Use of the conductivity curve of the induction-electrical log to define the depth at which a bore hole enters an abnormally pressured section, as outlined by Wallace (1964), prompted the writer to attempt-to develop a method for making quantitative pressure determinations from the conductivity curve.

A method of plotting conductivity readings of clean shales vs. depth was devised. This plot shows that a sloping straight line can be drawn through the points in the known normally pressured section, whereas the points in the abnormally pressured zone deviate toward higher conductivity. Causes for the straight line and the deviating portions of the plot were analyzed and it was concluded that the magnitude of the conductivity deviation is primarily a function of the pressure abnormality.

Plotting of pressure gradients (calculated from measured reservoir pressure and depths) vs. conductivity deviation in 41 abnormally pressured reservoirs in coastal and offshore Louisiana resulted in a basic curve which can be used to determine approximate pressures in future wells where electric logs can be made available but actual pressure measurements are lacking.

An example of the use of this curve is given for the case where a well passes out of an abnormally pressured section into a normally pressured zone. The lower pressures in the normally pressured zone are masked by the high mud weights required to hold the upper, higher pressures but can be detected by the proposed method.