Outcrops Never Looked So Good

To 'Tell the Truth' About Reservoirs

You may never see an outcrop the same again.

Thanks to years of research and newly cost-effective technology, 3-D outcrop analysis has become a viable tool for geologists.

"Literally in the past couple of months, we've made some really important breakthroughs," said Janok Bhattacharya, an associate professor and researcher at the University of Texas-Dallas.

Outcrops are cropping up in 3-D everywhere:

  • This year Schlumberger will conduct a 3-D outcrop project in Namibia.
  • Outcrop work is planned for both the Frontier Sandstone and the Lewis Shale in Wyoming.
  • AAPG's annual meeting in Denver will offer presentations about 3-D imagery and outcrops (see related story, page 20), including a poster session and an e-poster session by Schlumberger researchers involved in the Namibia project.

Call this, in part, the result of G-technology: GIS, GPR, GPS, GOCAD - or in long form, geological information systems, ground-penetrating radar, the Global Positioning System and geologically oriented computer-aided design.

Today's geologists and researchers use these tools to peer into outcrops to create three-dimensional studies, and to gather detail for digital 3-D photo-representations.

Outcrops remain the best guide to stratigraphy and provide key evidence of reservoir make-up, according to Roger Slatt, director of the University of Oklahoma School of Geology and Geophysics.

"My feeling in general is that outcrops tell the truth about what a reservoir is made up of," Slatt said. "They're the only way you can get continuous information about the reservoir."

Bhattacharya said work on 3-D outcrop imaging, including the use of GPR, began in the 1980s. UT-Dallas has spent years researching the problem, with funding from industry and the U.S. Department of Energy, he said.

After almost 20 years in the oil industry, Bhattacharya moved from Arco to UT-Dallas two and a half years ago. He thinks of outcrop studies as one step leading toward a better understanding of the reservoir in production.

The ultimate beneficiary is not the petroleum geologist, he said, but the reservoir engineer.

"Part of my work has been putting the fear of geology into engineers - explaining how complicated rocks are, but giving them the knowledge to understand how to work with those complexities," Bhattacharya said.

"What companies need are good descriptions of what the connectivities of sandstones are. For that, a lot of companies go to outcrops."

Challenges and Obstacles

Geologists probably have mapped outcrops for as long as they've seen outcrops. The resulting 2-D approach was useful, but limited as a predictor of rock volumes and other reservoir characteristics.

"The problem is, oil companies want to know, 'What is the volume of oil you can get out of there? What's the recovery factor?'

"We all know what we need are 3-D reservoir models," Bhattacharya continued, "but they are hard to come by."

Since outcrops are at the surface, ground-penetrating radar might seem like a natural choice for gathering near-subsurface data. However, use of GPR has proven problematic.

"People have tried to use GPR before, without much success," he explained. "You have to be able to process that data to get what you want and eliminate what you don't want."

GPR has some of the same features of seismic data, Bhattacharya said - lots of good data masked by useless data, misleading data and just plain noise.

At UT-Dallas, a team led by George McMechan has worked on developing algorithms to clean up GPR data, he said. The data can then be used in 3-D seismic-like visualizations.

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You may never see an outcrop the same again.

Thanks to years of research and newly cost-effective technology, 3-D outcrop analysis has become a viable tool for geologists.

"Literally in the past couple of months, we've made some really important breakthroughs," said Janok Bhattacharya, an associate professor and researcher at the University of Texas-Dallas.

Outcrops are cropping up in 3-D everywhere:

  • This year Schlumberger will conduct a 3-D outcrop project in Namibia.
  • Outcrop work is planned for both the Frontier Sandstone and the Lewis Shale in Wyoming.
  • AAPG's annual meeting in Denver will offer presentations about 3-D imagery and outcrops (see related story, page 20), including a poster session and an e-poster session by Schlumberger researchers involved in the Namibia project.

Call this, in part, the result of G-technology: GIS, GPR, GPS, GOCAD - or in long form, geological information systems, ground-penetrating radar, the Global Positioning System and geologically oriented computer-aided design.

Today's geologists and researchers use these tools to peer into outcrops to create three-dimensional studies, and to gather detail for digital 3-D photo-representations.

Outcrops remain the best guide to stratigraphy and provide key evidence of reservoir make-up, according to Roger Slatt, director of the University of Oklahoma School of Geology and Geophysics.

"My feeling in general is that outcrops tell the truth about what a reservoir is made up of," Slatt said. "They're the only way you can get continuous information about the reservoir."

Bhattacharya said work on 3-D outcrop imaging, including the use of GPR, began in the 1980s. UT-Dallas has spent years researching the problem, with funding from industry and the U.S. Department of Energy, he said.

