Compaction Induced Electrical Anisotropy in Sandstones

Using a novel electrical anisotropy measurement system we have measured VTI type anisotropy in clean (clay free), visually isotropic and layered sandstone samples. Ratios of mean horizontal to vertical resistivity of up to 1.2 have been observed. The minimum resistivity eigenvalue is normal to the bedding plane and normal to the mean orientation of grain axes; this is incompatible with established anisotropic grain shape effective medium models or layering models. Using finite element modelling of synthetic grain packs we show that this form of anisotropy can be simulated by gravity sorting during ballistic grain deposition and subsequent compaction during digenesis. Commonly isotropy is assumed for clean sands but may add considerable error to the estimation of fluid saturation and properties if not considered during the interpretation of well-log and CSEM data. This type of anisotropy might also prove a useful tool when monitoring the mechanical compaction response of reservoir sands.