Well log analysis of Vp and Vs in carbonates

Susan L. M. Miller

Well log data from three carbonate formations in Alberta were analyzed for relationships among Vp, Vs, porosity, density, and lithology. A review of the literature suggests that these relationships are highly variable among carbonates, and thus each geological setting should be characterized individually. In this well, Vp and Vs were inversely correlated with porosity in the Leduc formation. Velocity-porosity correlations were weak for the Pekisko and Wabamum carbonates. Vp/Vs did not appear to be correlated to porosity in any of the formations. Densities calculated from Gardner's relation were compared to measured well log densities. Calculated densities were close to measured densities in pure carbonates, but were underestimated in zones where anhydrite was present. Vs generally increased with density in pure carbonates, but samples with high anhydrite content had anomalously low velocities. Vp and Vs were linearly correlated in all the formations. The presence of anhydrite tended to raise the Vp/Vs ratio in the dolomites, and cause an overlap with the limestone Vp/Vs values. The Vp/Vs curve closely tracked the dolomite/anhydrite layers indicated by the photoelectron cross section index (PEF) log and the anhydrite fraction curve. The Vp/Vs curve also tracked the PEF and the limestone fraction curve across the boundary between the dolomite/anhydrite facies and a limestone formation. In the carbonates studied here, Vp/Vs appears to be a good lithology indicator which is independent of porosity effects.