Influence of lithology and Neogene uplift on seismic velocities in Denmark: implications for depth conversion of maps

A new velocity parameter, the velocity anomaly (dV), is defined as the deviation between a velocity measurement and a linear velocity-depth function for a formation.

The variation of the velocity anomaly of the lowermost Jurassic claystones in northern Denmark is interpreted as the combined effect of three types of uplift movements, and the magnitude of these are estimated. It is concluded that the pre-Quaternary surface in northern Denmark is an erosional surface, created by up to 1000 m of Neogene uplift and subsequent erosion. The Upper Cretaceous Danian Chalk Group and the Tertiary sequence must thus have been deposited far beyond their present extensions. Consequences of Neogene uplift for the source rock potential of the area are discussed.

In the Danish Central Trough, North Sea, the velocity anomaly of the Lower Cretaceous is shown to reflect lithological variations related to differential subsidence and to characterize the geological regions better than the interval velocity.

Since the primary influence of depth on velocity is removed, the velocity anomaly is an expression of physical factors such as lithology, overpressure, and gas content as well as uplift and erosion. Contoured maps based on well data of the velocity anomaly of a formation may thus be used to estimate the velocity anomaly at arbitrary locations. The velocity anomaly map, the seismic time maps, and the linear velocity parameters for each layer constitute the input for velocity-anomaly depth conversion. This depth-conversion method results in depth maps with perfect well ties and geologically reasonable depth estimates away from wells.