Investigation of multi-phase erosion using reconstructed shale trends based on sonic data

.Estimates from sonic data of removed overburden burial anomalies rely on identification of normal velocity–depth trends for relatively homogenous lithological units. In the North Sea, such trends are difficult to establish for Mesozoic formations that rarely are found at maximum burial and hydrostatic pressure. The analysis of erosional history based on sonic data can, however, be developed further where two homogeneous units are encountered in the same wells, because the two units may or may not have been simultaneously at maximum burial at a given location. If the two units were at maximum burial depth simultaneously prior to the most recent erosional event, the burial anomalies for the two units should be identical. If, however, the lower unit was at maximum burial before the upper unit, due to an intervening erosional event, the burial anomaly for the lower unit should exceed that of the upper unit. In the North Sea, Neogene erosion can be estimated from burial anomalies for the Upper Cretaceous–Danian Chalk Group. This allows corrections to be made of the depths of pre-Chalk formations to the situation prior to the onset of Neogene erosion. The normal trend for the pre-Chalk formation can now be traced more easily in a plot of velocity versus corrected depth, because the pre-Chalk formation was at maximum burial at more locations prior to Neogene erosion. This procedure is used to derive baselines for a Lower Jurassic shale unit, and for the Lower Triassic Bunter Shale and Bunter Sandstone using data from 123 British and Danish wells. The dominance of smectiterillite in the marine Lower Jurassi