Shale anisotropic rock-physics model incorporating the effect of compaction and nonplate mixtures on clay preferred orientation

Abstract

Compared with other sedimentary rocks, the strong elastic anisotropy of shale is extremely prominent, which is mainly caused by the preferred orientation and platy nature of its clay minerals. Especially in seismic reservoir characterization, a suitable and correct estimation of the shale elastic anisotropy can improve the accuracy of the shale seismic inversion and prediction. Due to the long-term compaction of shale and the rearrangement of minerals, its microstructure and macrostructure are more complex, resulting in obvious anisotropic characteristics of shale. Existing methods do not incorporate the impact of nonplate particles on clay platelets, or indirectly incorporate it through empirical formulas, resulting in poor applicability and errors in the rock-physics models. To reveal the main causes of the anisotropy of clay minerals, a theoretical model incorporating the effect of compaction and nonplate particles on the preferred orientation of clay platelets is developed using experimental data and electronic scanning results. Based on theoretical analysis, an orientation distribution function (ODF) based on the effect of compaction and nonplate particles is derived, which not only incorporates the influence of compaction but also further incorporates the effect of other nonplate particles, such as quartz, which makes the established shale anisotropic rock-physics model more reasonable and accurate. Then, an improved anisotropic shale rock-physics model is developed using the compaction and nonplate particles-based ODF. The prediction results indicate that the presence of nonplate particles has an inhibitory effect on the preferred orientation of clay platelets, which is verified by the measured experimental data and indicates that our method is reliable and effective.