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
business.
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
of hydrocarbons.
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
traps.
"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
indication.
"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
clastic rocks.
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."