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."