ABSTRACT

The fact that porosity can be measured accurately with the sonic log has prompted new procedures for estimating saturation, wherein data concerning the various permeable beds in a given well are compared.

1. In one approach a comparison is made of the values of the formation water resistivity computed from the resistivity log and from the sonic log. Actually, apparent formation water resistivities are calculated in assuming that all sands are wet. With the concept of continuity, this procedure makes possible a quick determination of zones of saturation in shaly sand and in cases where there are appreciable variations in formation water salinity with depth.
2. It has been found also that comparison of the apparent formation factor obtained from the sonic log with that computed from a short investigation resistivity log may reveal in many cases the presence of residual oil or gas and thus detect potentially productive formations. This procedure is valuable when true formation resistivity and the resistivity of the formation water are in doubt.

Although these two procedures permit only a qualitative interpretation of the log, they have the advantage of speed and simplicity. The quantitative interpretation that remains to be done can be performed very quickly since all the nonproductive formations have been eliminated by the above procedures. The first procedure is best adapted to formations of high porosity and in fresh mud. The second procedure works best in formations of low porosity and with little contrast between the mud and formation water resistivities.

Examples illustrate the application of these two techniques in the Miocene, Frio, and Wilcox sections of the Gulf Coast.