The spirit of collaboration resonates throughout the lifework of Robert Mitchum, the 2006 recipient of the Sidney Powers Memorial Award, AAPG’s most distinguished accolade.
Like Sidney Powers himself, Mitchum teamed with peers to convert groundbreaking theories into practical applications, greatly advancing petroleum geology with his definitive studies on seismic and sequence stratigraphy.
Ever the missionary for his beloved craft, Mitchum’s most telling tribute is the legacy of knowledge he continues to pass on to the numerous geoscientists he has educated and encouraged in search of their own scientific missions.
Pivotal Moments
Ten years. That’s how far ahead of his time many of Robert Mitchum’s contemporaries believe the geologist’s work to have been.
Mitchum, though, points to another significant period that was pivotal in defining his career.
As a Northwestern University doctoral student on a teaching fellowship in the early 1950s, Mitchum met students Peter Vail and John Sangree. Their lifelong friendship and collaboration would later generate one of geology’s most revolutionary predictive tools and transform the methodologies for petroleum exploration and production.
Newly indoctrinated, Mitchum accepted a position with Carter Research Lab (later Humble Oil Company) in Tulsa, where he met his wife, Naomi. Initially, Mitchum focused on fracture porosity in folded rocks, including the Big Horn Basin of Wyoming, and the Appalachian Belt in Maryland and Pennsylvania. At the time, the company was just beginning its research into the carbonate rocks of limestone and dolomites, and their reservoir capabilities.
“We looked at the reproductions on thin sections of carbonate,” Mitchum recalled. “We saw that there was a variety of grain types and matrix that is like sandstone except that it is made of calcium carbonate. There are some very huge hydrocarbon reservoirs in carbonates, especially in the Middle East.
“We tried to develop a technique that would predict the primary textures of the carbonates and where they would occur, in what part of a reef or what part of a carbonate bank.”
While with Exxon, Mitchum helped create a classification based on this carbonate work. The carbonate sojourn continued, taking the Nashville, Tenn., native to the Bahamas to examine modern environments, to the Delaware Basin for a comparison to ancient rocks and to the Ellenberger Field in Midland, Texas.
It was a study of the shallow sediments in the West Florida Slope of the Gulf of Mexico that would whet Mitchum’s appetite for predictive studies of another sort.
“We had very high-fidelity seismic lines that were just of the shallow sediments,” he said. “We had bore holes to show the cores and the lithologies. We had good paleontological age dates with the cores, so we were able to date the different surfaces that we saw and tie all of that together into integrated interpretation.
“It was such a beautiful match,” he said, “and it got me itching to do something more with seismic stratigraphy.”
Prolific Partnership
Some 13 years after their first meeting, the three Northwestern alums were transferred to Exxon Production Research in Houston. Their mission was to formulate a predictive geological theory using Vail’s thesis based on the rising and falling sea level, and the sediment packages that the changing sea levels left behind.
These packages were formed in predictive, synchronous sequences; hence the term Vail coined, “sequence stratigraphy.”
“You could predict from the seismic what the sediment distribution was,” Mitchum said. “Then we had wells to corroborate that, to calibrate with and show that our predictions were right. Then we could step out from the boreholes into areas that had no control, except the seismic. We could try to predict what the sediments were going to be.
“We saw the boundaries between these bundles of reflections, which were minor unconformities, and we could use fossils to date those and correlate them throughout the region based on the age,” he continued. “We would drill and find a surface in the Gulf of Mexico and date one in off-shore Africa, and they appeared to be time correlative. This led to the fact that there had to be some sort of worldwide control on these surfaces and packages of sediments that we saw.
“We found that within the resolution of the seismic data and within the resolution of the fossils that we could use for dating that they seemed to be synchronous all over the world,” he said. “That was quite a revelation.”
Exxon, and most of the scientific community at large, eventually agreed. Economic and intellectual resources were funneled into the landmark project. Paleontologists became experts at dating the surfaces; geophysicists promoted enhanced seismic data; the novel tool of a computer program was written to help organize data and make maps.
