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Reproduced with permission from the January, 1992 AAPG Explorer

Computer Massage Works Wonders

Old Data? SCAT! Now It's New!

By Mary Fritz
Explorer Staff Writer

Let's face it. The dipmeter just isn't very glamorous. It doesn't come with bells or whistles. Even geologists who use dipmeterdata a lot usually don't talk about it at cocktail parties.

Technological evolution, some say, pushed it aside, as if it were a box fan in an air-conditioned world. The dipmeter is an extremely useful tool, but simple and generally restricted to complex (and expensive) plays. Since it's not a tool most geologists use frequently, it doesn't get a lot of press, even within the industry.

But despite its low profile, the humble dipmeter is at the center of a story that spans more than two decades and illustrates how a technique developed by a geologist in the 1960s became the tool that a scientist of the next generation would use to carve out a new industry niche in the late 1980s.

Charles A. "Andy" Bengtson was an exploration geologist for Chevron, prospecting for gas in and around San Francisco's East Bay in the mid-'60s, when the volume of dipmeter data he was analyzing made him wish for a way to quickly convert tadpole plots into cross sections and contour maps.

That wish eventually resulted in the development of a methodology that he called SCAT, for Statistical Curvature Analysis Techniques.

About the same time on the eastern side of the United States, James Morse, now president of COMPUTATIONAL GEOLOGY Co. in Boulder, Colo. was a kid growing up in Vermont.

He had no idea what a dipmeter was--or that he would one day be a young geologist just getting established in his profession when oil prices would peak and then start a downward spiral on the way to a freefall.

There was no way he could have imagined the impact that a small, quietly done geological research project would have on his future one day.

Getting Started

Bengtson, now retired from Chevron, is a consultant residing in Walnut Creek, Calif., not far from the area he was working 25 years ago. Back then the computer age was still in its infancy.

"Continuous dipmeter logs had been introduced by Schlumberger in the 1950s and were still considered somewhat a new technology," he said recently, "but I had one for nearly every well Chevron had drilled in the area."

He also had several old lines of seismic and a lot of surface geology data, including dip and strike measurements.

All this data was filed in "one of the oldest computers in the world," he said. "A National Cash Register 101, I think it was. It was set up in its own separate room, a structure about two feet square that went from floor to ceiling. The whole thing was full of vacuum tubes that were constantly going out. There was an operator assigned to it full-time who spent most of it replacing vacuum tubes."

As he worked to compile and analyze the data so he could make structure maps, he recalled some earlier work by Clint Dahlstrom in the Canadian Thrust Belt where stereonet pole plots had been used to determine the dip azimuth and amount of plunge of a structure based on dipmeter data.

Bengtson decided to try it on East Bay.

"We ran the pole plots, but it was difficult to see any pattern on stereonet," he said. "The outcome didn't coincide with what we already knew from the surface data."

That was when he got the idea to convert the more complex, 3-D dipmeter information (depth, dip and azimuth), plotted as "tadpoles" in a 2- D display, into two simpler 2-D plots.

Through a series of calculations he separated the tadploe plots into longitudinal and transverse apparent dip plots, which he ultimately transformed into cross sections and contour maps, providing a much more detailed and precise picture of the subsurface structure.

After perfecting the method on data from his exploratory wells, Bengtson showed his results to management and from there, he recalled, "it sort of grew, like Topsy, so to speak."

Public Appearances

He was transferred to the La Habra research group for a year and a half, to program the method for computer operation.

La Habra's mainframe computers were up-to-date for the day, but megabytes away from today's workstations.

"A keypunch operator had to punch out all the data in the IBM format, and even a single dipmeter log includes a lot of data," Bengtson said.

"The technique I'd used in the beginning was to have a technical assistant take three dip curves at a time, as they were on rolls of paper, align them for correlation, mark depths and measure the deviation of the well.

"We found out it cost more for the computer to do than to have the T.A. to do it by hand, but we kept on doing it anyway.

"As computers got more powerful, we found we could do more with it," he said.

Since SCAT was more a concept about how to do something than it was an original design or product, lawyers said it was not patentable. Chevron kept it proprietary as long as possible, but as the boom of the late '70s lured scientists away from the majors, dozens of geologists trained in Chevron's structural schools by Bengtson himself took their knowledge of SCAT with them.

Company managers decided Bengtson and Chevron should at least go on the record as having developed the technique. Bengtson was urged to present a paper at the 1978 AAPG annual meeting.

Finally, he was free to talk about SCAT outside the company. After the AAPG paper an article was published by the Oil and Gas Journal. Bengtson's paper was published in the BULLETIN in 1981.

Other articles and papers followed, but as the boom went to bust, industry interest in thrust belts and other complex areas waned. Bengtson retired from Chevron in 1982 and started consulting, specializing in dipmeter analysis of structurally complex regions.

A Second Front Begins

Morse, meanwhile, had graduated from Texas A&M with a master's degree and was with Amoco in Denver. Working the thrust belt, he looked at a lot of dipmeter logs and he, like Bengtson years before, wished to see the data in a geologic format.

"I got to know Bill Brown, who is ex- Chevron, through the Rocky Mountain Association of Geologists," he said. "We were talking at a meeting one day about subsurface interpretation and he told me about Bengtson and SCAT and how it was THE BEST THING EVER TO COME OUT OF CHEVRON RESEARCH.

"I never forgot it."

In 1983 Morse struck out on his own as a consultant specializing in subsurface interpretation. One job presented some particularly difficult problems, for which he thought SCAT might offer solutions.

From Bengtson's published articles he and a colleague figured out the calculations, ran them and they worked. He was hooked.

