Selection of Porosity MethodS
The answers for all the porosity solutions will vary, and in some
cases be unreasonable or impossible to calculate due to lack of
data. In order of preference, we would choose:
1. Density neutron crossplot (if hole is good and if both logs
2. Sonic density crossplot if neutron is unavailable and no gas
3. Sonic neutron crossplot in carbonates or in bad hole where
density is not acceptable.
4. Density log corrected for shale (in good hole only).
5. Sonic log corrected for shale (in bad hole or if nothing else
6. Neutron log corrected for shale (in bad hole or if nothing
else is available).
7. Microresistivity, shallow or deep resistivity as a last resort.
8. Apply maximum porosity and material balance constraints to
Apply non-porous lithology triggers as needed.
Discard unreasonable answers, and/or revise shale or matrix assumptions
and re-compute if difference between methods is too large.
Normally you will settle upon a method that suits you and the
zone under consideration. You will not have time to compute results
from all methods. Use the above list as a guide to reduce your
effort and to gain a better chance for success on the first pass.
Log analysis is seldom satisfactory on the first pass in new areas,
so do not be bashful about trying several methods.
Then keep a record of which methods worked best in which areas.
Porosity is usually reported to the nearest 1/10 of one percent
(or 0.001 fractional) but this can be reduced to the nearest percent
(or 0.01 fractional) for hand calculations or for porosity greater
Computed Results for Mixed Lithology Example from six
different porosity models, compared
to core analysis porosity.
All porosity calculations need to be
calibrated to core data at some point, usually at an early stage in
an analysis project. Log analysis in isolation from other data is
pointless and dangerous.