Evolution of Anisotropic Acoustic Properties in Clay-Quartz Mixtures during Mechanical Compaction: Implications for Shale Burial

Shales exhibit significant seismic anisotropy, which can cause errors in seismic imaging, seismic attribute analyses and reservoir characterization. During deposition and burial, platy clay mineral particles tend to align along bedding planes, creating distinct direction-dependent seismic properties — a primary cause of anisotropy. Therefore, investigating the influence of clay on the evolution of shale anisotropy during compaction is crucial for enhancing seismic interpretation accuracy. However, quartz, serving as the primary rigid grain in shales, complicates this process. It forms a resisting framework that interferes with the preferred orientation of clay platelets. Hence, solely studying pure clay is insufficient, and investigating the compaction behavior of clay-quartz mixtures is of great value to accurately characterize the evolution of seismic anisotropy in realistic shale environments. Given their dominance in shale mineralogy, kaolinite and illite were chosen as the representative clay end-members for our study. We prepared eight clay-quartz mixtures (4 kaolinite-quartz mixtures and 4 illite-quartz mixtures) by mixing different amounts of clay and quartz: 100%, 80%, 60% and 40% of clay by weight. To simulate the natural burial process, these water-saturated loose sediments were subjected to uniaxial mechanical compaction experiments.

Experimental results indicate that for both groups, porosity decreases monotonically while seismic anisotropy increases with increasing compaction stress. However, the influence of quartz content differs significantly between the two mineral systems. In the kaolinite group, Thomsen parameters ε and γ show a negative correlation with quartz content at identical pressure levels. The illite group, conversely, exhibits more complex behavior: while γ remains negatively correlated with quartz fraction, ε displays a non-monotonic trend—initially decreasing, then increasing, and finally decreasing again as quartz content rises. Furthermore, the parameter δ serves as a distinct discriminator: kaolinite mixtures exhibit suppressed δ values (near zero or negative), whereas illite mixtures consistently display positive δ values (> 0.10), indicative of well-stratified compliant bedding.

These findings underscore that the "bridging effect" of quartz and the resulting anisotropy are strictly controlled by clay mineralogy. Specifically, the high positive δ of illite implies that conventional elliptical assumptions may cause significant errors in processing. Therefore, accurate seismic interpretation requires mineral-specific anisotropic models that account for the distinct structural evolution of kaolinite and illite during compaction.

Fig. 1 Thomsen’s anisotropy parameters ε, γ, and δ versus porosity for the K-series and I-series samples.