Putting Geology into the 'Squiggles'

DHI the Next Seismic 'Bright Spot'?

By KATHY SHIRLEY
EXPLORER Correspondent

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 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."

-- 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 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.