A million years in the making

By PATRICK SPRINGER

The Forum

FARGO, N.D.

The Bakken formation’s oil bonanza was triggered eons ago in a story involving a cast of millions, earthly upheavals, heat and pressure “lots and lots of pressure.” It’s actually a very depressing story. In a good way. Good for oil development, at least.

The setting inhabits the Williston Basin, an underground circular depression spread over 300,000 square miles, including parts of North Dakota, South Dakota, Montana, Saskatchewan and Manitoba.

Picture a giant bowl — actually, picture a series of bowls or saucers stacked on top of each other as layers in the earth’s crust.

Some of those rock layers, about 15 of them, hold oil and gas.

In the case of the Bakken formation, lots and lots of oil. The latest estimate is between 3 billion and 4.3 billion barrels of undiscovered but probably recoverable oil.

That 2008 government estimate is 25 times larger than a 1995 estimate, and a new study is under way that many oil developers believe should show the Bakken is even bigger.

How did all of that oil get there? It took a long, long time, a saga that began unfolding around 540 million years ago, when much of the earth was covered by ancient oceans separated by three barren continents.

In what later would become North America, a weak spot in the earth’s crust developed, and the area that became the Williston Basin began to sink, a process geologists call subsidence. Much like a bowl, the basin began to fill, with layers of sediment that kept piling up over millions of years.

The inland sea that covered what became the Williston Basin, including western North Dakota, fluctuated in depth and extent as millions of years passed, and even disappeared several times. Finally, the inland sea dried up around 60 million years ago, about the time dinosaurs came along. The depth of the inland sea was never very deep, probably never exceeding 100 to 250 feet.

Actually, it doesn’t take impressive oceanic depths to make an awful lot of oil. That’s because shallow sea water, bathed in sunlight, is abundant with marine life‚ö teeming with algae and plankton, tiny plants and animals. All containing carbon, which remained when the organisms died and drifted to the bottom of the sea, where they were buried by silt and other sediment from the ocean and the rivers emptying into it.

So, most oil and gas drilled today originated at the bottom of the sea from dead marine life.

“Land materials are much more likely to form coal beds,” said Allan Ashworth, a geology professor at North Dakota State University.

The process continued over millions of years. Carbon accumulated in layers of sediment that hardened over time, under heat and the enormous pressure from the weight of the overlying minerals.

“It kind of waxes and wanes like that through the tens and hundreds of millions of years,” Ashworth said.

Elsewhere, coinciding with the continued subsidence of the Williston Basin, uplifting occurred in the earth’s crust‚ö sometimes caused when land masses slammed into what became the Pacific Coast. Some of the more spectacular uplifts include the Rocky Mountains, which contributed lots of sediment eroded by wind, rain and ice action, much of which flowed into the Williston Basin.

In fact, the shifting of the plates in the earth’s crust explains why the climate at the time the ancient seas dried up was tropical. That’s because the area including the Williston Basin once was located much closer to the equator, but gradually drifted north.

Today, one of the most visible remnants of the enormous changes that literally reshaped the earth’s crust can be seen in a layer of limestone called the Deadwood formation. The formation, which dates back 542 million years, lies almost at the bottom of the Williston Basin, beneath the Bakken.

It can be seen as a layer of grayish-green limestone in outcroppings around Deadwood, in South Dakota’s Black Hills, which today has an elevation of 4,537 feet.

Ashworth has taken his students on field trips to the Deadwood formation, where they have dug out ancient marine fossils and found evidence of worm holes‚ö once living at the bottom of the inland sea.

By comparison, the Deadwood formation around the southwest North Dakota town of Medora in the Williston Basin is about 14,000 feet‚ö or almost three miles‚ö underground.

“The crust is in this strange motion, and it’s actually pretty poorly understood,” Ashworth said.

The subsidence of basins like Williston, however, clearly is associated with the weight of accumulating sediment.

Several thousand feet above the Deadwood formation is the Bakken formation, where oil flowed through pores in the sedimentary stone, migrating from what geologists call source stone to reservoir stone.

“It’s actually oil-bearing shale stone,” said Richard Suggs, a geologist with the North Dakota Department of Mineral Resources. “The key here with the Bakken is the organic shale. Those shales are very rich.”

The prime zone is about 9,000 feet down, said Julie LeFever, a geologist with the North Dakota Geological Survey. “That’s where the organic matter turns to oil,” she said.

The Three Forks formation lies directly beneath the Bakken. Above the Bakken is an impenetrable layer of limestone 600 feet thick called the Lodgepole formation‚ö which prevents the oil from escaping. The only way to extract the oil from the Bakken and Three Forks formations is to inject water, chemicals and sand under high pressure to form tiny fractures in the rock, a process called hydraulic fracturing, or fracking.

“That rock itself is kind of like a dark, dense concrete,” Suggs said of the Bakken shale stone embedded with oil.

As more and more drilling occurs, geologists are learning more about the Bakken. Also, as oil companies gain experience in the field, they can capture more oil.

Early in its development, oil developers were recovering 2 percent to 3 percent of the oil. Today, that is 5 percent or better, geologists say.

“The Bakken is the big target right now,” LeFever said, “but we have a lot of other layers that can generate oil, and have.”

But, she added, based on what’s known, “The Bakken’s probably the richest.”

How it got that way is a long story.