Meteorites Make Good Impression

Impact Craters Can Yield Reservoirs

P. Jan Cannon spent years studying the striking features of meteor impact craters.

He now pursues the impact of those craters on the oil and gas industry. And for petroleum geologists, Cannon has two basic messages about terrestrial meteor craters:

"The mechanics involved in forming an impact crater create brecciated rock that makes an excellent reservoir."

"More than one part of an impact crater has potential to produce hydrocarbons."

Cannon will present his views at the AAPG Annual Meeting in Houston, in his paper "Hydrocarbon Potential of Buried Impact Craters."

The association of meteor craters and hydrocarbons gives his work more than theoretical interest. Cannon now targets impact structures to help generate crater-prospects.

"The production potential is so fabulous, people would like one," he said. "It's difficult because geologists haven't been schooled to recognize them. There are a lot of circular things out there, but not all of them are impact craters."

Cannon is president of Planetary Data in Tecumseh, Okla.

His paper will be part of the March 11 morning session "NASA: Human Exploration of Earth, Moon and Mars," chaired by Bill Muehlberger, a University of Texas emeritus professor.

Muehlberger has his own story about the need for geologists to be better schooled in recognizing impact craters.

He recalled taking students on field trips through East Texas, where he would point out the edge of the Marquez Dome as an excellent example of a salt dome rim.

Study and analysis later revealed that the Marquez feature is part of a meteor impact structure.

"You talk about feeling dumb," Muehlberger said.

'You Have a Crater Here'

Impact craters and their potential seized the attention of the Oklahoma oil industry in the early 1990s. Several new, deep wells in the Sooner Trend produced exceptional amounts of oil and gas.

Why, no one knew for sure.

Rick Carlson, an independent petroleum geologist in Edmond, Okla., put together some of the first prospects in the deeper play. At that time he worked for DLB Oil and Gas.

"In the Sooner Trend in that area there's a lot of well control," he said. "There's Mississippi Hunton penetrations — you can map structural closure."

Image Caption

Perhaps America's best-known impact crater: Arizona's Barringer Crater, a 0.737-mile-diameter (1.186 km) crater that is a mere 49,000 years old.
Photo by D. Roddy/Lunar and Planetary Institute.

Please log in to read the full article

P. Jan Cannon spent years studying the striking features of meteor impact craters.

He now pursues the impact of those craters on the oil and gas industry. And for petroleum geologists, Cannon has two basic messages about terrestrial meteor craters:

"The mechanics involved in forming an impact crater create brecciated rock that makes an excellent reservoir."

"More than one part of an impact crater has potential to produce hydrocarbons."

Cannon will present his views at the AAPG Annual Meeting in Houston, in his paper "Hydrocarbon Potential of Buried Impact Craters."

The association of meteor craters and hydrocarbons gives his work more than theoretical interest. Cannon now targets impact structures to help generate crater-prospects.

"The production potential is so fabulous, people would like one," he said. "It's difficult because geologists haven't been schooled to recognize them. There are a lot of circular things out there, but not all of them are impact craters."

Cannon is president of Planetary Data in Tecumseh, Okla.

His paper will be part of the March 11 morning session "NASA: Human Exploration of Earth, Moon and Mars," chaired by Bill Muehlberger, a University of Texas emeritus professor.

Muehlberger has his own story about the need for geologists to be better schooled in recognizing impact craters.

He recalled taking students on field trips through East Texas, where he would point out the edge of the Marquez Dome as an excellent example of a salt dome rim.

Study and analysis later revealed that the Marquez feature is part of a meteor impact structure.

"You talk about feeling dumb," Muehlberger said.

'You Have a Crater Here'

Impact craters and their potential seized the attention of the Oklahoma oil industry in the early 1990s. Several new, deep wells in the Sooner Trend produced exceptional amounts of oil and gas.

Why, no one knew for sure.

Rick Carlson, an independent petroleum geologist in Edmond, Okla., put together some of the first prospects in the deeper play. At that time he worked for DLB Oil and Gas.

"In the Sooner Trend in that area there's a lot of well control," he said. "There's Mississippi Hunton penetrations — you can map structural closure."

The new wells produced from the Arbuckle formation, just above the Cambrian. One of Carlson's wells showed over 300 feet of gross pay on logs.

Great — but why?

"I couldn't map a structure that large," Carlson recalled. "Up to then, there had been only four wells drilled to the Arbuckle in Major County."

Some 2-D seismic existed, but its quality was too poor to help, he said. Computer simulations that aided later analysis weren't available at the time.

In 1991, the 1-20 Gregory in the new play targeted Arbuckle at about 8,800 feet. DBL and Continental Resources partnered with operator D&J Oil.

"We kept waiting for the Arbuckle to come up and it wouldn't come up," Carlson said. "We kept drilling shale and going lower. And lower. And lower."

Penetration rates suddenly increased. Cuttings included brecciated granite with good shows, as well as shattered quartz and feldspar with cleavage faces.

As it turned out, the Gregory had encountered a rich reservoir on a high near the center of an astrobleme.

