Anisotropy in the salt outcrop at Cardona, Catalonia – implications for seismic imaging

Abstract

The world’s largest oil discoveries in recent years have been pre-salt, in reservoirs located below or close to salt bodies. It is often assumed that the salt is homogeneous and that a near-constant isotropic velocity in the salt body can be used for seismic imaging. Here we show that this simplification may be too simple. Based on field observations from an old salt mine in Spain, we estimate anisotropic parameters describing the exploited salt diapir. In its outcrop we identify a regular pattern of alternating halite and clay layers, where the thickness of each layer is close to periodic. From this observation, we estimate corresponding anisotropic parameters for this salt outcrop and find that the degree of anisotropy is moderate, of the order of 5% difference between horizontal and vertical velocities. Furthermore, we identify potential shear zones within the salt outcrop that can be mapped over distances of several hundred metres. The thicknesses of these shear zones are of the order of metres. Based on these observations, it is a huge simplification to treat salt bodies as homogeneous and isotropic for seismic imaging purposes.