Long studied outcrops in Spain
that may hold secrets to understanding deepwater reservoirs are
providing new clues, thanks largely to new 3-D laser technologies.
Lasers coupled with sophisticated computer programs
are generating three-dimensional representations of the outcrops
that, compared to traditional field studies, significantly enhance
the information that can be gleaned from the exposed rocks.
Researchers from the Bureau of Economic Geology at
the University of Texas at Austin recently traveled to Spain to
study outcrops in the Ainsa and Tabernas Basins — rocks that are
analogous to deepwater deposits offshore West Africa. Their effort
is part of a larger program designed to develop an entire portfolio
of world-class examples of common reservoir types via 3-D representations
of outcrops — a virtual geology experience, said David Jennette
of the BEG and a researcher on the project.
The team has also studied outcrops in California,
West Texas and Chile to better understand deepwater reservoirs (see
April EXPLORER).
To do this, the scientists are using a light detection
and ranging (LIDAR) system, which has proven to be a valuable tool
for rapid, quantitative characterization of outcrop geology.
That technology — along with computer advances in
acquisition technology, data handling, data merging and visualization
— is providing a superior method of outcrop capture and analysis
when compared to traditional photograph-based methods, Jennette
said.
The combination allows outcrop facies to be readily
placed into navigable 3-D volumes that can be examined while in
the field and then later interpreted on a workstation or PC back
in the office.
The high resolution digital terrain models are draped
with conventional photographs and co-rendered with attributes such
as:
- Weathering profile.
- Laser intensity.
- Multi-spectral data to produce greatly
enhanced outcrop imagery.
Total Recall
LIDAR, built by Optech, is a portable system that emits
a laser pulse and then receives and measures the laser beam bouncing
off an outcrop face.
"The instrument collects 2,000 points a second and
is basically a mirror that captures the reflected laser or image
of the outcrop face," Jennette said. "We typically capture points
every two to five centimeters on an outcrop."
They then scan the outcrop looking for the geology
they want to capture as a very detailed digital elevation model.
Because this system captures so much spatial data, they are able
to fill out the XYZ points so the final image actually looks like
the outcrop surface — "and we can visualize almost all of the geology,"
Jennette said.
Using the LIDAR system Jennette and his colleagues
can produce a black and white photograph of the outcrop in three
dimensions because the instruments capture the intensity of the
reflection.
"It knows what energy is being emitted by the laser
and then measures the reflection strength that is returned," he
said. "Intensity values are closely tied to rock color and lithology."
The tool allows them to take several 10- to 15-minute
scans, then move the scanner over a few tens or hundreds of meters
and shoot another scan, almost as if taking a photograph.
The laser is just part of the system. Equally important
are the computer software programs that synthesize and then bring
meaning to the images.
"The software allows all these scans to be merged
within a matter of seconds," Jennette said. "We pick what appear
to be common points on two different scans, and the software merges
the two instantaneously within a statistical threshold that we establish.
That is how we capture kilometer long outcrops."
The 3-D outcrop volumes are compatible with visualization
technology, so in addition to viewing the 3-D imagery on the desktop,
scientists can cycle the volumes into the visualization centers
scattered throughout the world and be immersed in the outcrop.
Another benefit for geoscientists is recall of outcrops.
"When I worked at a major oil company research lab,
one of my responsibilities was building an analog system," Jennette
said. "One part of that training was to take people out to outcrops.
Unfortunately, retention of that information by an overworked geoscientist
sitting at a workstation a year later is minimal. Now they can have
instant recall through the 3-D images, and greater return on outcrop
training experiences can be demonstrated.
"That's what is great about this tool," he continued,
"putting it in the hands of geologists, so the people who collect
the data can actually do the processing, interpretation, visualization
and mentoring."
The Gain in Spain
For the Ainsa and Tabernas basins study in Spain,
conventional field study methods and tools with laser-generated
imagery were combined to create 3-D representations of several well-studied
outcrops.
The digital scans from the Ainsa 1 and Ainsa 2 channel
systems were manipulated to provide an elevated vantage point that
sites down bed dip.
"This approach revealed an abundance of strongly
layered bed and bed-set architecture that was not readily apparent
from the ground," Jennette said. "Small-scale accretionary beds
also are evident."
Scans of the Tabernas' Solitary Channel — a small,
incised submarine canyon of reservoir rock inside slope mudstones
— provided a 3-D digital framework from which lithofacies and time-significant
surfaces (high-frequency sequence boundaries and abandonment surfaces)
are better correlated along the two-kilometer outcrop belt.
A series of faults disrupt stratigraphic continuity
of the channel fill and digital removal of the faults simplified
oft-debated stratigraphic relationships.
Also, laser intensity data were integrated with outcrop
weathering patterns and RGB values from digital photographs to produce
a classification scheme that distinguished mudstones from sandstones,
and even differentiated sandstones having varying lithic-grain content.
"All of these data are being used to generate a cellular-based
geological model of a slope-channel reservoir analog in GOCAD,"
he said.
The Solitary Channel is an analog for the traps companies
are now exploring for and beginning to produce offshore West Africa.
"Even though the Solitary Channel is small — only
a couple hundred meters wide and 50 meters thick — it still offers
a very good view into the architecture of systems that we now know
exist in the subsurface and hold hydrocarbons," Jennette said.
Jennette and Bellian spent three days capturing the
entire multi-kilometer-long outcrop. The pair took 75 scans, which
were all merged in those three days.
"The software and merging system is so fast that
you can check your work in the evening and make sure you obtained
100 percent coverage," Jennette said. "If there is a hole in the
data you can go back the next day and re-scan it. The software gives
us that immediacy while conducting the survey."