“The group work I did with the sequence stratigraphy was the best example of personal collaboration that I’ve ever had,” Mitchum said. “Peter Vail was the conceptual giant. John and I had different theories in which we tried to make practical use of Pete’s ideas. We looked at different environments. We saw differences; we saw similarities, and we saw the unifying concepts that would tie them together. So with sequence stratigraphy, even though a section would look quite different, if we could find where the sequence boundaries were and where we were within the depositional environments, we could more or less make a predictive map of where the reservoirs might be.
“That group was not working as individuals,” he said, “but for the common goal of achieving this practicality.”
A Stratigrapher’s Heaven
In his 20 years on the sequence stratigraphy team, Mitchum assumed the role as troubleshooter, often with his expertise loaned to overseas Exxon affiliates, including the Europe-Africa Division, the Gulf of Alaska and Southern Atlantic (USA). His litany of basin studies encircles the globe and includes Argentina, Africa’s rift basins in Chad and Niger, the North Sea Paleocene deepwater, the Beaufort basin in northern Canada, the South Atlantic and numerous other sites.
“It just so happened that Exxon was making surveys along continental shelves around the whole world -- and we saw a great deal of frontier-type data,” Mitchum noted. “We were able to predict things that were later used in exploration ventures. We were able to make regional basin studies.
“When they didn’t understand what was going on in a seismic section, they would call Pete or John or me to try to help them,” he added. “So, we had a finger in the workings of a lot of these different affiliates all around the world.”
Mitchum’s extensive research has been ample fodder for the more than 40 papers he has authored or co-authored illuminating the complexities of seismic, sequence and classical stratigraphy.
The interpretative talents of the trailblazer were broadly recognized in 1977 upon the publication of the AAPG landmark Memoir 26, Seismic Stratigraphy, a collection of papers that detailed seismic and sequence stratigraphy. Credited with being the chief editor of Exxon’s contributions, Mitchum also was lead author on five of the papers, co-authored three others and helped defined the terminology that is still in widespread use today.
“Mitchum was the one who solved the problems so that they could be put on paper,” Vail later said. “Many of us contributed papers to Memoir 26, but Mitchum made them technically sound and readable.”
Likewise, contributions in AAPG’s Memoir 39, Memoir 58 and Methods in Exploration Number 7 are viewed as seminal documents because of their significance in establishing and clarifying sequence and seismic stratigraphic fundamentals. Beyond the clearness of interpretation, Mitchum relies on an innate ability to locate areas of exploratory interest.
“I could visualize the image of the rocks that I thought would be in a particular section,” Mitchum explained. “I could spot where the sequence boundaries were and then, in between those, the configuration would fit sort of a preconceived, flexible image of the model that I had in mind.
“I could look at a seismic section, very commonly, and sort of see shapes in it that remind me of the model that we have been using all these years,” he said. “Brud Leighton called it ‘calibrated eyeballs.’”
AAPG recognized the work of Mitchum, Vail and Sangree on the Memoir 26 by naming them as co-recipients of its President’s Award. In 1992, J.C. Van Wagoner, Mitchum and other co-authors were presented the Robert H. Dott Sr. Award for the AAPG special publication, “Methods in Exploration Number 7.”
Still Going, and Going ...
An AAPG member since 1956, Mitchum was made an Honorary Member in 2000. A self-proclaimed displaced Tennessean, he earned his bachelor’s and master’s degrees from Vanderbilt University.
Although Mitchum retired from Exxon in 1988, he continued the pursuit of the skill he has honed by teaching short courses with John Sangree and consulting with oil companies large and small. At 78, he still works as a full-time consultant for Chevron in the deep water of the Niger Delta.
“It is a sequence stratigrapher’s heaven,” Mitchum describes. “It’s sort of a self-contained unit, this huge deltaic mass that builds up onto the ocean crust. You can map the sequences and count them. They occur almost exactly in the order that Pete Vail had predicted in his cycle charts many years ago.
“It’s just another beautiful confirmation of Pete’s cyclic theory of sea level rising and falling.”
And as for tomorrow? Mitchum says he still feels great enthusiasm about geology -- and he plans to continue practicing as long as he enjoys it and feels he is making a contribution.