METHODS FOR ESTIMATION OF GAS HYDRATE AND FREE GAS SATURATIONS AND APPLICATION TO THE NORTHERN SLOPE OF SOUTH CHINA SEA
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摘要: 饱和度是评价天然气水合物资源量的重要参数之一，不同海域由于水合物微观赋存状态差异，评价方法不同。大量水合物钻探取心显示在珠江口盆地不同海域发现了不同类型的水合物，在东部海域发现了呈脉状、块状、结核状等裂隙内形成肉眼可视的水合物，而神狐海域发现了呈分散状充填在孔隙空间不同饱和度的水合物，两种不同类型水合物层的测井异常响应特征不同，评价饱和度方法不同。研究表明东部海域基于各向异性电阻率模型估算的水合物饱和度与压力取心吻合，近海底发育的水合物层饱和度相对较低，约为10%，而基于各向同性模型估算水合物饱和度高达60%，远大于压力取心饱和度。在碳酸盐岩层，基于未固结地层各向同性与各向异性估算水合物饱和度大于氯离子异常估算水合物饱和度，而利用胶结模式估算水合物饱和度与氯离子吻合。利用地震资料，以测井为约束利用稀疏脉冲反演地层的速度或波阻抗，结合不同岩石物理模型分析，再通过线性拟合或循环迭代方法可以估算水合物饱和度体，研究水合物的空间分布。通过沿稳定带底界提取水合物饱和度属性，发现神狐海域水合物呈不均匀分布，在局部峡谷脊部呈条带状特征。Abstract: Saturation is an important parameter for gas hydrate resource assessment. The methods for estimating gas hydrate saturation vary greatly according to the differences in morphological characteristics of gas hydrate and its contact relation with sediments. Fracture-filled gas hydrate occurred in forms of a vein, patch, or nodule has been found in the east part of the Pearl River Mouth Basin. However, the gas hydrate occurred as pore-fillings has been identified in the pressure core sample collected from the Shenhu area. The well log responses to fracture-filling and pore-filling gas hydrate are quite different indeed, and they show anisotropic and isotropic appearances respectively. Gas hydrate saturation estimated with anisotropic resistivity model fits well with gas hydrate saturation estimated by pore-water freshening. The gas hydrate saturation near seafloor is about 10% estimated according to anisotropic resistivity. However, it may reach about 60% if estimated from isotropic resistivity which is obviously greater than that estimated from pressure core degassing. Gas hydrate saturations in carbonate layers are similar no matter they are estimated with anisotropic and isotropic resistivity or velocity, which are higher than those estimated from chloride. However, the gas hydrate saturation at this layer estimated from cementation model C1 is near to the value estimated from chloride. The acoustic impedance or P-wave velocity was obtained from the constrained sparse spike inversion. The spatial distribution of gas hydrate saturation can be calculated with linear or iterative forward inversion of rock physics model based on seismic data and well log data. Gas hydrate saturation extracted from the base of gas hydrate stability zone with a time window of 50 ms showed a patchy or nonuniform distribution pattern in the Shenhu area. However, gas hydrate saturation showed linear distribution at the crest of canyons.
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Keywords:
- gas hydrate /
- saturation /
- anisotropic /
- isotropic /
- velocity /
- resistivity
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图 1 水合物微观形态及其与沉积物颗粒之间接触关系
Figure 1. Morphological characteristics of gas hydrate and its contact relation with sediments
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图 2 不同井位含水合物层电阻率与速度异常
Figure 2. The anomalies of resistivity and P-wave velocity at different sites
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图 3 GMGS2-08井井径、伽马、电阻率、纵波速度、密度和密度孔隙度测井曲线
Figure 3. The well logs from site GMGS2-08 showing the caliper, gamma ray, resistivity, P-wave velocity, density and density porosity
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图 4 各向同性电阻率(蓝线)、各向异性电阻率(黑线)与压力取心估算水合物饱和度
Figure 4. Gas hydrate saturation estimated from isotropy resistivity (blue line), anisotropy resistivity and pressure core degassing
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图 5 GMGS2-08井基于各向同性模型估算水合物饱和度[8]
Figure 5. Gas hydrate saturation estimated from isotropic velocity model at Site GMGS2-08
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图 6 GMGS2-08井基于各向异性速度模型估算水合物饱和度[8]
Figure 6. Gas hydrate saturation estimated from anisotropic velocity model at Site GMGS2-08
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图 7 基于各向异性模型，地层因子与速度联合反演GMGS2-08井饱和度与裂隙倾角
Figure 7. Gas hydrate saturation estimated from the pseudo joint inversion of formation factor and velocity using anisotropic model at Site GMGS2-08
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图 8 GMGS1航次不同站位波阻抗或纵波速度随电阻率反演水合物饱和度交会
Figure 8. The cross plots between acoustic impedance and P-wave velocity and gas hydrate saturation estimated from resistivity at GMGS1 expedition
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图 9 基于循环迭代反演水合物饱和度及沿稳定带底界(虚线)50 ms提取的水合物饱和度均方根振幅值
Figure 9. Gas hydrate saturation estimated from interative forward inversion and the root-mean-square attribute map extracted from a time window 50 ms along the base of gas hydrate stability zone (broken line)
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