基追踪弹性阻抗反演识别含气砂岩

Gas sand prediction using basis pursuit elastic impedance inversion

砂岩储层含气后地震波阻抗会显著降低,因此阻抗反演是对含气砂岩储层进行预测的常用方法。但砂岩储层的波阻抗值不仅取决于含流体性质,还与岩石的孔隙度、矿物结构组分等多种因素有关,即使砂岩储层含气,其波阻抗也可能高于高孔含水砂岩,因此,叠后地震阻抗反演结果在含气砂岩储层预测中存在较强的多解性。由于利用了含气砂岩的AVO异常特性,叠前弹性阻抗在含气砂岩储层预测中比叠后地震阻抗具有更高的可靠性。为了提高弹性反演结果的精度,将基追踪反演(basis pursuit inversion,BPI)算法引入叠前弹性阻抗反演。该方法是将地震信号投影到奇、偶子波库上,通过投影值求得每道的相对反射系数,然后进行道积分得到相对弹性波阻抗。相比于传统的稀疏脉冲反演(sparse spike inversion,SSI)算法,基追踪弹性反演算法不需要建立初始低频模型,可显著提高薄层分辨能力和反演精度。通过合成信号的试算和实际地震数据的反演表明,基追踪叠前弹性反演可有效地对砂岩储层的含气异常进行检测,并且相比于传统SSI算法具有更高的反演精度。

The impedance of sand reservoir will become lower if the pores are filled with natural gas,so the gas reservoir can be detected by using impedance inversion,which is a common used method.However,the impedance of sand reservoir can be influenced by many kinds of factors such as porosity and mineral composition.As a consequence,the impedance of gas sand may be higher than that of high porosity brine sandstone.In this case,mistake would be caused if post-stack impedance is only used to predict gas sand.Elastic impedance is more reliable than acoustic impedance,because gas sand reservoir can induce AVO anomaly.Simultaneously,in order to improve inversion resolution and accuracy,the authors introduce BP(Basis Pursuit)algorithm to complete elastic inversion.This algorithm is used to decompose seismic signal to even and odd wavelet dictionaries and then reflection coefficient can be obtained by the decomposition coefficients.The method proposed by the authors doesn't need initial low frequency model that traditional inversion method SSI(Sparse Spike Inversion)has to know beforehand.In this case,resolution and accuracy can be improved.The application to synthetic data and field data indicates that this method is more accurate than SSI.