Overpressured Cenozoic shale mapped from velocity anomalies relative to a baseline for marine shale, North Sea

A study of interval velocities from almost a thousand wells reveals basinwide differences in physical properties of the Cenozoic deposits of the North Sea Basin. These differences relate primarily to the sediments below the mid-Miocene unconformity as testified by a subdivision of a subset of these wells. Velocity-depth anomalies are mapped relative to a constrained, normal velocity-depth trend derived for marine Jurassic shale: tt = 465.e (super -z/2435) +180, where tt is transit time in mu s m (super -1) , and z is depth in metres below sea bed. The upper Cenozoic deposits are close to normal compaction, whereas anomalies for the lower Cenozoic sediments outline a zone of undercompaction in the Central North Sea that corresponds to the overpressure in the Upper Cretaceous-Danian Chalk. The overpressure results from a balance between the load of the upper Cenozoic deposits, and the draining determined by the thickness and sealing quality of the lower Cenozoic sediments. The shale trend may be more widely applicable to marine shale dominated by smectite/illite. This suggestion is based on the observed correspondence between velocity anomalies and pressure data, and due to the match between trends for marine shale of different ages in the North Sea and in the US Gulf Coast area over a significant velocity range.