THE NEW ROLE OF PETROPHYSICS IN GEOPHYSICAL
E. R. (Ross) Crain, P.Eng.
Spectrum 2000 Mindware Ltd.
403 – 845 – 2527 firstname.lastname@example.org
Published in CSEG Recorder Sept 2003
NEW ROLES – NEW INTERACTIONS – NEW INTEGRATION The role of petrophysics in seismic interpretation has taken a major leap forward in the past ten
years, resulting from important advances in seismic data processing techniques, particularly
seismic inversion, attribute analysis, and amplitude versus offset methods that showed we could
estimate reservoir properties from such data. Coupled with the recent advances in dipole shear
sonic logging, new vistas in seismic interpretation, dubbed seismic petrophysics, have opened.
Geophysical well logs suffer from many borehole and environmental problems that need to be
repaired before being used for calibrating seismic models or seismic interpretations. A primary
aim of the geophysicist/petrophysicist is to create a synthetic seismic trace from EDITED log data
that accurately represents the seismic response of the subsurface. This is accomplished by
editing, repairing, or reconstructing the log data. Using unedited logs for seismic purposes is a
waste of time and money and, in the worst case, can lead to very expensive exploration and
If the synthetic seismic trace is a good representation of the real seismic response, then the
edited logs can be used effectively as aids to interpretation of the advanced seismic products.
Consequently, the role of the petrophysicist has also evolved; she must now be competent in log
reconstruction as well as conventional log analysis, and must understand the petrophysical
needs and limitations of the inversion, attribute, or AVO results. Unfortunately, logs are not
perfect measures of in-situ rock properties and seismic data is severely band-limited compared to
log data, so there are many compromises to be made. A significant change in mindset is also
needed, as most of the log repairs (with the exception of fluid replacement) take place in the non-
reservoir intervals - intervals that are not usually of interest to petrophysicists.
Geophysicists engaged in seismic interpretation seldom use logs to their full advantage. This sad
state is caused, of course, by the fact that most geophysicists are not experts in log analysis.
They rely heavily on others to edit the logs and do the analysis for them. But, many
petrophysicists and log analysts have no ides what geophysicists need from logs, or even how to
obtain the desired results. That's a particularly vicious "Catch-22".
Education, practical solutions, appropriate software, and practice are the keys to success. In
order for geophysicists and petrophysicists to communicate well, each must know something of
the other's specialty. Chapters Twenty through Twenty-Five on my website www.spec20000.net
provide theory, practical methods, and case histories to accomplish this goal.
SEISMIC PETROPHYSICS AND SEISMIC MODELING Seismic petrophysics is a term used to describe the conversion of seismic data into meaningful
petrophysical or reservoir description information, such as porosity, lithology, or fluid content of
the reservoir. Until recently, this work was qualitative in nature, but as seismic acquisition and
processing have advanced, the results are becoming more quantitative. Calibrating this work to
well log “ground truth” can convert the seismic attributes into useful reservoir exploration and
development tools. Since there are an infinity of possible inversions, it is pretty important to find
the one that most closely matched the final edited logs or the computed results from those logs.
A seismic petrophysics study aimed at quantifying porosity is shown below in Figure 1.
FIGURE 1: Seismic petrophysics study for porosity
This example used a geostatistical package to distribute the dense "fuzzy" seismic attribute data
between the sparse, "accurate" well log data. The logs, or log analysis results, in turn are
calibrated to core, well test, and production data before being used to control seismic
interpretation. The use of geostatistics to map seismic attributes onto well logs is a relatively new
phenomenon and is showing great promise.
SEISMIC PETROPHYSICS AND WELL LOG MODELING
Unfortunately, it takes a fair amount of effort to compare seismic results to log data. The logs will
usually require some kind of editing or modeling or both. Comparison of seismic results to log
data may indicate that further processing of the seismic is needed, and the calibration cycle is
repeated – often several iterations are needed. In other cases, it is the logs that need further
Log modeling or editing is required because logs don’t see the same rock and fluid mixtures that
the seismic signal sees. Drilling fluid invasion removes gas or oil near the wellbore, replacing it
with water and altering the sonic and density log response from the reservoir's undisturbed
values. Compensating for invasion is called "fluid replacement". Fluid replacement calculations
are also used in "what-if" scenarios to see what a gas filled reservoir might look like on seismic.
Such models are usually run post-mortem, after a lovely seismic bright spot was drilled to find an
equally lovely porous water zone. Maybe the models should be run BEFORE drilling?
The author and John Boyd presented a practical solution for fluid replacement in 1979, based on
the log response equation and a "pseudo-travel time" for typical gases. Since then, at least a
dozen, more rigorous but less friendly, solutions have been published: Castagna, Greenberg and
Castagna, Aki and Richards, Batzie and Wang, Toksoz et al among others. Most are based on
extensions of early work (late 1950's) by Biot, Gassmann, and later, Domenico. The final tally is
not in yet on fluid replacement calculations for the sonic log, especially in shallow,
unconsolidated, or underpressured reservoirs.
Fluid replacement calculations for the density log are straight forward, with no pitfalls if the gas or
oil PVT properties are known. How well do you know the reservoir engineer down the hall?
Mechanical or chemical rock alteration due to drilling usually reduces sonic velocity and density
in the environment measured by the logging tool. This effect is somewhat subtle but pervasive or
it can be catastrophic as in hole breakouts. It can be repaired by using information from other log
curves (in the case of bad density data), or checkshot or VSP data to calibrate the sonic log. But
many common sense rules for using checkshots are ignored because the software doesn't think
like a human petrophysicist.
