Precision in Pore Pressure Modeling, Part 2

Through a single depositional sequence there are two primary factors that cause the resistivity and sonic logs to not be a straight line through a shale member, grain size sorting caused by varying energy level and carbonate content caused by varying density of planktonic “rain.” In Precision in Pore Pressure Modeling Part 1 I described grain size sorting in shale and referenced the resistivity log. Part 2 discusses both resistivity and sonic logs and gives a more comprehensive discussion of the sonic log.

Both logs should be studied and compared looking for a trend.

The resistivity log is more sensitive to the rate of planktonic “rain” than the sonic log. Given the same porosity, the difference between the resistivity of a weathered clay lattice and a calcium carbonate test is sufficient to cause the resistivity log to vary through a shale member due to varying rates of planktonic “rain” more so than the sonic log which “sees” the porosity but not the geochemical difference. The rate of planktonic “rain” will vary. When drilling a vertical well through a shale member, the same rate of planktonic “rain” will be repeatedly encountered. The logs must be studied in detail to select the relative same rate of planktonic “rain” which has dissimilar effects on the different logs.

When selecting shale points, study and compare both resistivity log and sonic logs, then create one shale point data base for the resistivity model and a different shale point data base for the sonic model.