In some instances, induction logs indicate the presence of fractures. This is possible when there are resistivity anomalies in an interval and sufficient resistivity contrast with adjacent beds (Timko, 1966).

Figure 3 9 presents gamma ray and induction logs for a Gulf Coast well run at three different times. Figure 3 9A shows the gamma ray and induction logs run between 15,850 and 16,020 ft following drilling with 17.1 lb/gal inverted oil emulsion mud. When the well was deepened to 16,294 ft, a pressure kick occurred. As a result the mud density was increased to 17.5 lb/gal. This new density allowed the mud to flow in the formation, losing 275 b of mud. Figure 3 9B shows the logs obtained between 15,850 and 16,020 ft following the mud density transition from 17.1 to 17.5 lb/gal.

Figure 3 9: Resistivity logs, Gulf Coast Well (after Timko, 1966)

Then the mud density decreased to 17.3 lb/gal and the mud flowed back from the formation. Figure 3 9C shows the logs obtained following the reduction in mud weight.

Various interesting features are to be noted from this example. First, there is a general increase in resistivity (R _Il) in Figure 3 9B compared with Figure 3 9A especially between 15,910 and 15,925 ft. This can be interpreted by saying that the formation was fractured (or maybe an existing hairline fracture was opened) as a result of increasing the mud density from 17.1 to 17.5 lb/gal. The 275 b of nonconductive inverted oil emulsion mud lost to the...