Page 41 - Well Logging and Formation Evaluation

Quicklook Log Interpretation 31

can be clearly identified as zones where the density lies to the right of the
neutron, typically by 6 or more neutron porosity units.
The greater the crossover between the density and neutron logs, the
better the quality of the reservoir. However, gas zones will exhibit a
greater crossover for a given porosity than oil or water zones. Because
both the neutron and density logs are statistical measurements (i.e., they
rely on random arrivals of gamma rays in detectors), they will “wiggle”
even in completely homogeneous formations. Therefore, it is dangerous
to make a hard rule that the density curve must cross the neutron curve
for the formation to be designated as net sand. For most reservoirs, the
following approach is safer (see Figure 2.2.1):

1. Determine an average GR reading in clean sands (GR sa ) and a value
for shales (GR sh ). For GR sh , do not take the highest reading observed,
but rather the mode of the values observed.

0.45 –0.15
Neutron (frac)
GR (API) Density (g/cc)
0 150 1.95 2.95
5000

5005

5010

5015
Depth
5020

5025

5030

5035

Figure 2.2.1 Identifying Net Reservoir