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
real geology?
A new seismic interpretation technique is offering
just that — plus important implications for advancing the science
of exploration.
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
context.
"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.