Seismic anisotropy: Geological causes and its implications to reservoir geophysics

The primary focus of this dissertation is to improve the applicability of rock physics models for elastic anisotropy through useful approximations, empirical relations, and practical workflows considering the geological origins of rock anisotropy.;Anisotropy arises from aligned heterogeneities at scales smaller than the scale of measurement. Ignoring elastic anisotropy may lead to poor seismic imaging, inaccurate well-ties, and incorrect interpretation of seismic data. Directional dependency of seismic wave propagation has become even more important with the routine acquisition of long offset, wide azimuth P-wave, S-wave and converted wave seismic data.;The mathematics of rock anisotropy is far more advanced than what we can apply in practice. This is because we do not make enough measurements to completely characterize all of the input parameters necessary for anisotropic rock physics modeling. The main objective of this dissertation is to improve the applicability of anisotropic rock physics models considering the geological origin of elastic anisotropy. We present simplified linearization of the anisotropic models, provide empirical constraints on the input parameters and present practical workflows to model elastic anisotropy considering their geological cause.;We consider three important geological origins of elastic anisotropy in sedimentary rocks: (a) anisotropy due to shale, (b) anisotropy due to stress and fractures, and (c) anisotropy due to fine laminations. Additionally, we explore the effect of fluids in modifying the anisotropy resulting from these causes.;First, we present rock-physics modeling strategies for elastic anisotropy in (a) organic-rich source rocks; and (b) shallow compacting shales. Our laboratory measurements on compacted pure clay minerals show increasing velocity anisotropy with increasing compaction. However, our experiments suggest that a simple compaction-dependent clay orientation model may not always be valid. A compilation of ultrasonic velocity measurements was used to obtain useful links between the anisotropy parameters and commonly measured vertical velocities. Furthermore, we present simplified equations linking textural orientation in shale to anisotropic Thomsen's parameters.;Second, we present a method to compute the third order elastic coefficients from isotropic measurements, combining a compliant-porosity based stress-induced anisotropy model with the third order elasticity formalism. In addition, we invert the third order elastic coefficients from our compiled database on shale anisotropy. The third order coefficients in shale do not show any apparent inter-relationships. However, we show that in highly anisotropic organic shales, the third order coefficients increase with increasing thermal maturity of source rock.;Third, we derive simplified equations for Walton's contact-based model for stress-induced anisotropy in unconsolidated sandstones. Such simplifications make the application of the anisotropic model simpler and computationally more efficient. We extend this granular, contact-based model to sandstones with pressure solution.;Finally, we derive an approximate form of the Gassmann's anisotropic fluid substitution equations for vertical velocities, and present the fluid substitution equations in terms of the Thomsen's parameters. Our approximation enables one to perform anisotropic fluid substitution for vertical velocities with fewer anisotropy parameters. It reveals that, in a VTI medium, it is the Thomsen's parameter, delta, that controls the anisotropic contribution to the vertical velocity during fluid substitution.