After almost 20 years in the oil industry, Bhattacharya moved from Arco to UT-Dallas two and a half years ago. He thinks of outcrop studies as one step leading toward a better understanding of the reservoir in production.

The ultimate beneficiary is not the petroleum geologist, he said, but the reservoir engineer.

"Part of my work has been putting the fear of geology into engineers - explaining how complicated rocks are, but giving them the knowledge to understand how to work with those complexities," Bhattacharya said.

"What companies need are good descriptions of what the connectivities of sandstones are. For that, a lot of companies go to outcrops."

Challenges and Obstacles

Geologists probably have mapped outcrops for as long as they've seen outcrops. The resulting 2-D approach was useful, but limited as a predictor of rock volumes and other reservoir characteristics.

"The problem is, oil companies want to know, 'What is the volume of oil you can get out of there? What's the recovery factor?'

"We all know what we need are 3-D reservoir models," Bhattacharya continued, "but they are hard to come by."

Since outcrops are at the surface, ground-penetrating radar might seem like a natural choice for gathering near-subsurface data. However, use of GPR has proven problematic.

"People have tried to use GPR before, without much success," he explained. "You have to be able to process that data to get what you want and eliminate what you don't want."

GPR has some of the same features of seismic data, Bhattacharya said - lots of good data masked by useless data, misleading data and just plain noise.

At UT-Dallas, a team led by George McMechan has worked on developing algorithms to clean up GPR data, he said. The data can then be used in 3-D seismic-like visualizations.

"In some cases, it's taken us a year to write the algorithms to process that data," McMechan said, "but that's an absolutely critical step."

Drilling and coring wells behind an outcrop adds valuable information, as well as providing a reality check for the 3-D picture, he added.

But the cost of that approach - Bhattacharya estimated $15,000-$20,000 for the GPR and another $15,000-$20,000 for the holes to be cored - can be prohibitive for an academic research program.

"For industry that's not so expensive," he said. "For us, as a university, that is expensive."

Carlos Aiken and Xueming Xu at UT-Dallas have developed a 3-D, photorealistic virtual outcrop of the Austin Chalk, according to Bhattacharya.

"This outcrop already has been placed within the immersive visualization rooms at Norsk Hydro and Exxon," he said.

"We also have nearly finished work on a 3-D interpretation of deep-water deposits of the Jackfork Sandstone outcrop at Big Quarry, Ark., based on integrating digital terrane maps with digital photomosaics."

Let's Get Real

In the past, the industry's efforts usually focused on obtaining detailed 2-D outcrop representations, said David McCormick, senior research scientist for Schlumberger-Doll Research in Ridgefield, Conn.

"One of the things that has always been hard for industry consortia is that they produce mosaic panels (of outcrops) and lots of detailed sedimentological work," he said, "but in the end those are 2-D and they're on paper."

Now Schlumberger and Shell in Oman, with other industry partners, are funding a 3-D study of outcrops about 150 miles south of Namibia's capital, Windhoek. McCormick gets to conduct the outcrop survey, and he's excited.

"In these sections in Namibia are the world's oldest shelly fossils," he said. "It's just before the Cambrian explosion everybody knows about. It's a scientifically hot place to be working."

McCormick said the project aims to provide an analog for deep drilling targets in Oman that are poorly imaged on seismic. John Grotzinger and two post-doctoral researchers from the Massachusetts Institute of Technology also will conduct associated studies in Namibia, he said.

And McCormick has another reason to look forward to the trip:

"For me, personally," he said, "this will be the first time I've taken an outcrop all the way through to reservoir simulation."

Schlumberger will use real-time kinetic (RTK) differential GPS and laser-optical measurement instruments to obtain outcrop data. McCormick said he'll wear a backpack with equipment and gather readings as he moves.

"You can do it very quickly and you can combine volumetrics with purely geological information," he said. "And you can do that in an extremely efficient way. You can map as fast as you can walk."

RTK solves and corrects for wavelength ambiguities in GPS signals. It maintains the correct algorithms and locks onto a specific set of GPS satellites throughout a survey, using one or multiple positioning instruments.

Its major advantages are accuracy and fineness of scale. Schlumberger's Namibia outcrop survey will have 2-centimeter resolution, according to McCormick.

Data can be loaded into a portable computer and processed in the field, allowing the geologist to work with real-world readings in real time, McCormick noted. Once a digital topography map is generated, it can be combined with aerial photos, photogrammetry or other digital photos.

"If you're concerned about shales as barriers and baffles, you can go out there (to an outcrop) and measure the important characteristics and the distribution of the shales," he said.

"The process is both time-effective and cost-effective - the prices have come down to where even academic groups can use these tools."

