论煤层气AVO分析对地震勘探偏移距的要求

使用Zeoppritz方程组弹性模量近似式研究了煤层气储层顶板界面P波反射系数R(θ)随入射角和折射角平均值θ的变化及梯度d(R(θ))/dθ，并与天然气储层顶板界面的对应量比较，以确定煤层气AVO分析对地震资料偏移距的要求。由于煤层气储层顶板界面的d(R(θ))/dθ有极值点，而天然气储层顶板界面R(θ)是随θ单调变化的，因此，煤层气AVO分析对地震资料偏移距的要求不同于天然气AVO分析，小偏移距也能用来做煤层气AVO。建议根据煤层气储层顶板界面d(R(θ))/dθ的极值点确定煤层气AVO分析需要的最佳偏移距，并结合实例确定了最佳偏移距的3种方法。

Requirements of seismic offset data for CBM AVO analysis

In order to determine the optimal offset for CBM AVO(amplitude versus offset) analysis，the variation of P-wave reflection coefficient R(θ) versus θ，the average of incidence and transmission angles，the top boundary of a CBM (Coalbed Methane) reservoir and its derivative d(R(θ))/dθ are explored with the help of geomodulus approximation of Zoeppritz equations，and compared with their counterparts of a C-gas (conventional gas) reservoir.d(R(θ))/dθ has extreme value for the top boundary of a CBM reservoir，while R(θ) at the top boundary of a C-gas reservoir varies monotonously with increment in θ.Thus，the requirement for maximum offset by CBM AVO analysis is different from that required by C-gas AVO analysis，the rule for C-gas AVO analysis，i.e.，“the bigger the maximum offset the better the AVO analysis achievements”，is not applicable for CBM AVO analysis，and the small offset can be used for CBM AVO.It is suggested that the optimal offset for CBM AVO analysis be determined according to the extreme value of d(R(θ))/dθ.Three methods for determining the optimal offset for CBM AVO analysis are proposed in this study.