四川盆地川中地区沙溪庙组致密含气砂岩地震预测方法及应用

Seismic prediction technology of tight sandstone gas reservoirs in Middle Jurassic Shaximiao Formation,central Sichuan Basin

四川盆地川中地区侏罗系沙溪庙组一段发育三角洲广覆式分布砂体,砂体孔隙度变化大、非均质性强、井震匹配差,传统基于叠后地震的砂体识别的,储层预测以及含气性检测方法在该层段内应用效果有限,制约了勘探开发研究的进展和效果。为支撑该地区井位目标优选,开展叠前地震属性定性预测与反演定量方法攻关,针对沙一段形成了一套致密砂岩气藏的叠前地震综合预测技术。研究结果表明:①中速砂体叠前振幅随入射角变化会出现极性反转情况,导致同向叠加后与传统代表自激自收的纵波阻抗标定(AI)存在相位差异,井震匹配效果较差,使用考虑了振幅随入射角变化情况的弹性阻抗标定(EI)可针对该类型砂体获得更好的井震匹配关系;②针对叠后地震反射复杂的砂体,使用叠前AVO截距(P)和梯度(G)之差反映的横波阻抗变化率属性以及叠前同时反演的横波阻抗结果完成砂体地定性和定量识别,砂体岩性预测吻合率显著提高,可达95%;③使用反映纵波阻抗变化率的叠前AVO截距(P)以及孔隙度反演结果对达到储层标准的砂体进行定性和定量识别,使用反映纵横波速度比的叠前AVO截距(P)和梯度(G)之和属性以及纵横波速度比反演结果对砂体含气性进行定性定量识别。根据不同类型砂体叠前属性反射特征,结合反演定量预测结果,本地震综合预测方法系列为川中地区沙一段探井部署提供了有力技术支撑,助力该层系致密砂岩气藏勘探开发取得新的突破。

There develops extensive delta sand bodies in the first member of Middle Jurassic Shaximiao Formation(Sha 1 Member)in central Sichuan Basin,characterized by great variation of porosity,strong heterogeneity and poor well-seismic matching.The traditional post-stack seismic-based methods for sand body identification,reservoir prediction and gas detection are inapplicable to the Sha 1 Member,which restricts the researches on the exploration and development.To support the well placement optimization in this area,a pre-stack seismic prediction technology for tight sandstone gas reservoir has been developed by comprehensive studies on prestack seismic attribute qualitative predication and quantitative inversion.Results indicate that,(1)the pre-stack amplitude changing with incidence angle of medium-velocity sand body exhibits polarity inversion,resulting in a phase diversity with traditional P-wave impedance calibration(which represents self-excitation and self-receiving)after in-phase stacking,and poor well-seismic matching results;in contrast,the elastic impedance(EI)calibration which considers the amplitude changing with incidence angle can obtain better well-seismic matching;(2)for sand bodies with complex post-stack seismic reflection,the S-wave impedance change rate reflected by the difference between the pre-stack AVO intercept(P)and gradient(G)and the S-wave impedance derived from pre-stack simultaneous inversion are used for qualitative and quantitative identification;it remarkably enhances the coincidence(up to 95%)with the lithologic prediction of sand bodies;and(3)for sand bodies which meet the standard of reservoir,the pre-stack AVO intercept(P)reflecting the P-wave impedance change rate and the inverted porosity are used for qualitative and quantitative identification.The sum of pre-stack AVO intercept(P)and gradient(G)which reflects the Vp/Vs and the inversion results of Vp/Vs are used for qualitative and quantitative identification of gas-bearing potential of the sand bodies.Based on the pre-stack attribute reflections of different types of sand bodies,together with the results of quantitative inversion,the proposed seismic prediction technology provides a powerful support for the exploratory well deployment in the Sha 1 Member,and further to achieve new breakthrough of tight sandstone gas exploration and development.