Finding What's Important

Outcrop imaging in 3-D provides a useful tool, but one that's secondary to a three-dimensional look at reservoir geology, according to McCormick.

"Visualization is an important and enabling technology that we use to support the work we do," he said, "but the point is to quantify geology in 3-D — connect a variety of measurements and interpretations to their 3-D spatial position.

"Reservoir simulators don't care about visualization," he continued, "they care about where, in 3-D, the porosity, permeability, fluid saturations and pressures are."

Imaging is an important tool for spatial insight, "but the engineering and economic exercise is driven by the 3-D locations of key data and information," he added.

McCormick and Peter Kaufman, also a Schlumberger geologist, will present information about digital geology and outcrop studies at the 2001 AAPG annual meeting.

Continuity: The Key Issue

Slatt, who before coming to OU, directed the geology and geological engineering program at the Colorado School of Mines (CSM), has done pioneering work in outcrop studies.

"Years ago, I helped to popularize gamma-ray logging of outcrops," he said. "I also have championed going behind the outcrop and drilling wells, then logging the wells, including borehole image logs, and coring them to get a picture of reservoir continuity beyond the outcrop."

And continuity, not just rock volume, determines the character of a reservoir and provides a primary focus for outcrop analysis.

"Continuity is a real key issue," Slatt noted. "You can get a high volume of sands, but if it's all discontinuous and broken up by shale, you're not going to have a good reservoir."

Slatt still works on outcrop studies with colleague Roger Young at OU and AAPG Elected Editor Neil Hurley at CSM. He's leading CSM/OU's Lewis Shale Consortium, sponsored by 16 companies, which will produce 3-D visualizations in Wyoming.

GPR has proven very successful in the Lewis Shale, as excellent 3-D images of channel sandstones have been obtained from behind two outcrops.

That project also received "tremendous support" from the Gas Research Institute, now the Gas Technology Institute, he added.

A special area of interest for Slatt is reservoir characterization on subseismic scale.

"A lot of features that control production, fluid flow, show up on the subseismic scale," he said.

His research mainly extends to deep-water reservoir types.

Because of those interests, he's intrigued by the kind of insights outcrop studies can provide.

"There are some key questions in deep-water development," he said. "For instance, 'How widely spaced must our wells be when going into production?' That's a question addressed by the lateral continuity of the reservoir."

Companies typically take seismic data and control from two or three of the wells, then try to draw up a comprehensive production plan for an offshore reservoir, according to Slatt.

He quoted one company that realized the importance of outcrop analogs too late:

"They evaluated their work later on (after beginning production) and said, 'If we had looked at the continuity and character of the reservoirs, we could have saved ourselves a whole lot of money.'"

Now the largest companies maintain catalogues of outcrops, he said, based on extensive research and analysis.

Research Continues

At UT-Dallas, Bhattacharya said, researchers take 3-D seismic models and "feed them to our reservoir engineering cohorts," including Chris White at Louisiana State University.

"What heterogeneity really matters? That's up to the engineers, and that work is really just starting," Bhattacharya said. "It's more of a matter of cost right now."

McCormick also acknowledged that 3-D outcrop studies have gained importance as guides for engineering, not fancy geology.

"I get paid to help the bottom line," he said. "It became clear to me early on that the people who hold the purse strings are the reservoir engineers and the drillers."

Several new trends promise to enhance the move to outcrop visualization:

♦   Researchers are now building 3-D outcrop models with GOCAD software, developed in France.

The GOCAD Research Program was begun by the computer science group of the National School of Geology in Nancy, France, and is managed by the Association Scientifique pour la Geologie et ses Applications (ASGA).

Bhattacharya said use of GOCAD has been a breakthrough application at UT-Dallas, and McCormick said he hopes to load GOCAD for his Namibia project.

♦  Another breakthrough development might be use of seismic instead of ground radar for deeper penetration of outcrops.

"The problem with GPR is that it doesn't penetrate very far," Bhattacharya said.

♦  And in the future, you may walk into a 3-D outcrop display instead of walking up to an outcrop.

"Most of the biggest corporations have these visualization rooms," Bhattacharya observed. "They're realizing those are good places to visualize the guts of these reservoirs." (See related story, page 12.)

Technologies for 3-D outcrop studies may have become less expensive, but that's probably not the major consideration for industry, Bhattacharya said. Getting good, applicable information in a timely manner makes the difference.

"Companies can't afford to send their best geologists into the field for three months to gather outcrop data," he said. "The big cost for the oil companies is in time."

He has no doubt 3-D outcrop visualization will become a standard for the industry. You may never see outcrops in 2-D, even mentally, again.

"Once a technology is developed," he said, "everybody wants it."

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