Sediments with gas hydrates: Internal structure from seismic AVO
Sediments with gas hydrates: Internal structure from seismic AVO
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DOI：
10.1190/1.1444462
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发表时间：
1998-09-01
期刊：
影响因子：
3.3
通讯作者：
Nur, A
中科院分区：
文献类型：
--
作者：
Ecker, C;Dvorkin, J;Nur, A
利用基于岩石物理的合成地震模型，对佛罗里达近海海底模拟反射层（BSR）的振幅随炮检距变化（AVO）资料进行了解释。先前对现场地震数据进行的速度和AVO分析表明，BSR将含水合物沉积物与含游离甲烷的沉积物分开。BSR处的振幅随着偏移的增加而越来越负。这一现象的解释是：BSR上方的纵波速度大于BSR下方的纵波速度，而BSR上方的横波速度小于BSR下方的横波速度。我们使用这些AVO和速度的结果来推断的内部结构的水化沉积物。要做到这一点，我们研究了两个微观力学模型，对应于两个极端的情况下，水合物沉积在孔隙空间：（1）水合物水泥颗粒接触，并强烈加强沉积物，（2）水合物位于远离颗粒接触，并不影响刚度的沉积物框架。只有第二个模型才能定性地再现所观测到的AVO响应。由此推断的含水合物沉积物的内部结构意味着：（1）BSR上方的沉积物未胶结，因此机械强度较弱;（2）由于水合物堵塞了大孔隙通道，其渗透率非常低。后者解释了为什么自由气体被困在BSR下面。地震数据还表明，在水合物层的顶部没有强烈的反射。这一事实表明，在BSR上方的沉积物中的高浓度的水合物随着深度的减少而逐渐减少。这种效应与BSR上方的低渗透性水合沉积物阻止游离甲烷向上迁移的事实一致。
We interpret amplitude variation with offset (AVO) data from a bottom simulating reflector (BSR) offshore Florida by using rock-physics-based synthetic seismic models. A previously conducted velocity and AVO analysis of the in-situ seismic data showed that the BSR separates hydrate-bearing sediments from sediments containing free methane. The amplitudes at the BSR are increasingly negative with increasing offset. This behavior was explained by P-wave velocity above the BSR being larger than that below the BSR, and S-wave velocity above the BSR being smaller than that below the BSR. We use these AVO and velocity results to infer the internal structure of the hydrated sediment. To do so, we examine two micromechanical models that correspond to the two extreme cases of hydrate deposition in the pore space: (1) the hydrate cements grain contacts and strongly reinforces the sediment, and (2) the hydrate is located away from grain contacts and does not affect the stiffness of the sediment frame. Only the second model can qualitatively reproduce the observed AVO response. Thus inferred internal structure of the hydrate-bearing sediment means that (1) the sediment above the BSR is uncemented and, thereby, mechanically weak, and (2) its permeability is very low because the hydrate clogs large pore-space conduits. The latter explains why free gas is trapped underneath the BSR. The seismic data also indicate the absence of strong reflections at the top of the hydrate layer. This fact suggests that the high concentration of hydrates in the sediment just above the BSR gradually decreases with decreasing depth. This effect is consistent with the fact that the low-permeability hydrated sediments above the BSR prevent free methane from migrating upwards.