Abstract

Because the Electrical Survey (ES) log did not work well in salt-mud-filled boreholes, laterologs were introduced around 1950. By using guard or focusing electrodes the current beam from the surveying electrode was forced into the formation. These older laterologs came in two configurations, one with three electrodes (Laterolog-3) and another with seven electrodes (Laterolog-7). When logging in the Permian Delaware Mountain Group of west Texas and New Mexico or the Permian Rotliegend Sandstones of the North Sea a problem with these older laterologs was noted. This problem is referred to as the “Delaware Effect”. The Delaware Effect occurs when the older laterologs were used below a very thick high-resistivity bed. The return current from the surveying electrode (Ao) to the logging cable (B) located in the high-resistivity bed has a high resistance path, this causes the apparent resistivity to be too high. On the log the Delaware Effect appeared as a rapid increase in laterolog resistivity starting at some distance below the thick high-resistivity bed. This distance was a function of the distance between the Ao and the B return electrode.

The service companies minimized the Delaware Effect in two ways: 1) for the LL-7, the B return electrode was placed on the surface; and 2) for the LL-3, the distance from Ao to the B return electrode was shortened when compared to laterologs with Delaware Effect. Laterolog examples from the Ramsey sandstone in the Ford Geraldine Unit of Reeves and Culberson Counties suggest that the laterologs in the Ford Geraldine Unit were logged with the Delaware Effect minimized.