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DHI the Next Seismic 'Bright Spot'?

Putting Geology into the 'Squiggles'

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."

Image Caption

New federal edicts for the Gulf of Mexico, intended to protect marinelife, are presenting new challenges for seismic crews.
Photo courtesy of Fairfield Industries

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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.

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