EMPIRICAL INVESTIGATION OF SOME FACTORS AFFECTING ELASTIC WAVE VELOCITIES IN CARBONATE ROCKS *

Continuous velocity log data from the Upper Cretaceous section of about 65 wells from NW. Germany have been studied in order to find some factors which affect the behaviour of elastic wave velocities in carbonate rocks. It could be assumed for these particular rocks that the velocities they exhibit may be subject predominantly to their state of compaction and their lithology (shale-carbonate ratio). Considering the behaviour of the VL-curve, several types could be distinguished: The basic type r for which the shale-carbonate ratio remains almost invariable over a large depth range (as suggested i.a. by a constant degree of radiation in the accompanying Gamma Ray log) discloses a clear relationship between interval velocity (measured as travel time per meter) and overburden pressure. Velocity profiles of log type r and also “peak” velocities from “pure” limestones, plotted versus depth indicate an increase rate almost identical to that of Jurassic shales. It is shown that the lithologic constant can be extracted from the Velocity Log data and then used for mapping fades changes in an area with sufficient well control. In certain cases it is even possible to determine the rate of uplifting (or the original maximum burial position) for a carbonate rock which has an anomalous high velocity in respect to its present depth.