By KATHY SHIRLEY
EXPLORER Correspondent

Barcelona Posters See West Africa Analogies

Spain Outcrops Hold Deep Secrets

David Jennette and co-author Jerome Bellian, both with the Bureau of Economic Geology at the University of Texas at Austin, will present two posters dealing with their Spain 3-D laser-outcrop studies during the AAPG International Conference and Exhibition in Barcelona, to be held Sept. 21-24.

"Three-D Digital Characterization and Visualization of the Solitary Channel Complex, Tabernas Basin, Southern Spain," will be presented Monday afternoon, Sept. 22.

"Application of Laser-Scanned Outcrop Data to Build Models of Deepwater Reservoirs: Examples from the Tabernas and Ainsa Basins, Northern and Southern Spain," will be presented Wednesday afternoon, Sept. 24.

Analog Database to Expand

Oil companies also see the potential value of a technique for producing 3-D outcrop analogs. Several major oil companies are supporting the Bureau of Economic Geology's research.

The BEG team plans to expand its outcrop analog database over the next several years to include important outcrops of both clastic and carbonate systems. This summer David Jennette and his team will be in Ireland.

"One of the areas we would like to advance to is very large rock outcrop systems like the Brushy Canyon of West Texas or Karoo in South Africa," he said. "We can cover tens of square kilometers in an afternoon with aerial LIDAR tools and implant ground based, high-resolution surveys into that data."

He views this as an opportunity to bring together big aerial surveys and highly focused ground based surveys to build the basis for a much more accurate geologic characterization of the subsurface.

"Once this data is captured it is captured for good," he said. "It isn't a document gathering dust on somebody's bookshelf. It is something that can constantly be made evergreen through new analyses and new types of applications, and new company Web-based learning tools.

"Given the cost associated with studying these far afield places, once the images are captured there is a continuing return on the investment."

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. BACKGROUND

"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."