DENVER – Addressing the USGS 2013 reassessment of the Bakken and Three Forks plays, research geologist Stephanie Gaswirth said, “Although the volume of oil we reported in 2008 for the Bakken is similar to the number we determined during the 2013 assessment, the fact is that more than 500 million barrels have been produced since 2008 from Bakken. If you consider that, the numbers have actually gone up for the Bakken.”
The USGS’ Dr. Gaswirth, lunch speaker at the Rocky Mountain Association of Geologist’s August meeting in Denver, also noted that “operators are changing the ways these wells produce and the production curves over time. The other thing that’s different this time around is the increased cooperation from industry regarding geology and what they’re seeing in the areas where they’re drilling.”
Williston Basin Province, Bakken total petroleum system (TPS), and the Bakken formation Assessment Units (AUs). Inset map shows location of the Bakken TPS.
The USGS study looked at two different types of accumulations — conventional (vertical drilling with standard production methods) and continuous (horizontal drilling, with unconventional production techniques like hydraulic fracturing, sliding-sleeve completion techniques, etc.).
According to the 2013 report, the undiscovered, technically recoverable resource from continuous oil resources in six assessment units equals 7.4 billion barrels of oil, 6.726 trillion cubic feet of associated gas, and 0.527 billion barrels natural gas liquids. The conventional oil resources in two assessment units equal 0.008 billion barrels of oil.
The 2013 reassessment of the Bakken and Three Forks plays in the Williston Basin incorporates the Three Forks, which was previously not included.
“As part of incorporating the Three Forks formation for 2013, we had to adjust our total petroleum system (TPS) boundary from the 2008 assessment to account for potential undiscovered resource in the Three Forks,” she said. “In the 2008 assessment, Three Forks wasn’t included so it was largely based on the extent of the Upper Bakken shale and possible accumulations in the Lodgepole. For 2013, the boundary was extended to include Three Forks.”
The other new piece of data that was heavily used in this assessment was updated thermal maturation data that was analyzed by geochemistry experts at the USGS labs from a series of kerogen isolation experiments from samples collected at the USGS and the North Dakota Geological Survey core facility samples, according to Gaswirth.
The 2013 Bakken assessment was divided into six assessment units (AUs): Middle Bakken (conventional, Montana, North Dakota); Northwest Transitional (continuous); Central Basin (continuous); Elm Coulee-Billings Nose (conventional); Nesson-Little Knife (continuous); and Eastern Transitional (continuous).
Bakken formation assessment results in billions of barrels of Oil (BBO).
These AU lines were based on structural elements, source rock maturity of Upper Bakken shale, and resistivity data. For unconventional oil resources, the USGS estimated a total mean resource of 3.65 billion barrels of oil, and 4 billion barrels of oil for conventional resources.
“Instead of the 400 hydrogen index number they used in the 2008 assessment, they used a 450 hydrogen index as the area of the Bakken that is mature for oil. The assessment also looked to define sweet spots versus non-sweet spots by using geologic factors such as the Brockton Froid fault,” she said.
“In the 2008 assessment, we had very little production and geologic information in the Three Forks. It was largely considered a nonentity of the Bakken play at that time, but fast-forwarding to 2012 it’s easy to see that since the end of our last assessment period the Three Forks has just exploded.”
Three Forks results in billions of barrels of oil (BBO).
The Three Forks was broken into two units, a conventional and continuous unit. The study also accounted for recent completions in the second bench (Burlington Resources’ #11-1TF-2SH Sunline and Continental Resources’ #2-22H Charlotte), the third bench (Continental Resources’ #3-22H Charlotte) and other benches of the Three Forks.
“We also heard from operators that if they put a well into Upper Three Forks and fracture it, you produce oil from the overlying Pronghorn, and that is why we assess it with the Three Forks. However, the Pronghorn member is geologically and stratigraphically part of the Bakken formation,” Gaswirth said.
“A minor amount of the conventional AU extends into South Dakota, but we believe that the geology in this area isn’t cooperating to create reservoirs, and oil migration appears to be to the north instead of the south.”
A copy of the USGS report, Assessment of undiscovered oil resources in the Bakken and Three Forks formations, Williston Basin Province, Montana, North Dakota, and South Dakota, 2013 can be found online.