全入射角叠前密度反演在白云岩优质储层预测中的应用

Application of full-angle prestack density inversion for deep tidal-flatthin dolomite reservoirs

川西坳陷三叠系雷口坡组雷四上亚段发育潮坪相低渗致密白云岩储层,该套储层埋藏深(5600~5900 m)、厚度薄(相对地震分辨率)、纵向多层叠置、横向储层厚度和物性变化快、储层与围岩波阻抗差异小,白云岩优质储层预测难度大。研究区测井数据的岩石物理分析结果表明,密度对该区优质储层最敏感。首先基于目的层储盖层的测井数据开展了Zoeppritz近似线性方程误差分析、AVO道集正演和全入射角道集叠前密度反演的可行性研究,并与基于部分叠加的叠前密度反演方法即叠前同时反演方法进行了对比。在对实际叠前偏移道集优化处理的基础上,采用适用于研究区的Zoeppritz近似线性方程,并优选参数(包括v_(P)/v_(S)、道集切除参数和入射角参数)开展全入射角叠前密度反演,得到高质量的密度数据体,并与部分叠加的叠前密度反演得到的密度数据体进行了对比。将全入射角叠前密度反演得到的密度反射率数据应用于后续的密度高分辨率反演,该反演结果得到了雷四上亚段两套优质储层预测结果的证实,11口后钻井揭示的储层厚度和物性的符合率均超过85%。

Low-permeability tidal-flat dolomite reservoirs occur in the upper part of the fourth member of the Triassic Leikoupo Formation in the western Sichuan Depression.It is very difficult to predict promising dolomite reservoirs due to large buried depth(5600~5900 m),small thickness(compared with seismic resolution),rapid lateral variations of reservoir thickness and porosity,and small P-impedance contrast between reservoirs and non-reservoirs.According to log-based petrophysical analysis,density is most sensitive to high-quality reservoirs.First,the error analysis of the Zoeppritz approximate equations is conducted using log data,followed by AVO modeling and a feasibility study of full-angle prestack density inversion.We also compare full-angle density inversion with prestack density inversion using partial stacks.Based on gather conditioning and parameter optimization(v_(P)/v_(S),muting parameter,and incident angle),we use the Fatti equation for full-angle prestack inversion to obtain high-quality density reflectivity,which is applied to high-resolution density inversion for the prediction of high-quality reservoir thickness and porosity.The accuracy of prediction(thickness and porosity)exceeds 85%for two target zones(TL 3-24 and TL 3-34)as per drilling results of 11 wells.