Chalk background velocity - Influence of effective stress and texture

The relative variations in acoustic impedance and hence porosity may be determined by inversion of seismic data. The estimation of the absolute level of these parametres, however, requires a priori knowledge about the low-frequent, background velocity. We find that the background velocity of the North Sea chalk is primarily controlled by effective stress: velocity and porosity log data for chalk in two wells plotted versus effective depth match published reference lines for normally compacted chalk above c. 1.8 km. Effective depth is depth corrected for the effect of overpressure. Below that depth we observe a significant drop in porosity and an increase in velocity that possibly may be explained by a decrease in the Biot factor leading to an increase in effective stress and hence in pore-filling cementation originating from pressure dissolution along stylolites. Moreover, we observe different velocity-porosity relation for the chalk in the two wells leading to differences in porosity of some 5% for identical values of velocity for apparently pure chalk. These variations may be related to differences in contact cementation between the poorly cemented and suffer pore shapes (possibly affected by quartz-overcoating). The non-unique velocity-porosity relation for chalk will lead to variability in the estimation of porosity from seismic data because one impedance value may represent a range of porosities.