机热解成因甲烷流体是通过断层、气烟囱等运移通道,在流体超压的驱动下渗漏、扩散至浅表层地层中,并在海底形成了泥火山以及羽状流等构造。

Three important geological elements,i.e.high sedimentation rate,strong tectonic activity and high heat flow,promote the wide distribution of hydrocarbon fluid seeps and leakage structures such as mud volcanos and gas chimneys on the western slope of Okinawa Trough.Several previous studies have been conducted to reveal the mud volcano geomorphology and forming conditions,in particular the bottom simulation reflectors of gas hydrate accumulated in the mud volcano.However,rare researches focused on the geochemistry of the pore fluids and the origin of methane saturated in the pore water of sediments recovered from the mud volcano.In this work,seafloor drilling rig was deployed for two shallow holes to recover sediment and pore water samples from the mud volcanos.Geochemical analysis of pore water molecular ratios and isotopic compositions show that C1/C2 vary from 960.53 to 1120.75 in the core 18-01 and from 1064.66 to 1546.74 in the core 18-05,while the carbon isotopic values of pore water methane exhibit a variation from-36.07‰ to-56.60‰ V-PDB in the core 18-01 and from-36.10‰ to-62.92‰ V-PDB in the core 18-05 respectively.Combined with molecular ratios,stable carbon and hydrogen isotopic compositions,we conclude that the pore water methane is derived from thermal degradation of organic matter.As the result of conversion from smectite to illite induced overpressure in deep sediments,thermogenic methane was driven along gas chimney and tectonic conduits and migrated to the shallow sediments,then formed mud volcanos and associated methane plumes in the overlying water.