Page 85 - Well Logging and Formation Evaluation

Advanced Log Interpretation Techniques 75

formation in variable amounts. I have worked in some fields having
varying amounts of limestone, marl, anhydrite, dolomite, siderite, pyrite,
quartz, and clays where a conventional approach using deterministic equa-
tions is not reliable. In such a situation, the best approach is to adopt a
multimineral/statistical model. The basic way programs using this tech-
nique operate is as follows:

1. The various minerals and fluids to be included in the model are
determined.
2. The response of each of these minerals/fluids for a variety of parame-
ters as measured by logging tools is specified by the user.
3. The program finds the combination of mineral/fluid volume fractions
that most closely matches the observed log responses, such as to a
variety of criteria and constraints specified by the user, such as the:
relative importance (weighting) of various tools,
measurement error for each tool,
relative saturations of fluids in the invaded zone as opposed to the
virgin zone,
relative amounts of various minerals, and
resistivity response relative to fluid saturations (e.g., Archie, dual-
water, Waxman-Smits, etc.).

Needless to say, it is not possible to have a greater number of
minerals/fluids than the number of tool response equations (although the
fact that the sum of all the volume fractions must equal 1 effectively pro-
vides an extra equation).
Overall, this sounds like a very rigorous approach to a conventional
sort of deterministic evaluation, which would be preferred in all cases.
Reasons why it is not are as follows:

1. The fact that the program is capable of calculating back the correct log
responses does not mean that the results are necessarily correct. If a
bad choice of minerals/fluids is made or their properties are incorrect,
a solution may be found that is completely wrong.
2. The program does not have any depth-dependent reasoning capability.
Hence, one may frequently find “a bit of everything everywhere,” with
gas below oil and oil below water and minerals popping up all over
the place, often more in response to the hole quality than anything else.
3. Many of the tool responses for minerals in the formation are not known
accurately and recourse is made to standard tables of typical minerals.