Acoustic frequency differences have to be accounted for, especially when shear velocity is
measured. High frequency shear velocity (lab measurements and sometimes sonic log data) is
faster than low frequency (seismic) data. Anderson's 1984 paper provides useful information but
is weak on specific recommendations.
Poor log response due to bad hole condition or
faulty logs may be an even more serious problem,
as in Figure 2. Check-shots, offset well data, other
logs, and common sense are used to correct for
Figure 2: Rough sonic log corrected where it needs
it – delta-T minimum method
The log should be edited only where it needs it
using common sense rules grounded in local and
regional trends. Few practitioners have hip pockets
full of sonic and density trend data applicable to
their current projects.
Again, at least a dozen authors have provided more
or less practical solutions, such as Ausburn, Faust,
Smith, Fischer and Good, Crain and Boyd, Patchett.
Calibration methods come in three flavours: good, bad, and really ugly. Block shifting a log is
really ugly. Rescaling and delta-T minimum methods are better but still a little bit ugly. Discreet
editing where the log needs it, or more sophisticated curve fitting techniques based on other logs,
are pretty good approaches. The ugly methods are fast and mostly useless, as most of the false
reflectivity is still there. The good methods take more effort, but you get what you pay for.
In other cases, no appropriate logs exist, so sonic and density data have to be created by
transforming some other available log. Most of the methods used to repair bad hole effects will
also generate complete sonic or density logs. In the worst case, a set of geological tops, lithology
descriptions, and an offset well log will suffice, especially if only the density log is missing.
Some models are made by "cut and paste", for example thickening or thinning a reef or pinching-
out a sand bar to see what happens to the seismic signature. Splicing realistic data from one well
to another in a geologically sensible manner can create any number of plausible models. The
more models you create, the more likely you will find one that matches your seismic.
Smoothing and filtering may also be performed on raw or edited logs to extract only those
frequencies that are likely to be recorded in real seismic data. Cut and paste, and filtering, are
fairly obvious operations and are not dealt with further here.
A competent petrophysicist working closely with the geophysicist can provide the needed
expertise to solve these problems and generate useful log data. When integrated with the
geologist and reservoir engineering members of the team, very credible interpretations will result.
LOGS USED TO AID SEISMIC PETROPHYSICS
The two logs most used by geophysicists are the sonic (also called acoustic) log) and the density
log, because these two rock properties determine the acoustic impedance and hence the
reflection coefficients of the rock layers. A synthetic seismogram can be calculated from these
Raw logs should NEVER be used for this purpose - editing and
modeling are nearly always required.
Most other log curves are useful to the geophysicist. For example,
the neutron, density, photoelectric effect, and spectral gamma ray
(both (natural and induced) can be used to determine lithology
quite accurately. This knowledge assists seismic modeling and
inversion or attribute interpretation.
Even the lowly gamma ray log plotted on a two-way time scale on
a seismic section can be an invaluable aid to horizon picking and
interpretation, since it is one of the best shale indicators available.
Computed log analysis results, such as shale volume, porosity,
lithology, and hydrocarbon fill are very informative when
displayed on a seismic section, as shown in in Figure 3. Notice the
strong reflections caused by even thin gas zones (pink colour
on the log analysis).
Figure 3: Log analysis results showing hydrocarbon fill (pink)
plotted on two-way time scale with VSP data.
These properties are all derived from appropriate log analysis
techniques. They are generally called log analysis results,
petrophysical properties, or computer processed interpretations
(CPI). They often provide the "ground truth" for calibrating
attribute or inversion interpretation.
Modern sonic logs, called full wave, array, or
dipole sonic tools, record the complete sonic waveform instead of
just the travel time of the first arrival. This allows us to process
each wavetrain to determine shear wave and Stoneley wave travel
time (and hence velocity) as well as the more usual compressional
wave travel time.
Thus shear wave synthetics can be constructed to calibrate shear wave seismic sections.
Lithology analysis and direct hydrocarbon detection are sometimes possible from a comparison
of compressional and shear velocities. These can be verified by the compressional and shear
synthetic seismograms. A transform of shear and compressional data, either from logs or seismic,
into Poisson's Ratio helps distinguish between hydrocarbon and lithology variations.
The key to interpretation of seismic inversion, attribute analysis, and AVO is Petrophysics. Your
friendly, neighbourhood petrophysicist can help you integrate geophysics and petrophysics for
more effective exploration and development. Try it, you’ll like it!
ABOUT THE AUTHOR
Mr Crain is a Professional Engineer with over 35
years of experience in reservoir description,
petrophysical analysis, and management. He has
been a specialist in the integration of well log
analysis and petrophysics with geophysical,
geological, engineering, and simulation phases of
oil and gas exploration and exploitation, with
widespread Canadian and Overseas experience.
He has an Engineering degree from McGill
University in Montral and is a registered engineer in
Alberta. He wrote “The Log Analysis Handbook”,
published by Pennwell, and offers seminars,
mentoring, or petrophysical consulting to oil
companies, government agencies, and consulting
service companies around the world.
Ross is credited with the invention of the first
desktop log analysis system (LOG/MATE) in 1976, 5
years before IBM invented the PC. He continues to
advise and train people on software design,
implementation, and training. For his consulting practice, he uses his own proprietary software (META/LOG), and is familiar with most commercial
He has won Best Paper Awards from CWLS and CSEG and has authored more than 30 technical
papers. He is currently building an Interactive Learning Center for petrophysics on the World Wide
Web. Mr Crain was installed as an Honourary Member of the Canadian Well Logging Society for
his contributions to the science of well log analysis.