Salt-induced stress anomalies: an explanation for variations in seismic velocity and reservoir quality

Accurate rock property prediction is often a critical success factor for wells targeting hydrocarbons. This applies not only to reservoir porosity and permeability affecting productivity directly, but also to acoustic velocity, seismic time-to-depth conversion and depth prognosis. A detailed analysis of variation in the overburden rock velocity in the Southern North Sea has shown that Triassic velocity variations of up to 18% occur within short distances (e.g. <1 km). A correlation was found between increased acoustic velocities and the presence of an underlying salt weld. Salt Induced Stress Anomaly (SISA), a geomechanical model, is presented that can explain these observations and is based on the principles of buoyancy and point-loading. In the initial state, prior to salt movement, the vertical effective rock stresses resulting from the overburden weight are transmitted uniformly and cause laterally even compaction in the sediments. However, once the salt layer is able to flow and redistribute itself under the influence of buoyancy forces, the overburden stress will concentrate itself near the salt welds. This locally increased stress gives rise to higher velocities in the overburden. The same stress concentration model can also explain deterioration in the porosities of the Rotliegend reservoir as observed underneath salt welds.