"I was fascinated by how well SCAT worked--and amazed that so may of my colleagues either hadn't heard of it or had only a vague idea of how it worked," Morse said.

He wasn't alone in his enthusiasm for SCAT, however. BROWN, WHO TOLD MORSE ABOUT THE TECHNIQUES, SAID SCAT IS "ONE OF THE BEST TOOLS FOR GEOMETRIC ANALYSIS OF FOLDS THERE IS.

"IT'S FAR MORE COMPREHENSIVE THAN THE STANDARD DIPMETER LOGS," HE SAID. "WITH IT YOU CAN CLEARLY IDENTIFY AXIAL PLANE VS. CRESTAL PLANE DIRECTIONS; MEASURE THE AMOUNT OF PLUNGE; PICK OUT FAULTS AND UNCONFORMITIES."

Peter Verral of Chevron added that SCAT also is useful to analyze surface dips and pointed out that in areas of significant structure where seismic data are not available or not clear enough, dip plots are inexpensive to generate.

Thrust belt wells may cost upward from $10 million to drill. ACCURATE SUBSURFACE ANALYSIS HELPS AVOID DRILLING OFF STRUCTURE AND DRY HOLE COST SAVINGS ADD UP FAST. VERRAL ESTIMATED SCAT HAD PROBABLY SAVED CHEVRON HUNDREDS OF MILLIONS OF DOLLARS ALREADY.

On the flip side, Morse said SCAT analysis also may suggest better locations to find reserves, due to the improved subsurface image it provides.

Meeting of the Minds

Morse finally had an opportunity to meet Bengtson in 1986, at AAPG's annual meeting in Atlanta. Bengtson said Morse approached him following his presentation of a paper on SCAT and introduced himself. A two-hour conversation ensued.

"He was very enthusiastic about the possibility of developing a software program based on SCAT," he said.

The two began to correspond and ultimately, co-authored a series of dipmeter analysis articles for the BULLETIN which were published in 1988, 1989, and 1990.

"The possibilities SCAT presented for development of a computer software package really intrigued me," Morse said. "It was a fairly complex undertaking though, and I had neither the programming expertise nor the money to swing the project myself."

He talked the project up everywhere he went, though, and eventually, through a mutual acquaintance, found the perfect partner for his proposed venture . . . .

The two hit it off, and by the end of 1990, had a software package ready to take on the road.

"We knew we had a good product, and we got enthusiastic responses to our demonstrations," Morse said . . . . Although the process is calculation intensive, the basic steps of the process are straightforward.

Once the computer has split the dip vectors in two directional components, it creates dip "isogons"--lines through the structure connecting points of equal dip. Then the dots are connected to construct a 2-D cross section, producing a picture of the subsurface structure in the vicinity of the wellbore that is based on actual wellbore control . . . .

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NOTES:

Emphasis (uppercase) added.

James D. Morse is one of the founders of Computational Geology.

'Old' Dipmeter Data

Turned Alaskan Data into Gold

Bill Sercombe can testify to the truth of the claim that old dipmeter data, properly massaged, can be worth its weight in gold thousands of times over.

Sercombe, with Amoco Production in Houston, was assigned during the late '80s to develop additional reserves in offshore Alaska's Granite point and Middle Ground Shoal fields, Cook Inlet Basin.

Originally discovered in 1957, most of Cook Inlet's known accumulations of oil have been associated with closed anticlines with steep flank dips, complicated by thrust and transverse faults.

Seismic quality was poor, sketchy at best, in the two fields, each of which already had about 70 wells drilled.

"I had old dipmeter data from the early '60s for about 80 of the wells, but no adequate interpretation of tadpole plots had ever been performed. Basically, the data had been archived and never used BECAUSE TADPOLE PLOTS ARE NOT EASILY QUANTIFIABLE."

Sercombe's task, the, was to plot dip and dip azimuth for each wellbore at each of the nine mapped producing horizons.

He employed Bengtson's SCAT method and plotted quantitatively calculated azimuth and dips on the structure maps.

"The results were very good. FAULTS, BOTH LARGE AND SMALL-SCALE, BECAME VERY APPARENT," HE SAID. "THAT INFORMATION CAN BE HIDDEN ON A TADPOLE PLOT.

"WE ALSO WERE ABLE TO SEE IN AN UNAMBIGUOUS WAY THE CRESTS OF STRUCTURES, AND LOCATE AXIAL AND INFLECTION HINGES."

The overall subsurface picture that finally emerged revealed tight, asymmetric structures instead of the wider folds mapped originally.

That new insight into the true structural style of the fields showed them to be much larger than previously thought, he said. Almost all of the earlier Granite Point wells were located on the east flank of the structure. The steep west flank had not been developed.

Sercombe proposed drilling on the flanks of the structures, and thanks to some old dipmeter data, between 20 and 30 million barrels of oil were added to Granite Point resource estimates.

Obviously, SCAT has snared yet another advocate.

"It's really very a simple, intuitive way of analyzing dipmeter data," he said.

"IT WORKS, IT'S EFFECTIVE, AND IT MAKES YOUR JOB EASIER."

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NOTE: Emphasis (uppercase) added.

Updip Update

Using Computational Geology's Geodes® software, Bill Sercombe has gone on to apply SCAT elsewhere, including the northern Pakistan foreland and, most recently, the Gulf of Suez, Egypt. See Sercombe and others, 1987, "Significant structural reinterpretation of the subsalt, giant October Field, Gulf of Suez, Egypt, using SCAT, isogon-based sections and maps, and 3-D seismic": The Leading Edge, August, 1997, p. 1143-1150. Reprints of this article are available at cost from Computational Geology. E-mail us at info@CGeology.com.

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