Retarded compaction due to overpressure deduced from a seismic velocity/depth conversion study in the Danish Central Trough, North Sea

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As a result of overpressuring below the Upper Miocene, seismic velocities below this level are low in the Danish Central Trough, North Sea compared with areas outside the trough. Using the top Middle Miocene as a datum, velocity-depth functions match those functions established east of the study area and lead to improved depth conversion. It is proposed that the present day velocity-depth (porosity-depth) profiles below the top Middle Miocene reflect the 'frozen' pre-late Miocene profiles. The compaction effect of late Cenozoic burial was retarded, with the exception of areas where the sealing capacity of the lower Tertiary sediments was reduced due to permeable sandy deposits or possibly due to fracturing caused by salt diapirism. In the study area, the thick Late Cretaceous-Danian Chalk Group constitutes a significant high velocity layer. Depth conversion of maps covering the Danish Central Trough down to the top Middle Jurassic, an important deep exploration target, should include at least four layers: the Post Chalk Group (Cenozoic, excluding Danian), the Chalk Group and the Cromer Knoll Group (Lower Cretaceous) and the Upper Jurassic. 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, optimum depth conversion with constant velocity is obtained with velocities of 2050, 4400, 2950 and 2800 m/s for the four layers. The relative success of different depth conversion methods is 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 V0-k method) is shown to be an effective way to convert travel times to depth, when interval velocity increases with depth of burial. In this method velocity is assumed to increase linearly with depth and laterally velocity is calibrated to wells. Velocity variations in the Upper Jurassic impose the major problem for depth conversion in the area.

Sider (fra-til)715-733
Antal sider19
TidsskriftMarine and Petroleum Geology
Udgave nummer6
StatusUdgivet - dec. 1994


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