Pore water anomalies of submarine gas-hydrate zones as tool to assess hydrate abundance and distribution in the subsurface
Pore water anomalies of submarine gas-hydrate zones as tool to assess hydrate abundance and distribution in the subsurface
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DOI：
10.1016/s0012-8252(02)00117-4
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发表时间：
2003-04
影响因子：
12.1
通讯作者：
R. Hesse
中科院分区：
文献类型：
--
作者：
R. Hesse
在过去十年中，对天然气水合物研究最重要的新贡献来自大洋钻探计划的发现，特别是112、141、146和164航次，以及较小程度上的131、160、170和186航次。特别是，第164航次致力于在西大西洋的布莱克海岭天然气水合物油田进行天然气水合物钻探，这是一项前所未有的多学科研究工作[Paull等人，1996.海洋钻探程序计划，初始报告164，623页;保罗CK，松本，R.，华莱士，P. J.，Dillon，W.P.（Eds.），2000年a。海洋钻探程序程序科学结果164，大洋钻探方案，德克萨斯州学院站，459页]。与天然气水合物有关的水化学研究的进展中最重要的是，人们越来越认识到扩散调制平流过程在水合物带中形成化学和同位素孔隙水剖面的重要性。首先是从钻探（含水合物的）秘鲁和（无水合物的）巴巴多斯活动陆缘获得的平流的定性证据，接着是南海海槽加积棱体、日本海沟斜坡和卡斯卡迪亚及哥斯达黎加活动陆缘，最后是Egeberg和狄更斯[《地质化学》153（1999年）53]的定量平流扩散模型。布莱克山脊的被动边缘主动和被动边缘的对流流态是不同的，因为在主动边缘的流有一种趋势，即沿着叠瓦状冲断岩席楔形体中的沿着向陆倾斜的冲断面和断层集中，并在孔隙水剖面的阶梯模式中找到表达。对于被动陆缘和一些活动陆缘，平流来自钻孔剖面下方的源，通过近垂直断层（被动陆缘）或来自滑脱带（活动陆缘）。我们已经了解到，众所周知的耦合孔隙水异常归因于水合物解离-向下氯度降低与δ 18 O增加的结合[黑森和哈里森，1981年，地球行星。Sci. Lett. 55（1981）453.]在水合物存在的情况下，同位素效应不一定同时发生，因为同位素效应可能被其他反应（如火山灰蚀变或低δ 18 O流体的平流）的影响所叠加。然而，如果异常出现，水合物几乎总是存在的（平流低Cl −/高δ 18 O沃茨除外）。平流和扩散的影响，允许成功地模拟简单的水合物影响的孔隙水剖面被动的利润。建模工具是氯同位素，它提供了一个有效的工具来评估平流率。Egeberg和狄更斯模型允许估计地下的水合物浓度和分布，因为它分离了平流、扩散和水合物分解的影响，但关键取决于在原位压力和温度条件下采集的样品。模拟活动边缘更复杂的孔隙水剖面是未来的一个挑战。与估算水合物浓度的地球物理方法相比，地球化学方法给出的量最小。
The most significant new contributions to the study of natural gas hydrates in the past decade have come from findings of the Ocean Drilling Program (ODP), notably legs 112, 141, 146 and 164, and to a lesser extent legs 131, 160, 170 and 186. Leg 164, in particular, was dedicated to gas-hydrate drilling in the Blake Ridge gas-hydrate field in the West Atlantic in an unprecedented multidisciplinary research effort [Paull et al., 1996. Proc. Ocean Drill. Program, Initial Rep. 164, 623 pp.; Paull C.K., Matsumoto, R., Wallace, P.J., Dillon, W.P. (Eds.), 2000a. Proc. Ocean Drill. Program Sci. Results 164, Ocean Drilling Program, College Station, TX, 459 pp.]. Most important for the progress of hydrochemical studies related to gas hydrates has been the growing awareness of the significance of diffusion-modulated advective processes shaping the chemical and isotopic pore water profiles in hydrate zones. This started with qualitative evidence for advective flow from drilling the (hydrate-bearing) Peru and (hydrate-free) Barbados active margins, continued with the Nankai Trough accretionary prism, the Japan Trench Slope and the Cascadia and Costa Rica active margins and culminated in the quantitative advection-diffusion model of Egeberg and Dickens [Chem. Geol. 153 (1999) 53.] for the passive margin setting of the Blake Ridge. Advective flow regimes are different at active and passive margins, as there is a tendency for the flow at active margins to be focussed along landward-dipping thrust planes and faults in the wedge of imbricated thrust sheets that finds expression in a step-pattern of the pore water profiles. For passive margins, but also for some active margin sites, advection is from sources below the drilled section, either through subvertical faults (passive margins) or from the décollement zone (active margins). We have learned that the well-known coupled pore water anomalies that are ascribed to hydrate dissociation—downward chlorinity decrease combined with δ18O increase [Hesse and Harrison, 1981, Earth Planet. Sci. Lett. 55 (1981) 453.]—need not occur together in the presence of hydrates because the isotope effect may be overprinted by the effects of other reactions such as volcanic ash alteration or by the advection of low-δ18O fluids. However, if the anomalies show up, hydrates are present almost invariably (with the exception of advected low-Cl−/high-δ18O waters). Coming to terms with the effects of advection and diffusion has allowed successful modeling of the simpler hydrate-affected pore water profiles at passive margins. Instrumental for modeling are chlorine isotopes, which provide an effective tool to assess advection rates. The Egeberg and Dickens model allows estimation of hydrate concentration and distribution in the subsurface because it separates the effects of advection, diffusion and hydrate dissociation but critically depends on samples taken under in situ pressure and temperature conditions. Modeling the more complex pore water profiles of active margins is a challenge for the future. Compared to geophysical methods to estimate hydrate concentration, the geochemical method gives minimum amounts.