Development of Seismic Anisotropy in Deforming Salt Bodies

Physical properties of salt, such as its low permeability, density and viscosity make it an important component in various industrial applications, e.g., natural salt deposits can form hydrocarbon traps in the subsurface. Many polycrystals develop a texture (lattice preferred orientation, LPO) and become seismically anisotropic when subject to plastic deformation. The potential of deformed rock salt to generate such anisotropy, the impact on seismic waveforms and seismic images is investigated, by using both, seismic data and sophisticated elasticity modelling. A vertical seismic profile (VSP) dataset from the Mahogany salt body (located in the Northern Gulf of Mexico) is investigated using shear-wave splitting analysis. The results show that shear wave phases, converted at the top of the salt body show significantly higher splitting than those, converted at the base of the salt body. The observations are clear signs of seismic anisotropy within the salt. Various scenarios are tested to explain the anisotropy by creating simple forward models. We conclude that LPO of halite is the most likely explanation of the splitting observed, although others causes, such as the presence of aligned water inclusions can not be disregarded. Motivated by the results, we use texture plasticity modelling to predict LPOs in halite polycrystalline aggregates and associated seismic anisotropy. First, rock salt texture deformed in simple deformation regimes (simple shear and uniaxial compression) is simulated and we analyse the ability to use texture modelling to predict anisotropy in naturally deformed rock salt. Seismic anisotropy predicted is around 10% for S-waves and 6% for P-wave. We develop a work-flow, based on calculating strain history of deformed salt formations, to estimate salt anisotropy due to deformation plasticity in more complex deformation scenarios, and demonstrate its adequacy on a salt diapir deformation model. The results show that the anisotropy pattern is complex, with higher anisotropy in regions which experienced larger deformation. The predicted anisotropy is significant and would falsify seismic images, if not accounted for properly. Then, the workflow is applied to a realistic Mahogany salt deformation model. A synthetic VSP-dataset is created, based on the calculated elasticity of the deformed salt. Synthetic shear-wave splitting results are consistent with the VSP field-data set results from Mahogany. This is strong evidence that the splitting observed is due to LPO of constituent rock salt crystals. Salt formation experience large deformation and can show, consequently, seismic anisotropy. Usually, salt anisotropy is not accounted for is seismic data, and therefore this study has potentially large impact on traditional seismic imaging in rock salt settings, which experienced strong deformation. This study places the common assumption of salt isotropy on unsafe ground.