Velocity-depth trends in Mesozoic and Cenozoic sediments from the Norwegian Shelf: Discussion

Abstract

Storvoll et al. (2005) present log data from 60 wells on the Norwegian Shelf to investigate velocity-depth trends in sedimentary rocks. They present an interesting analysis of, e.g., the sonic-log velocities of overpressured Jurassic sediments and of source rock intervals. The authors do, however, reach the conclusion that “no general velocity-depth function can be used when performing more accurate analyses like depth conversion of seismic data, pore-pressure prediction, or basin modeling” (Storvoll et al., 2005, p. 359). This conclusion is surprising for two reasons: first, because it is not tested by the authors, and second, because the article presents very good evidence for the existence of normal velocity-depth trends for shale (i.e., functions that describe how sonic velocity increases with depth in relatively homogenous, sedimentary formations as porosity is reduced during normal compaction). The authors present log data from three areas along the Norwegian Shelf (but no examples of pressure versus depth): the northern North Sea, Haltenbanken, and the Barents Sea (about 60°N, 65°N, and 72°N, respectively). The main figures show velocity measurements versus depth for several wells in the three study areas (their figures 6, 8, 12). The authors compare the data with a linear velocity-depth trend: V p = 1477 + 0.57 × Z , which is similar to the normal velocity-depth trend for marine shale dominated by smectite-illite published by Japsen (2000): ![Formula][1] where V p is velocity in meters per second, and Z is depth in meters below seabed or ground level. This trend is almost identical to that developed by Scherbaum (1982) for Lower Jurassic shale in the Northwest German Basin and to that presented by Hansen (1996) based on data for Cretaceous–Cenozoic shales on the Norwegian Shelf (for depths less than about 2.5 km [1.6 mi]). It is interesting that the presented shale … [1]: /embed/graphic-1.gif