Electrical logs have now been used for some 25 years in the Gulf Coast and have become a standard in the oil industry. Used at the beginning for correlation and simple qualitative interpretation, they now have many applications, thanks to the various tools and techniques that have been introduced in the last few years.

Structural Studies

The art of correlation with electrical logs is an old one, and little will have to be said here. The SP curve and the amplified normal are the curves most often used for this purpose. It is likely that the conductivity scale of the induction log will be preferred to the amplified normal in the future because of its greater detail.

Fresh muds are necessary when using the electrical log for correlation. The mud filtrate resistivity should be at least four times the average formation water resistivity (Rmf = Rw). It is true that good radioactivity logs can be run in salt mud, but they have not been found to be as efficient in the Gulf Coast for correlation studies.

Dipmeter surveys remain a great help in structural studies. This is particularly true since the introduction of the continuous dipmeter.

Determination of Boundaries and Sand Count

In the early days, only the SP curve and the resistivity logs were available for this purpose. Increased accuracy has resulted from the introduction of the Microlog and Microlaterolog. Because of the small spacings used in the microdevices, sharp definition of bed boundaries is possible. The MicroLog has shown that many sands believed to be uniform are often broken by tight lenses or members. Detailed sand count studies are now possible, and the Microlog has helped greatly in the sidewall sampling program by the elimination of hard or shaly intervals (Example 1).

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