Optimizing depth conversion in the Danish Central Trough, North Sea

In the Danish Central Trough, North Sea, the thick Late Cretaceous-Danian Chalk Group constitutes a significant high velocity layer. Depth conversion to top Middle Jurassic, an important deep exploration target, should include at least three layers: The Post Chalk Group (Cenozoic excl. Danian), the Chalk Group and the Upper Jurassic-Lower Cretaceous. Analysis of time-depth data from 76 wells shows that velocities in the study area vary as a function of depth, thickness, lithology, presence of gas, and overpressure (caused by rapid Late Cenozoic deposition and by gas generation). For wells drilled away from salt diapirs, optimal depth conversion with constant velocity is obtained with velocities of 2050, 4400 and 2850 m/s for the three layers. The relative success of different depth conversion methods was evaluated by comparing the mean absolute error on the estimated thicknesses for the layers individually, assuming each well in turn to be undrilled. Velocity-anomaly depth conversion (or the Vo. - k method) is shown to be an effective way to convert traveltimes to depth, when the interval velocity of the layer increases with depth. In this method velocity is assumed to increase linearly with depth and laterally velocity is calibrated to wells. Due to overpressuring below the Upper Miocene, seismic velocities below this level are low when compared to areas outside the trough. Using top Middle Miocene as datum, the velocity functions match those functions established east of the study area and lead to improved depth conversion.