A shale gas-type approach

Bakken Tricks Work on Three Forks

Steve Sonnenberg
Steve Sonnenberg

A better understanding of the Three Forks Formation is enhancing shale oil prospects in the Bakken Shale play area.

As in, up to another two billion barrels of oil.

The Three Forks is “part of the Bakken petroleum system, and all of a sudden this year we’re seeing a lot more activity,” said Steve Sonnenberg, professor of petroleum geology at the Colorado School of Mines in Golden, Colo.

Sonnenberg heads the school’s Bakken Research Consortium, with 20 member companies and a grant from the National Energy Technology Laboratory.

Explorers have targeted the Bakken system for oil and gas for many years.

The contemporary Bakken story began in Montana in 2000, when horizontal drilling started to open up the shale oil play.

In 2006, discovery of the Parshall Field in Mountrail County, North Dakota, created a second front of intense drilling activity.

Source beds for Three Forks.
Source beds for Three Forks.

Both the Bakken and the Three Forks have long been known as productive oil targets, but with mostly hit-and-miss economics – until the advent of a new approach to unconventional reservoirs.

“Some of the earlier attempts to get it out met with success, and some with not so much success,” Sonnenberg noted.

Operators needed both geology and luck on their side for success in the Bakken, even after the introduction of hydraulic fracturing to the play area in the 1960s.

It took long-lateral drilling and multi-stage fracks to tap the full potential of shale-oil production, first in the Bakken Shale, now in the Three Forks.

“It’s interesting because the Three Forks is some of the oldest production in the Williston Basin – it goes back to the 1950s,” Sonnenberg said.

“It has taken 50 years for the technology to catch up with this reservoir.”

Regional fractures for the Three Forks Formation play.
Regional fractures for the Three Forks Formation play.
Geologic Details

Current thinking puts recoverable oil from the Bakken Shale at just over two billion barrels, and from the Three Forks Formation at just under two billion.

The U.S. Geological earlier estimated mean, technically recoverable, undiscovered volumes in the Bakken system of 3.65 billion barrels of oil, 1.85 trillion cubic feet of associated/dissolved natural gas and 148 million barrels of natural gas liquids.

Sonnenberg said the main source rock for the upper Three Forks is the lower Bakken Shale, although where the lower and middle Bakken thin in the southern Williston Basin, the upper Bakken is the main source.

The Three Forks underlies the Bakken, separated by the Sanish Formation.

“The Sanish is a very thin dolomitic sandstone that occurs right on the very top of the Three Forks,” he said. “There are some people who want to include that as part of the Bakken – it’s very thin, and it’s not present everywhere.”

Three main facies can be found in the upper Three Forks, according to Sonnenberg:

  • Massive to chaotic bedded dolostone.
  • Interbedded dolostone with green mudstones.
  • Bioturbated dolostone to sandstone (the Sanish).

Reservoir quality is poor, he noted, with porosities generally less than 8 percent and permeabilities less than 0.1 millidarcys.

That dictates a shale-gas-type approach to oil development in Three Forks wells, with the learning curve continuing.

“They're being drilled with 10,000-foot laterals, most of them, and with the same orientation,” Sonnenberg said. “Generally that's in a northwest-southeast direction.”

Success Stories

Whiting Petroleum Corp. of Denver drills both Bakken and Three Forks wells in the Sanish Field in Mountrail County.

(The term Sanish is used in several different contexts in North Dakota. Confusion over the name led one local geologist to declare, “The Sanish should vanish,” Sonnenberg said.)

Last year, Whiting completed 65 operated Bakken wells and seven operated Three Forks wells in the Sanish Field. The company said initial production for the Three Forks wells completed in 2010 averaged 1,302 barrels of oil equivalent per day

In 2011, Whiting intends to drill 95 operated wells (54 net) in the field, with 70 planned Three Forks wells, 15 cross-unit Bakken wells, seven Bakken infill wells and three wing wells, at a capital expenditure of $352 million.

A decision to drill three Three Forks wells per 1,280-acre unit instead of two has added 80 potential gross well locations in the Sanish Field, Whiting said.

The company estimated that total completed well costs in the field will come in below $5.5 million per well for recently completed wells.

Its recent wells reached a total measured depth of about 20,000 feet, including 10,000 feet of lateral, in an average of 22 days – with a company record from spud-to-total depth of just under 14 days.

Adding frack stages and using more proppant and frack fluid have proved key to increased production for Whiting.

Before last year, its wells in the Sanish field were fracked in 10 stages. In 2010, Whiting fracture stimulated its wells with 15 to 30 separate fracks, averaging 21 frack stages per well.

The company also began using 30,000-45,000 barrels of frack fluid and 2.4 million to 3.3 million pounds of sand for a 30-stage frack. And Whiting has run a sliding sleeve assembly in a Sanish Field well for a planned 40-stage frack.

Test Patterns

Production from both the Bakken and the Three Forks at the same place isn’t unusual, according to Sonnenberg.

The formations are present in much of the same area, with the Three Forks extending farther to the south in North Dakota.

Both the Bakken and Three Forks extend north over the Canadian border, primarily in Saskatchewan, although Manitoba also has a Three Forks play.

“The source beds are thought to be more mature in Canada, so you are dealing more with migrated hydrocarbons, while the thermal area, or ‘cooking pot,’ is in the United States,” Sonnenberg said.

Shale gas plays tend to develop a core area or fairway of best production, but it isn’t clear if shale oil plays will follow the same pattern.

“It’s early in the development process. We’re seeing some very good wells along the Nesson Anticline and in the Parshall Field area,” Sonnenberg said. “It’s early to say where all the sweet spots are.”

As more exploration extends into the Three Forks and Bakken, those development efforts could become a pattern for shale oil and tight-oil drilling elsewhere.

“The model one develops from looking at this is going to be the classic tight-oil system for the world,” Sonnenberg said.

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At ACE 2011 ...

AAPG Honorary Member and past president Steve Sonnenberg will present the paper "Petroleum Potential of the Upper Three Forks Formation, Williston Basin, USA" at 4:40 p.m. Tuesday, April 12 at the AAPG Annual Convention and Exhibition in Houston.

His co-authors are AAPG members Rick Sarg, an AAPG Levorsen Award winner, and Allaudin Gantyno. All are at the Colorado School of Mines in Golden, Colo.

Sonnenberg's paper is part of the session titled "U.S. Active and Emerging Plays – Paleozoic Basins and Cretaceous of Rockies." Other titles in the nine-paper session include:

Unlocking Value in an Unconventional Gas Shale Play by Returning to Conventional Basics: Regional Analysis of the Woodford Shale, Arkoma Basin

An Overview of Some Key Factors Controlling Well Productivity in Core Areas of the Appalachian Basin Marcellus Shale Play.

Potential Paleozoic Shale Gas Resources in Utah.

The Barnett Shale Oil Model of North Texas.