Sonic velocity of chalk, sandstone and marine shale controlled by effective stress: Velocity-depth anomalies as a proxy for vertical movements

Sonic velocities of sedimentary rocks can provide a simple measure of the vertical movements of the rocks relative to the earth's surface. During progressive burial histories when effective stress increases, sonic velocities of many sedimentary units of uniform lithology show a progressive increase that defines a baseline or a normal velocity-depth trend. Departures from such a trend may reflect velocities that are low relative to depth (positive burial anomaly) due to overpressuring or velocities that are high relative to depth (negative burial anomaly) due to deeper burial followed by exhumation. Maps of such anomalies across the North Sea Basin based on data from about 1000 wells for the Upper Cretaceous–Danian Chalk Group and for the overlying, lower Cenozoic, shale-dominated deposits, reveal an east-west oriented, sinusoidal pattern that vary between − 1 and + 2 km with a wavelength of about 1000 km. The anomalies primarily reflect two physical processes that cause deviations from normal compaction. (1) Removal of up to 1 km of sediments along the western and eastern margins of the basin due to Cenozoic uplift of the British Isles and Scandinavia. (2) Overpressure of up to 20 MPa in the formations below the mid-Miocene unconformity in the central North Sea due to disequilibrium compaction. In order to define such patterns with confidence it is essential that sonic velocity data are compared with well-constrained, normal velocity-depth trends for suitable lithologies; e.g. baselines for chalk, marine shale and sandstone that are reviewed here. The mechanisms behind such vertical movements as those observed for the North Sea Basin are debated, but the material presented here documents the tectonic nature of these processes by revealing their variation in time and space.