Study on Forward Numerical Simulation and Instantaneous Seismic Attributes of Natural Gas Hydrate in Permafrost Area
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摘要: 天然气水合物是一种具有巨大潜能的新型能源，研究冻土区天然气水合物的地震响应特征，对我国陆域天然气水合物的勘探和开发具有重要意义。本文运用Kelvin粘弹性介质模型，基于祁连山冻土区的实际地质地层条件，建立理论地质-地球物理模型；采用交错网格有限差分法进行正演数值模拟，并对自激自收地震记录进行波场特征分析和提取瞬时地震属性。研究结果表明：地震波通过天然气水合物地层时，反射振幅能量较弱；在瞬时频率属性剖面可分辨层厚的范围内，瞬时频率随着层厚增加，频率在小幅度衰减；地震波通过含天然气地层时，反射波表现为强反射特征，瞬时频率能量明显增大；瞬时地震属性对波阻抗界面有更好的分辨能力，特别是瞬时相位属性剖面，作用明显。因此，综合分析波场特征与瞬时属性特征可以为陆域天然气水合物的识别、预测提供依据。Abstract: Natural gas hydrate is a new energy source with great potential. Studying the seismic response characteristics of natural gas hydrate in permafrost area is crucial to the exploration and development of natural gas hydrate in China's land area. Based on the Kelvin viscoelastic media model, our work establishes a theoretical geological-geophysical model based on the actual geological and stratigraphic conditions of the Qilian Mountain permafrost, uses the staggered-grid finite difference method to perform forward numerical simulation, and performs wave field characteristic analysis and instantaneous seismic attribute extraction of self-excitation and self-receiving seismic records. The results show that the reflected amplitude energy is weaker when the seismic wave passes through the gas hydrate formation. In the range where the instantaneous frequency attribute profile can distinguish the layer thickness, the instantaneous frequency decreases marginally with the increase of the layer thickness. When seismic waves pass through natural gas-bearing formations, the reflected waves show strong reflection characteristics, and the instantaneous frequency energy increases. The instantaneous seismic attribute has better resolution for the wave-impedance interface, especially the instantaneous phase attribute profile. Therefore, the comprehensive analysis of the instantaneous attribute characteristics can provide a basis for the identification and prediction of terrestrial gas hydrates.
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表 1 祁连山DK-1、DK-3和DK-4三个井孔含水合物和不含水合物层段速度和密度[18]
Table 1. Velocities and densities of the hydrate segments and segments without hydrate in the holes of DK-1, DK-3, and DK-4
井孔 深度h/m 岩性 平均纵波速度Vp/（m/s） 平均密度ρ/（g/cm3） 含水合物段 不含水合物段 含水合物 不含水合物 含水合物 不含水合物 DK-1 133.90～134.86 92.05～94.75 细砂岩 4728 4204 2.34 2.53 143.35～144.30 49.20～69.70 细砂岩 4676 4171 2.38 2.57 DK-3 139.05～154.45 195.05～196.95 泥岩 2996 2867 2.32 2.43 DK-4 134.40～131.70 151.50～152.45 泥岩 4071 2822 2.25 2.31 165.75～167.25 112.45～115.35 粉砂岩 3823 3356 2.22 2.36 
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表 2 楔状地质-地球物理模型参数表
Table 2. The parameters of wedge-shaped geological-geophysical model
模型编号 Vp/（m/s） Vs/（m/s） ρ/（g/cm3） Qp Qs 层厚/m 冻土层① 3250 1950 2.31 187.2 139.5 54 稳定沉积物② 4000 2000 2.37 295.6 153.0 100 稳定沉积物③ 4450 2130 2.55 373.7 214.8 - 天然气水合物④ 4750 2330 2.29 431.4 373.7 0～40
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