Gas hydrate estimates in muddy sediments from the oxygen isotope of water fraction
Gas hydrate estimates in muddy sediments from the oxygen isotope of water fraction
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根据水部分的氧同位素估算泥质沉积物中的天然气水合物
DOI：
10.1016/j.chemgeo.2017.08.027
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发表时间：
2017
期刊：
影响因子：
3.9
通讯作者：
Matsumoto Ryo
中科院分区：
文献类型：
--
作者：
Kano Akihiro;Miyahara Reina;Yanagawa Katsunori;Mori Taiki;Owari Satoko;Tomaru Hitoshi;Kakizaki Yoshihiro;Snyder Glen;Shimono Takaya;Kakuwa Yoshihiro;Matsumoto Ryo
深海沉积物中的天然气水合物是一种潜在的能源，一直是钻探研究的重点。然而，直接评估海洋沉积物中天然气水合物的数量一直是困难的，因为在船上回收过程中，由于压力下降和温度上升，回收的沉积物岩心中的天然气水合物至少部分解离。在这项研究中，我们应用了一种新的方法，基于水合物和泥浆亚样(分别为δ18OH和δ18OM)的H2O组分的氧同位素组成，以评估在日本近海Joetsu和Oki附近采集的岩心剖面中天然气水合物的体积百分比，这些岩心剖面在半深海泥浆中含有不同组构的水合物。我们测量了从岩心沉积物中仔细分离并密封在玻璃瓶中的小尺寸(通常为0.3立方厘米)子样与H2O平衡的CO2的同位素组成。天然气水合物的体积百分比(H in%)是根据泥浆亚样的孔隙度和从一定长度的岩心沉积物(通常为20 cm)中挤压出来的主体孔隙水(δ18OPW)的氧同位素组成来确定的，其中包括分解的水合物。在检查的29个岩心剖面中，有28个表明了三个组分的同位素值之间的关系，这三个组分是δ180Hgt；δ18OPW>；δ18Om，正如从水合物组分中富含18O的同位素分馏所预期的那样。28个剖面的H值范围为1.0%~95.4%，大部分剖面的H值明显大于岩心图像上水合物分布的估计值。我们的新方法能够以一种简单的方式校正由于岩心搬运过程中的解离而导致的水合物量的低估。我们的水合物和泥浆样品的氧同位素数据与假定在封闭系统中进行瑞利分馏的同位素演化曲线很不吻合。这意味着孔隙水的同位素组成可能是由于周围沉积物中18O贫化程度较低的孔隙水的扩散和平流而均一化的。在朱埃苏遗址的一些剖面中，怀疑泥浆基质中存在微尺度水合物，这表明δ180H和δ180M之间存在微小的差异，顶空气体中的CH2/CO2比值很高。我们认为，如果在船上进行仔细而快速的采样，这种方法适合于根据海泥中存在的水合物数量来估计天然气水合物作为一种能源。
Gas hydrate in deep-sea sediments is a potential energy resource, and has been the focus of extensive drilling research. However, direct evaluation of the amount of the gas hydrate in marine sediments has been difficult because the gas hydrate in recovered sediment cores is at least partly dissociated due to the drop in pressure and increase in temperature during onboard recovery. In this study, we apply a new method based on oxygen isotopic composition of the H2O fraction of both hydrate and mud sub-samples (δ18OHand δ18OM, respectively) in order to evaluate the volume percentage of gas hydrate in core sections collected from the Japan Sea off Joetsu and Oki, which contain different fabrics of the hydrate within hemipelagic mud. We measured isotopic composition of CO2equilibrated with H2O of the sub-samples of a small size (typically 0.3 cm3) carefully separated from the core sediments and sealed in glass vials. The volume percentage of gas hydrate (H in %) was determined using porosity of the mud sub-samples and oxygen isotopic composition of the bulk pore water (δ18OPW) squeezed from a certain length of core sediment (typically 20 cm) including dissociated hydrate. 28 out of the 29 examined core sections indicate the relation in the isotopic values of the three components, δ18OH> δ18OPW> δ18OM, as expected from isotopic fractionation that enriches18O in the hydrate component. Evaluated H-values of the 28 sections ranged from 1.0% to 95.4% and, for most of the section, the H-value was clearly larger than the value estimated by the hydrate distribution on core images. Our new method can, in a simple manner, correct for the underestimation of hydrate amount caused as a result of dissociation during core handling. Our oxygen isotopic data of the hydrate and mud sub-samples fits poorly with the isotopic evolutional curve that assumes Rayleigh fractionation in a closed system. This implies that the pore water isotopic composition may have been homogenized by diffusion and advection of less18O-depleted pore water from the surrounding sediments. Presence of micro-scale hydrate in the mud matrix was suspected for some sections from the Joetsu site, which present a small difference between δ18OHand δ18OMas well as high CH2/CO2ratios in headspace gas. We suggest that this method, if carried out with careful and quick onboard sampling, is appropriate for the estimation of gas hydrate as an energy resource based on the amount of hydrate present in marine mud.