Page 47 - Well Logging and Formation Evaluation

Quicklook Log Interpretation 37

reservoir. In general I do not recommend calculating f eff at all as part of
any quicklook evaluation. At this point I would like to make it clear that
I never favor making use of the neutron/density crossplot log for calcu-
lating porosity in sandstones. My reasons for this are as follows:

1. Both the neutron and density logs are statistical devices and vary ran-
domly within certain limits determined by the logging speed, detector
physics, source strength, and borehole effects. The error introduced
when two such random devices are compared is much higher than
when one such device is used on its own.
2. The neutron is severely influenced by the amount of chlorine atoms in
the formation, occurring either in the formation water or in the clay
minerals. This means that the neutron porosity is only very loosely
related to the true porosity (as observed when it is compared with the
density log in sand/shale sequences!).
3. The neutron is also affected in an unpredictable way by gas (unlike the
density, for which a correction can be made using the appropriate rho f ).
4. I have never had much faith in the overlays presented on standard
neutron/density crossplots by the contractors. In practice, when real
data are plotted, the overlays typically predict all kinds of minerals,
from dolomite to limestone, to be present when in fact one is dealing
with a clay/quartz combination.

When I do a quicklook I use the neutron log for only two things: (1)
qualitative identification (using the density) of shale/sand zones and (2)
identification of gas zones. I also do not favor the use of the sonic log for
porosity determination under any circumstances. In my view you are
better off just making an informed guess at the formation porosity based
on the general log response and regional information, rather than relying
on any quantitative calculation based on the compressional sonic.

2.5 CALCULATING HYDROCARBON SATURATION

In most quicklook evaluations of clastic reservoirs, it is sufficient to use
Archie’s equation (see above) to calculate saturations, using the deepest
reading resistivity tool directly as R t . In the absence of any regional core
values, I would recommend using m = n = 2. Note that I have not chosen
to include the so-called Humble constant (a), since this can just as easily
be incorporated within R w.