Density estimations using density-velocity relations and seismic inversion
Maria Fernanda Quijada, Robert R. Stewart
Density can be an important parameter to differentiate lithologies and estimate other petrophysical properties, such as porosity or fluid content. Density in the area has a complex behavior, not related directly to lithology; with the best lithological indicator being the Vp/Vs ratio. Also, density changes within the interval of interest can generate significant changes in the amplitudes and their variations with offset. Density, gamma ray, P- and S-wave sonic data from a well in Manitou Lake, Saskatchewan were used to evaluate different density-velocity relations and their parameters. Default values for a and m in Gardner's equation are 0.31 and 0.25, respectively. Using a local fit for sands and shales, differentiated using the GR log, results in values of a and m of 0.51 and 0.19 for shales, and 0.22 and 0.28 for sands. Using Lindseth's approach the biggest change is on parameter d, which changes from 1054 to 855 for sands, and to 459 for shales. The RMS value of the residuals using Gardner's equation is improved from 0.1147 to 0.0879 g/cm3 just by estimating parameters from a local fit, while the difference is even greater when using Lindseth's equation, changing from 0.4550 to 0.0879 g/cm3. Densities estimated from impedance show similar RMS errors, but using the parameters from the single fit results in an averaged log with no variations associated to the geology.