STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

974 Drilling and Well Completions
Figure 4-270. Cross-section of an MWD focused gamma ray tool. (Courtesy
SPWLA [112].)
Figure 4-272 shows an example of a MWD spectral GR log. On the left track,
SGR is the total GR count, and CGR is this total count minus the uranium count.
On the right side of Figure 4-272 the wireline spectral gamma ray in the same
interval is displayed. The curves are similar but some differences occur in the
amplitude of the three curves.
The main field applications of this log are:
1. Clay content evaluation: Some formations may contain nonclayey radioactive
materials. Then the curve GR-U or GR-K may give a better clay content estimate.
2. Clay type identification: A plot of thorium versus potassium will indicate
what type of clay is present. The thorium/potassium ratio can also be used.
3. Source rock potential of shale: A relation exists between the uranium-topotassium
ratio and the organic carbon content. The source rock potential
of shale can thus be evaluated.
Resistivity Logs
Four types of resistivity logs are currently run while drilling:
1. short normal resistivity
2. focused current resistivity