Geology Reference
In-Depth Information
7
6
A
5
B
4
R
2
= 0.44
3
0
0.05
0.10
0.15
Porosity
Figure 8.63
Porosity vs velocity for low-porosity clean sandstones from the same formation but different wells (after Smith et al.,
2009
).
Colours represent different wells. It is evident that in general there is a poor correlation between velocity and porosity and a large scatter of
P wave velocity. Two groups of data are identified ('A' and 'B') and it is inferred that the principal reason for the difference is that group B have a
greater proportion of compliant microcracks than group A. Note how a different effective mineral velocity would be predicted for data groups
A and B.
The definition of litho-facies, for example based
on discrimination cut-offs, should involve a
careful consideration of the geological
environment. It is not often that a geological facies
scheme, for example based on detailed thin section
analysis, can be translated directly into rock
physics litho-facies. There is usually some
simplification required.
data. Equally, understanding the stratigraphic and
effective pressure context of the data prior to
averaging is of paramount importance.
Once the rock physics model has been established
it is tempting to believe that subsequent fluid or
lithology substitution is valid. It is possible that
the chosen model is too simplistic. Different
models may fit the same data but have different
implications. Smith (
2011
) highlights the need to
understand the reasons for porosity change before
settling on an appropriate rock physics
relationship to describe the data.
When assessing variability in elastic parameters
between wells and determining statistics for
later use in probabilistic applications a
distinction needs to be drawn between the natural
variability of log data and true geological variability.
Caution is needed in using rock physics models
outside the measured range, for example at depths
below which there is no measured log data, or for
lithologies that have not been encountered in the
wells. Rock physics models can be captivating; one
should keep the data in sight.
There are a number of important issues in the
construction of depth trends from multiple wells.
Averaging should be specific to individual litho-
facies and it would usually be inappropriate, for
example, to combine clean sand and shaley sand
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