By KATHY SHIRLEY
DHI: Overcoming 'Mindless' Approaches
Proper Time, Effort Brings Success
In a perfect world, direct hydrocarbon indicators would
be just that -- direct hydrocarbon indicators. But that term is
a misnomer that since its inception has plagued the exploration
That's not to say that seismic hydrocarbon indicators
are not important -- but those anomalies on the data require sophisticated
processing and analysis techniques to accurately predict the presence
They are, after all, indicators.
"Seismic direct hydrocarbon indication started out
simply by looking for bright spots and has been plagued by this
mentality ever since," said AAPG member John Castagna, a professor
at the University of Oklahoma and a world renowned expert on direct
hydrocarbon indication. "The desire is for a single universal indicator
that can be displayed as an attribute and colored red.
"When occurring in the right structural context that
approach can be powerful, even without much geophysical understanding,"
he said. "Unfortunately, this mindless approach to direct hydrocarbon
indication has led to many dry holes, especially when hunting stratigraphic
"Many companies successfully utilize direct hydrocarbon
indicators," he continued. "What those companies have in common
is a willingness to take the time and effort to geophysically and
petrophysically understand the attribute they are interpreting."
In recent years advancements in seismic processing
and interpretation have been achieved with amplitude variation with
offset, or AVO, and that data has impacted the value of direct hydrocarbon
"Major strides have been made in AVO analysis, including
AVO cross plotting, proper AVO inversion, utilization of long offset,
etc.," he said. "Such techniques are helping us not only detect
hydrocarbons, but with a strong understanding of rock physics, identify
fluid type and saturation."
Unfortunately, a lot of "snake oil" is still being
peddled, he said.
"There has been a proliferation of new AVO attributes
that have been advertised as having magical properties, which they
do not," he said. "In my travels I have been appalled at the expense
people have gone through to do unnecessary additional analysis and,
worse, the dry holes that have been drilled because naïve interpreters
believed in magic.
"My advice to interpreters is, ask yourself if the
new attribute really contains any additional information," he said.
"If it doesn't, you probably aren't gaining more than you could
by just changing your color bar."
Needed: More Research
Despite these pitfalls, the use of direct hydrocarbon
indicators is widespread.
"Amplitude and attribute analysis is now almost universal,"
he said, "AVO is applied routinely in most clastic basins, and in
places it has been used well in carbonates and Paleozoic rocks.
"The circumstances have to be just right, however,"
he said, adding that the most useful setting for the technique is
still for gas detection in young, shallow, porous, poorly consolidated
Castagna believes much research remains to be done
to advance the applicability and scope of direct hydrocarbon indication.
"The effects of anisotropy can no longer be ignored,
and we are just starting to learn how to interpret critical and
post-critical angle reflections," he said. "More effective pre-stack
noise suppression will always be needed.
"Pre-stack spectral decomposition is only in its
infancy and is showing great promise," he said. "But probably our
biggest problem is poor imaging at far offsets. In my experience,
no matter what the claims, our pre-stack migration methodologies
are not good enough at wide angles.
"Eventually, imaging researchers will have to start
being careful with amplitudes and will have to concentrate on more
than just getting a good structural picture."
-- KATHY SHIRLEY
There has always been a slight element
of mystery surrounding those seismic squiggles for geologists, even
for all the geologists who know how to work with seismic data.
Wouldn't it be nice to turn those squiggles into
A new seismic interpretation technique is offering
just that -- plus important implications for advancing the science
It's all an outgrowth of direct hydrocarbon indication
and the ongoing effort to better understand from a geologic perspective
the anomalies that crop up on seismic data.
"I have been working with seismic all my professional
life, mostly in the Gulf of Mexico," said geologist Gordon Van Swearingen
with Houston-based eSeis Inc. "In the early, exciting days of bright
spots I was actually told that geologists were just needed to give
tops to the geophysicists, because every well we drilled offshore
in the Gulf had a show within 100 feet of a bright spot.
"I had to explain that on average every well in the
Gulf of Mexico had a show every 100 feet," he said, "but a show
and a commercial well are two different things."
Advancements in the understanding of direct hydrocarbon
indicators -- and particularly amplitude variation with offset --
are advancing the knowledge that can be gleaned from seismic data.
"What we do is look at AVO information and translate
that to lithology, porosity and fluid," Van swearingen said. "You
can get an amplitude indicator off of regular stacked data, but
you will never know if that indicator is a result of a lithology,
porosity or fluid change.
"Using AVO the information is better," he added,
"but you still can't differentiate between those elements. You have
an amplitude, but you don't know what's causing it."
As an example he cited a trend in south Texas, where
the operator is getting stacked amplitudes caused mainly by porosity.
"The company has drilled a lot of these amplitudes
and the reservoir is nice and porous, but there is no gas," he said.
"We try to put more geology into that amplitude to
achieve a better understanding -- I like to call it a seismic outcrop."
Place and Pay
Roger Young, the chief technical officer for eSeis,
is a petrophysicist who wanted to view seismic as petrophysical
data as opposed to geophysical data.
The key technique in petrophysics is cross plotting
where two independent pieces of information can be calibrated back
to lithology in one direction and porosity in the other. He wanted
a way to use that same technique with seismic data.
After studying seismic data he realized that by using
pre-stack time migrated data he could extract both compressional
and shear information from the gathers and cross plot those to get
lithology and porosity.
"We take amplitudes one step further and look at
what's actually causing that amplitude in terms of lithology, porosity
and fluid," Young said. "With that information you can go back and
apply geology to the seismic.
"What we do through inversion and AVO techniques
is split apart the contributions from porosity, lithology and fluids,"
Young continued. "Through this process the dataset is put in geologic
"Referring to a change in AVO gradient or a change
in acoustic impedance doesn't tell a geologist much," he said. "But
if that can be related as a change in lithology, porosity or fluids,
then it means something to geologists."
With this technique interpreters can first place
the reservoir and then add the pay potential.
"The Gulf of Mexico is a good example," Van swearingen
said. "You can get a hydrocarbon show every100 feet -- there is
plenty of gas out there, but whether or not it's economic gas is
the key, and that is controlled by the quality of the reservoir,
among other factors."
Finding Meaningful Estimates
In order to predict lithology, pore fluids and porosity,
an understanding of specific rock properties as found in the seismic
data is required.
One set of properties is found in the seismic waveforms
themselves -- the measure of p-compressional wave energy and s-shear
wave energy. Once estimated, the relationship between the two types
of seismic energy may be used to analyze specific rock properties.
Analyzing the rock properties of different classes
of lithology indicates shales on one end of the extreme and high
porosity sandstones on the other.
"When we analyze shales, we find that they have a
very low relative compressibility and a very high relative shearability,"
Van swearingen said. "Conversely, a porous sandstone with a high
gas content has a high relative compressibility as well as a high
relative shearability. Tight sandstones and carbonates have a low
relative compressibility and a low relative shearability."
The relative compressibility and relative shearability
of the rock can be measured via the seismic wave energy. Petrophysical
techniques can then be applied to aid in understanding the relationship
between the two, and generate a meaningful estimate of rock types
and pore fluids.