Origin from Massive Yellowstone Volcanoes?
Ash Deposits Can be DeceivingIn and around the Gulf of Mexico one can expect to find an unruly mix of jazz, heartache, gumbo, prolific petroleum reserves and, if a new study is to be believed, volcanic ash.
And this volcanic ash, according to one researcher, has the potential to contribute to the rising cost of petroleum exploration by playing a kind of cat and mouse game with the industry.
The theory, according to AAPG member Matthew Totten, an associate professor at Kansas State University, is that seismic readings of ash deposits look a lot like those of petroleum reserves and, to put it simply, it’s tough to tell the difference.
And, apparently, the Gulf has been collecting this ash for millions of years.
“In the beginning we were a little incredulous ourselves,” says Totten, who recently authored a study that indicated large beds of volcanic ash are in the Louisiana bayou.
Totten believes that massive volcanic eruptions (3,000 to 4,000 times more powerful than Mount St. Helens) that occurred two million years ago in what is now known as Yellowstone National Park covered parts of the Midwest, including Kansas and Nebraska, made its way, via the Mississippi River, to the Gulf of Mexico.
(As to the effects of Mount St. Helens, Totten says it was too small of an eruption and too far west to be significantly concentrated in the Mississippi River drainage.)
“The magnitude of these eruptions is outside normal experience,” Totten said. “When you find thick deposits in Kansas, halfway between Wyoming and Louisiana, you realize that the drainage system must have been full of ash for some time.”
The Fresh Connection
Totten, a former University of New Orleans professor who left for KSU shortly before Hurricane Katrina slammed into the Gulf Coast, has been studying the ash-bayou connection for years.
“Volcanic ash was spread over a tremendous area of the United States, and has been recovered from a wide variety of locations in the western U.S., the Great Plains, the Gulf of Mexico and even the Pacific Ocean. The ash outcrops in Kansas show evidence of a system overwhelmed with ash ... and record a time when the Mississippi River drainage system was literally choked with ash.”
Totten says that over time, the ash reached the depocenters in the Gulf of Mexico.
How much time?
“Based upon the paleomarker we used in our most recent study (GCAGS, 2006),” Totten said, “and assuming it is correctly dated, then the maximum amount of time from eruption to final deposition in the deepwater GOM, is about 100,000 years.”
Why this is important, is that if the Gulf is filled with ash and if that ash “looks” like oil, industry may spend millions of dollars looking -- and drilling -- in the wrong places.
“The seismic reflection of sands with about 20 percent ash seems similar to a hydrocarbon-rich sand,” Totten said, adding that not enough is known about the minimum amount of ash within a sand to lower the density enough to generate this response.
While saying one of the byproducts of finding all this ash is how much it will likely affect climate, water chemistry, etc., Totten believes the more pressing implication to the industry is the possible misinterpretation of an ash-rich bed as a DHI (direct hydrocarbon indicator).
“The ash is very low density,” Totten said. “A mixture of low-density ash within sand can give an amplitude anomaly similar to hydrocarbon-bearing sand. So avoiding this is important.”
A Tough Sell
At first, his findings were a tough sell.
“Some of what we were battling was the conception that all of the ash was airfall, instead of reworked airfall into the river system,” he said. “Airfall deposits are much too thin to be found in the 300-foot-thick beds we see. Airfall ashes from Central America are reported in the Gulf, but millimeters thick -- and we can’t think of a mechanism to rework the ash up the continental slope into the deposits we see.
“I think some of the well-publicized TV documentaries on super-volcanoes have helped. Once you recognize the magnitude of these eruptions, the occurrence of ash in the Gulf of Mexico seems inevitable, at least to me.
“Also, as more work on the hotspot became widely known, and as we also realized the extent of the ash across the continent, it became easier to convince the industry that it almost has to come from this area.”
Here, There and ... Everywhere?
Totten says he hasn’t been able to look at enough of the Gulf to determine the full impact, but does say, “it isn’t concentrated everywhere. We reported that it seems to occur within higher order channel deposits in amalgamated channels.
“That being said,” he continued, “it does seem to occur in at least trace levels in every sample we looked at of the appropriate age.”
And this isn’t just an academic exercise. Totten points to two instances where industry has drilled false positives: One happened in the late 1970s, when Shell explored an area in the Green Canyon, and the other involved a project conducted by Spirit 76 a few years ago.
“I met an independent at GCAGS (in September) that had just drilled an ash-filled false positive, but I was not told exactly where,” he said.
Totten believes that once “we get all of this worked out,” industry will have the ability to “determine what sediment pathways are active during the time period of ash deposition.”
As for now, Totten says there isn’t a device that can uniformly and consistently detect the differences between a positive finding of oil reserves and a concentration of ash, which is why finding out where the deposits are beforehand are so important.
“After the well is drilled,” he said, “ash is reasonably easy to identify on well logs.”