An update to its 2008 study

New USGS Bakken Assessment On Its Way

The Upper Devonian-Lower Mississippian age Bakken shale play in the Williston Basin in North Dakota and Montana has become a really Big Deal in the world of shale oil production.

So big that the U.S. Geological Survey is completing a two-year reassessment of the Bakken formation, following closely on the heels of the initial assessment undertaken by the organization in 2008.

Noting that it’s rare to reassess so quickly, the agency commented that the prolific Bakken is an unusual reservoir, emphasizing that what is technically recoverable has changed over the course of a short time period.

This is because as drilling escalated, knowledge of the geology improved – and significant information was derived from production data.

The 2008 evaluation estimated a total mean resource of 3.65 Bbo for unconventional oil resources and four MMbo for conventional resources, according to Stephanie Gaswirth, USGS research geologist and task chief for the new assessment.

It is anticipated the study will become available to the public later this year.

The Big(ger) Picture

Noted oilman Harold Hamm, CEO of Continental Resources, is on record as saying that a conservative estimate for the Bakken is 24 billion barrels recoverable. He asserted that it’s the largest field found in the world in 40 years.

Continental is the largest leaseholder and driller in the Bakken.

The Bakken petroleum system has been a target for the explorers for many years.

Its big breakout moment might be said to date back to 1995, when explorer and AAPG member Dick Findley determined there was good porosity and a likely oil zone in the fractured dolomitic middle section of the shale.

This eventually led to development of the giant Elm Coulee oil field in the Bakken in eastern Montana – and for Findley it eventually led to him receiving the AAPG Explorer of the Year award for his insight and efforts.

Today, lengthy laterals along with multi-stage fracturing technology might be called the icing on the proverbial cake when it comes to economical, prolific production from the shale.

The Three Forks formation underlies the Bakken, separated in some areas by the Pronghorn Member of the Bakken Formation (“Sanish Sand”). It’s a part of the Bakken petroleum system and is some of the oldest production in the Williston Basin, dating back to the 1950s.

The 2008 assessment did not evaluate the Three Forks, but it is included in the 2013 assessment.

“This time, there will be undiscovered, technically recoverable resource numbers for the Bakken and the Three Forks as well,” Gaswirth said.

Team Effort

The USGS is mandated by law from the Energy Policy Conservation Act of 2000 to provide assessments of 32 priority basins in the United States, according to Rich Pollastro, retired USGS geologist who was the task leader for the 2008 Bakken assessment.

The subsequent Energy Policy Act of 2005 states that the same methodology must be used in producing these assessments.

Pollastro noted, however, that the methodology used to assess continuous, i.e. unconventional, accumulations differs from the conventional.

In the case of conventionals, the researchers look at sizes and numbers for the accumulations. With the continuous type accumulations, they assume the hydrocarbons are regional in extent, and it’s a matter of how successful operators will be with recovery.

To get at the 2008 numbers, the area of the oil generation window for the Bakken continuous reservoir was determined and then divided into five continuous assessment units (AU). A sixth hypothetical conventional middle sandstone member AU was defined external to the area of oil generation. The final assessment numbers included all AUs.

The AUs were defined based on structural elements, source rock maturity of the Upper Bakken shale, and resistivity data.

“The reassessment of the Bakken formation redefines the AUs based on updated thermal maturity data, structural controls and the mapped extent of Bakken source rocks,” Gaswirth said. “Continuous AUs are refined given the large amount of new production data available following 2008, particularly in the central basin, northeast Montana and northwest North Dakota.

“The Parshall and Sanish fields have also been developed substantially since 2008, yielding longer production histories and more robust EUR data.”

Gaswirth has kudos for the industry and others.

“I talked with more than a dozen different companies, and they were really helpful,” she said. “The geological surveys in North Dakota and Montana also were very cooperative and helpful.

“There was very good collaboration all around.” 

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Presentation

Research geologist Stephanie Gaswirth will present the paper “Reassessment of Undiscovered Resources in the Bakken Formation, Williston Basin, North Dakota and Montana,” at 9:05 a.m. Tuesday, May 21, at the AAPG Annual Convention and Exhibition in Pittsburgh.

Gaswirth is with the U.S. Geological Survey in Denver. Her co-authors are AAPG members Kristen Marra, also with the USGS in Denver, and Troy Cook, with the Department of Energy.

Her talk is part of an AAPG-EMD session on the Bakken Petroleum System.

Emphasis: Shale Horizons