韵律型薄互层视各向异性的适用性分析

Thin inter-bed is an important reservoir type of continental hydrocarbon-bearing basins in China. For gas-oil exploration and exploitation, it is a key and difficult problem about quantitative analysis of thin inter-beds by seismic data. Due to numerous elastic parameters involved, super underdetermined problem limits thin-bed inversion. Previous works show that, under the assumption that the dominant wavelength is long enough compared to the layer thicknesses, thin layered medium can be replaced by an equivalent non-dispersive transversely isotropic medium, i.e. apparent anisotropy of thin inter-beds. The existing research results about apparent anisotropy lie on the three aspects as follow: In the theoretical research, approximation formulas of apparent anisotropy are derived. In the rock physical tests, velocity anisotropy characteristics are just verified by several simple rhythm thin inter-beds. In the numerical simulation, applicability analysis of apparent anisotropy is almost about thin-bed thickness. However, lack of comprehensive applicability analysis of thickness, elastic parameters of the surrounding rocks and thin beds, frequency limits the application of apparent anisotropy. For typical and representative rhythm thin inter-beds in China, we approximate elastic parameters of thin inter-beds by apparent anisotropy and simulate thin-bed approximate seismic responses by the reflectivity theory of anisotropic medium. Compared with accurate seismic responses simulated by thin-bed reflectivity theory, we discuss the comprehensive applicability of apparent anisotropy systematically from different perspectives, such as thin-bed thickness, inter-bed structure, elastic parameters and frequency. It aims at higher precision approximation formulas and comprehensive applicability condition of the rhythm thin inter-beds, which may provide theory guidance for the seismic inversion of the rhythm thin inter-beds.

薄互层储层是我国陆相含油气盆地的重要储层类型之一，利用地震资料对其进行定量的预测是油气勘探开发需要解决的重点和难点问题。由于薄互层涉及弹性参数众多，超欠定问题使得薄互层反演难以实现。前人的研究已发现，在地震长波长假设下，薄互层可以等效为横向各向同性介质，即薄互层视各向异性。但已有的研究成果仅给出了视各向异性近似公式；岩石物理实验也仅从几种简单韵律型薄互层的角度证实了速度的各向异性特征，数值模拟研究大多是针对于互层厚度的讨论，缺乏对上覆与下伏围岩、互层物性与频率参数的综合适用性分析。针对我国典型的、代表性韵律型薄互层，申请拟利用视各向异性对薄互层进行近似处理，通过薄互层近似与精确地震响应的对比，系统分析互层结构与厚度、薄层物性与频率对视各向异性近似的影响和适用条件，给出更高精度的韵律型薄互层视各向异性的近似公式，为实现韵律型薄互层反演提供理论基础。

随着我国油气、煤炭等化石能源勘探开发的不断深入，简单的构造储层已开发殆尽，目前面临的勘探开发目标多为岩性储层，且这些储层均较薄，在地震频带下以薄层的形式存在。由于薄互层地震反射影响参数众多，超欠定问题使得薄互层反演难以实现。长波长假设下，薄互层可以近似等效成VTI介质，利用5个弹性参数与1个等效密度参数近似N层薄互层的3N个参数，大大降低了参数的个数。然而，现有的视各向异性理论对薄互层的适用性讨论仅限于互层的厚度，因此本项目针对韵律型薄互层模型从多个角度展开薄互层视各向异性的适用性讨论。在理论研究方面，本项目针对韵律型薄互层模型，简化薄互层视各向异性近似公式，并建立储层累计厚度与薄互层总厚度比值和等效VTI介质弹性参数和各向异性参数的关系式。在数值分析方面，本项目建立两类典型韵律型薄互层模型，即砂泥岩韵律型薄互层和煤层-高速层韵律型薄互层，通过改变不同参数，全面展开韵律型薄互层视各向异性适用性分析。课题研究过程中获得如下进展和认识：(1)韵律型薄互层中两种介质累计厚度比值一定时，利用视各向异性近似等效所得的VTI介质的弹性参数相同；(2)韵律型薄互层中两种介质累计厚度相同时，视各向异性近似所得的纵波各向异性最强；(3)视各向异性近似误差随着阻抗差的增加而增大；(4)当薄互层总厚度仍在薄层范围内，互层单元数越多，薄互层视各向异性近似精度越高，此时，视各向异性近似所得的薄互层反射系数随入射角的变化与实际情况相符。(5)入射波频率的变化也会对视各向异性的近似精度产生影响，主要体现在薄互层总厚度较小的情况下，入射波频率越低，视各向异性近似精度越高，且PS波反射系数近似精度高于PP波情况。