Salt Induced Stress-Arching Controlling Rock Properties

Hoetz, Guido, Nederlandse Aardolie Maatschappij B.V, Assen, Netherlands

Accurate depth conversion of seismic data is often a critical success factor for wells, particularly in mature areas such as Europe. Remaining drilling targets are regularly charac­terized by relatively small HC columns. As a consequence, these targets are very sensitive to depth errors. A detailed analysis of variation of overburden rock velocity showed that sometimes, velocity variations up to 8% can occur within short distances (e.g. 1 km) for no apparent reason. By carefully studying the settings where these anomalies occur, a common factor in the geometries revealed an explanation. It appeared that where the Triassic pack­age is residing directly on top of the pre-Zechstein salt substratum (e.g. the Rotliegend), the Triassic velocities are higher than in surrounding areas. A geomechanical model has been developed that can explain these observations using the concepts of isostasy and stress arching. The vertical stresses resulting from the overburden weight are normally transmit­ted uniformly and cause laterally uniform compaction in the sediments. However if the salt layer is able to deform plastically and to redistribute itself under influence of buoyancy forces, the overburden stress will concentrate itself at the locations of the saltwelds. This locally increased stress, together with its related strain (including de-watering of the sedi­ments) will lead to higher seismic velocities in the overburden. The same stress concentra­tion phenomena can also explain certain variations in the quality of underlying reservoir rocks. Rotliegend reservoir rock underneath saltwelds show lower porosities than in near­by offset wells.