Dependence of sonic velocity on effective stress in fine-grained sediments
Overpressure estimation methods that use sonic velocity as a proxy for porosity only account for excess pressure due to disequilibrium compaction; the influence of unloading processes in generating larger excess pressure observed in most basins is ignored. Wireline log data and pore pressure measurements from wells across the Central Graben and the East Shetland Basin, North Sea, have been used to find out whether velocity is sensitive to the contribution of unloading processes to observed overpressures. The approach was to focus on fine-grained sediments, chalk and mudstones, and establish a relationship between sonic velocity and other petrophysical parameters, necessarily including porosity and vertical effective stress, when the latter variables are treated as independent. Investigation of the Chalk in the Central Graben has shown that velocity has no detectable dependence on vertical effective stress when porosity and effective stress are treated as independent variables. The significance is that velocity in Chalk cannot be used to detect the presence of any overpressure caused by unloading. It is suggested that the absence of an observable velocity reduction in unloaded Chalk is due to cementation. Analyses in the Lower Cretaceous and Jurassic mudstones show that gamma ray count and depth can usefully be taken as additional parameters in overpressure estimation. In both the Cromer Knoll and in the Heather formation, there is a small but significant dependence of velocity on vertical effective stress when porosity and effective stress are taken as independent variables together with gamma ray count and depth. The sensitivity factor is 21.8 m/s/MPa in the Cromer Knoll and 17.4 m/s/MPa in the Heather. The contribution of the vertical effective stress with associated independent variables (gamma-ray and depth) produced RMS errors between measured and forward-calculated values of sonic velocity of 101 m/s for the Cromer Knoll and 107 m/s for the Heather Formation. The discrepancies may be attributed to the contributions of other rock parameters that were not taken into account.