Improved Transition Zone Identification Using Relations Between Shear Wave Velocity and Density

In this study, we present a method for identification of the transition zone between mechanical and chemical compaction in data from shaly lithologies. By utilizing crossplots of wireline log-measured shear wave velocity (Vs) and bulk density (ρb), we observe an increased sensitivity compared to compressional wave velocity (Vp) to the onset of chemical compaction/cementation, while eliminating uncertainties related to porosity estimations. A clear change in the velocity (Vs) gradient with increasing density is shown to occur in data from eight exploration wells. Data above and below a certain velocity value (Vs = 1350 m/s) show substantially different behavior which is expected to be a result of cementation. A linear relation in the Vs –density domain is derived from the data and suggested as a representative trend for mechanical compaction in the study area. Our suggestion is that whenever recorded with sufficient vertical coverage, direct measurements of Vs and density are trust-worthy parameters for identifying the transition zone between mechanical and chemical compaction.