"Nobody at the time knew this was an impact crater," Carlson said. "Dr. Cannon was the first one who said emphatically to me, 'You have a crater here.'"

Today, the structure is famous as the Ames Hole (May, 1992 EXPLORER). Carlson believes a meteor or asteroid the size of a football field shattered 1,000 feet of Arbuckle and another 1,000 feet of basement rock, producing a crater more than 10 kilometers across.

Subsequent research indicated that the Ames crater began to fill with fine sediments, ultimately producing a thick, source-rock shale.

"Impacts will create their own reservoir rock," Carlson noted. "They can create their own source rock. They create their own faulting and closure."

Overlooking the Obvious

Impact craters may go unnoticed because they are subtle, buried structures and geologists tend to have little training in identifying them, according to Cannon.

"It's like the Ames Hole," he said. "Most everybody at first accepted the theory that it was a buried caldera.

"In the conterminous United States, we should find about 105 craters (similar to the Ames Hole). So far, we've only found five to seven of them."

Craters offer clues to their true origin, Cannon explained.

One of the clinching proofs for the origin of the Ames Hole came from the presence of coesite and maskelynite, minerals associated with meteor impacts.

Craters display morphological changes that occur as certain thresholds of impact energy are crossed, Cannon said. Larger craters show the effects of backward reflection of the original shock wave.

"The brecciated center actually rebounds — this is what causes the porosity," he said. "The shockwave comes back and pulls up the rock in the middle."

In strictly scientific terms, Cannon calls the result "an incredible, fractured mess."

"The morphology is a real key," he said. "On Earth you start to see that effect at about three kilometers. But on the moon, you don't start to see it until 23 kilometers."

Looking Up, Looking Down

Oddly, Cannon's introduction to impact craters had nothing to do with Earth, much less oil and gas.

He earned his doctorate in geology at the University of Arizona and went to work at the U.S. Geological Survey Astrogeology Center in Flagstaff, Ariz. Cannon became a member of the NASA Remote Sensing Applications Program and later mapped lunar landing sites for Apollo missions.

"We mapped impact craters, because that's the primary feature on the moon," he said. "They're the most common landform in the universe."

Later, while teaching at the University of Alaska, he used satellite images to identify an impact crater in central Alaska.

"The water draining out of this lake had a high nickel content," he said. "The USGS had found a high nickel anomaly in the streams.

"People didn't really know what to look for," Cannon explained, but he knew the nickel anomaly could indicate a meteor impact.

The Alaska discovery began his use of remote imaging as a tool for crater hunting.

"I find them in satellite imagery because there's an overprinting on the surface," he said. "We feel the mechanism driving that overprinting is gravity tides."

His work with NASA helped him understand the results of impact events. In analyzing the geology of Earth craters, he noted that "we're still on the learning side.

"Our studies of the moon tell us that there were a whole lot more of them in the Precambrian than there are now."

Buried impact craters can develop reservoir rock in the brecciated central uplift, brecciated floor materials, fractured rim materials and unconsolidated ejecta piles, Cannon said.

Those come from enormous force. Cannon said meteors travel at 22 kilometers per second or faster, and typically gain speed when they enter a planet's gravity well.

"That's because 22 kilometers per second is the cut-off point for orbital velocity," he explained. "It turns out to be very simple. Anything moving slower, hundreds of millions of years ago would have fallen into the sun."

The force of a large meteor impact produces pressures that exceed the Hugoniot elastic limit (HEL) for crustal rocks and minerals. Permanent deformation of rock occurs from stress waves beyond that point.

Stealth-Like Features

Existing features in the geology at an impact site can affect the nature of, and effect the shape of, the resulting crater, according to Cannon. For instance, at Meteor Crater, Ariz., the crater appears to be square.

"The fracture sets actually controlled the polygonality of the impact crater," he said.

Successful development of a crater structure depends on recognition of its morphology and degree of preservation, Cannon said. Deeply buried craters, however, are not simple to discover and analyze.

Carlson said the subtlety of impact crater geology appeals to him, and keeps him interested in discovering a structure similar to the Ames Hole.

"They're kind of stealth-like. The Ames Hole was in the backyard of the Sooner Trend, where 1,000 wells had been drilled," he said. "There it was sitting on the Anadarko shelf for a long, long time, and nobody found it."

But evidence existed — for geologists trained to see it.

Carlson said the Ames crater can be discerned in Permian formations draped over the structure.

He recalled an earlier well drilled at the crater site.

Conventional thinking led the operator to abandon the attempt after encountering wet sands.

"It was the driest dry hole you can imagine," he said. "If they had drilled 500 feet deeper, they would have found the Gregory well reservoir."

Cannon believes much more scientific study is needed to understand how meteor impacts have affected the present crustal structure of Earth.

"Just like Chicxulub, we can no longer ignore the importance of impact craters on the history of our planet," he said.

Examining craters as potential hydrocarbon reservoirs adds to their geological significance, although Cannon never expected to see that kind of exploration on his resume.

"When I was mapping the Apollo landing sites," he said, "hydrocarbons were the furthest thing from my mind."