Elastic and resistivity anisotropy of shale during compaction and diagenesis: Joint effective medium modeling and field observations

Abstract

Shales are anisotropic materials which have been observed to have both electrical and elastic anisotropy. Shales also comprise most of the sedimentary column in clastic basins, and thus, the ability to model shale response to electrical and seismic fields may improve our ability to better resolve anisotropy by using different measurements. Compacting shale-effective media modeling, which captures the geological process of porosity reduction and geochemical compaction, was formulated in this research in microstructural parameters associated with particle alignment and pore-shape deformation. The state of compaction, captured by the porosity of the soft sediment, governs the pore aspect ratio and the amount of particle alignment. Diagenetic processes are captured using temperature-dependent mineral elasticity and conductivity. The model uses the same microstructural parameters and equivalent effective medium approximation scheme to predict electrical and elastic anisotropy. In situ measurements of electrical anisotropy and seismic based estimates of elastic anisotropy in shales in the same basin compare favorably with modeling results. The joint-modeling formulation provides ways to study electrical resistivity and elastic-anisotropy jointly and explore the use of electrical resistivity anisotropy measurements to constrain elastic anisotropy, and vice versa.