P-SV波AVO响应特征分析

Analysis of P-SV wave AVO response feature

本文首先回顾了P-P波AVO响应特征及其分类,随后从P-SV波反射系数及振幅特征、归一化的P-SV波反射系数及截距-梯度特征、地层参数变化趋势与AVO类型的关系等方面系统地阐述了P-SV波AVO响应特征。文中选用(Castagna、Hilterman给出的四种AVO类型的岩性参数,利用佐普里兹方程计算了P-SV波反射系数,得出以下认识:⑴对于不同类型气层,P-SV波AVO响应特征比较复杂,利用单一的反射系数曲线和振幅特征无法同时区分四类AVO类型,表明P-SV波AVO响应对储层流体成分并不敏感;②通过引入伪转换波阻抗的概念,归一化反射系数曲线呈现单调性,同时放大了弱AVO异常。对于Ⅰ、Ⅱ、Ⅲ类AVO类型的气层,无论储层含水还是含气,垂直入射时归一化反射系数随入射角单调递增;对于Ⅳ类AVO类型的气层,储层流体的性质不影响反射系数的变化趋势,反射系数都为正值且随入射角单调递减;③Ⅰ、Ⅱ类气层具有负截距和正梯度,Ⅲ类气层的截距和梯度都接近于零,Ⅳ类气层具有正截距和负梯度。文中利用P-SV波AVO截距-梯度理论对不同AVO类型和Castagna给出的25种含(油)气砂岩模型进行了截距-梯度分析,进一步验证了P-SV波AVO响应特征及其对不同气层的分辨能力。本文的研究对AVO正演模拟、储层流体替换、多波AVO分析和参数反演都具有重要意义。

The paper first reviewed the P-P wave AVO response feature and its classification, then systematically expounded the P-SV wave AVO response feature from P-SV wave reflection coefficients and amplitude feature, normalized P-SV wave reflection coefficients and intercept-gradient feature and relationship between the variation trends of strata parameters and AVO types. The paper gained the following knowledge by selecting lithological parameters of 4 AVO types given by Castagna and Hilterman and using Zoeppritz equation to compute the P-SV reflection coefficients: (1) there is more complex P-SV wave AVO response feature in different kind of gas-beds, using single reflection coefficient curve and amplitude feature cannot differentiate from each other among 4 AVO types, which indicated the insensitive to fluids component of reservoir by P-SV wave AVO response; the normalized reflection coefficient curve is monotonic character when introducing the conception of pseudo converted wave impedance and amplifies weak AVO anomaly, which showed that the normalized reflection coefficients in vertical incidence are monotone increasing with incident angles for I, II and III AVO types either in water-bearing reservoir or in gas-bearing reservoir; for IV AVO type of gas beds, the feature of fluids in the reservoir does not affect the variation trends of reflection coefficients that are positive values and monotone decreasing with incident angle; (3) I and I types of gas beds are characteristics of negative intercepts and positive gradients, the intercepts and gradients of I type of gas beds approach to zero and IV type of gas beds are characteristics of positive intercepts and negative gradients. The paper uses P-SV wave AVO intercept-gradient theory to implement the intercept-gradient analysis for different AVO types and 25 gas-(or oil-) bearing sandstone samples given by Castagna, which further proved the P-SV wave AVO response feature and the ability to identify different gas-beds. The study in the paper has important meaning for AVO forward modeling, replacement of reservoir fluids, AVO analysis and parameters inversion of multi-waves.