Gas hydrate, fluid flow and free gas: Formation of the bottom-simulating reflector
Gas hydrate, fluid flow and free gas: Formation of the bottom-simulating reflector
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DOI：
10.1016/j.epsl.2007.07.008
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发表时间：
2007-09-30
影响因子：
5.3
通讯作者：
Hyndman, Roy D.
中科院分区：
文献类型：
--
作者：
Haacke, R. Ross;Westbrook, Graham K.;Hyndman, Roy D.
大陆边缘的天然气水合物通常由海底几百米以下的一个突出的海底模拟地震反射器（BSR）来指示。BSR标志着上面含有天然气水合物的沉积物和下面含有游离气体的沉积物之间的界限。大部分反射振幅是由下伏自由气体引起的。然而，天然气水合物可以在没有BSR的情况下发生，并且对其形成的控制尚不清楚。本文描述了天然气水合物稳定带（GHSZ）下自由气聚集的两种互补机制。首先是公认的水合物再循环机制，当GHSZ的底部相对于含水合物的沉积物向上移动时，从解离的水合物中产生气体。第二个是最近确定的机制，其中溶解气体的平流和扩散与局部溶解度曲线之间的关系允许当水合物存在于GHSZ底部附近时，液相在GHSZ下方的厚层中变得饱和。这种产气机制（称为溶解度-曲率机制）在这样的系统中是可能的，在这些系统中，扩散的影响相对于平流的影响变得重要，并且气-水溶解度降低到GHSZ以下几百米的最低水平。我们研究了一些天然气水合物发生的地区，以确定天然气形成由溶解度-曲率机制主导的地方，以及由水合物再循环主导的地方。研究表明，前者在流体向上流动速率低（如古老的裂陷大陆边缘）、海底隆升速率低、地热梯度和/或压力高的地区占主导地位。相反，在流体向上流动和海底隆升速度较快的地方（如俯冲带增生楔），水合物再循环主导着游离气的形成。利用这两种机制来研究大陆边缘天然气水合物下游离气体的形成，我们能够解决许多问题。这些问题包括，为什么亚bsr无气带通常在会聚边缘较薄，而在被动边缘较厚，以及为什么墨西哥湾和麦肯齐三角洲等地区没有区域性bsr。最后，我们利用我们对BSR如何形成的理解来表明，在具有可观测BSR的地区之外的被动大陆边缘，区域天然气水合物不太可能大量存在。(c) 2007 Elsevier B.V.版权所有
Gas hydrate in continental margins is commonly indicated by a prominent bottom-simulating seismic reflector (BSR) that occurs a few hundred metres below the seabed. The BSR marks the boundary between sediments containing gas hydrate above and free gas below. Most of the reflection amplitude is caused by the underlying free gas. Gas hydrate can occur without a BSR, however, and the controls on its formation are not well understood. Here we describe two complementary mechanisms for free gas accumulation beneath the gas hydrate stability zone (GHSZ). The first is the well-recognised hydrate recycling mechanism that generates gas from dissociating hydrate when the base of the GHSZ moves upward relative to hydrate-bearing sediment. The second is a recently identified mechanism in which the relationship between the advection and diffusion of dissolved gas with the local solubility curve allows the liquid phase to become saturated in a thick layer beneath the GHSZ when hydrate is present near its base. This mechanism for gas production (called the solubility-curvature mechanism) is possible in systems where the influence of diffision becomes important relative to the influence of advection and where the gas-water solubility decreases to a minimum several hundred metres below the GHSZ. We investigate a number of areas in which gas hydrate occurs to determine where gas formation is dominated by the solubility-curvature mechanism and where it is dominated by hydrate recycling.We show that the former is dominant in areas with low rates of upward fluid flow (such as old, rifted continental margins), low rates of seafloor uplift, and high geothermal gradient and/or pressure. Conversely, free-gas formation is dominated by hydrate recycling where there are rapid rates of upward fluid flow and seabed uplift (such as in subduction zone accretionary wedges).Using these two mechanisms to investigate the formation of free gas beneath gas hydrate in continental margins, we are able to resolve a number of problems. These include why sub-BSR free-gas zones are generally thin in convergent margins and thick in passive margins, and why areas such as the Gulf of Mexico and Mackenzie Delta do not have regional BSR.s. Finally, we use our understanding of how BSRs are formed to show that regional gas hydrate is unlikely to exist in substantial quantities in passive continental margins outside areas with an observable BSR. (c) 2007 Elsevier B.V. All rights reserved.