Estimation of sonic velocity in shales in abnormally-pressured formations from resistivity data

The writers proposed a method for estimating (1) the sonic velocity υa in abnormally-pressured shales using resistivity logs and, thus, (2) shale bulk density. Analysis of the well-log data for productive strata in Azerbaijan showed that there is a poor correlation between the sonic velocities and other well-log data, such as resistivity, SP, neutron and gamma-ray. Introducing normal trend of sonic velocity υn and resistivity ϱn allowed us to express sonic velocity (and, therefore, the bulk density of the shale) as a nonlinear function of resistivity with good correlation between the normalized velocity and normalized resistivity. The best-fit regression equation is of the following form: υaυn = 0.63 + 0.37(ϱaϱn)3.26 (where ϱa and ϱn are the resisitvities of abnormally-pressured and normally-compacted shales, respectively). efficient of correlation between parameters \̃gn[= υaυn] and \̃g9[= (ϱaϱn)3.26] is 0.87 and the mean-squared error for υa is 190 m/s. The average relative error of velocity estimation is 6% — ranging from zero to a maximum of 14%. Thus, υa can be estimated from the ϱaϱn ratio and the υn obtained from the normal compaction trend in the area studied.