Most Bakken petroleum system oil in the Williston Basin is currently produced from the middle Bakken tight oil reservoir, consisting of an interbedded sequence of siliciclastics and carbonates that is sandwiched between two zones of shale, the most common source rock worldwide for petroleum. There is also some production from the upper bench of Three Forks formation, another non-shale, tight, reservoir, made up of an interbedded sequence of green mudstones and tan silty to sandy dolostones. (Red beds and anhydrites occur in the lower half of the Three Forks — i.e. the third and fourth benches.)
Just how much production comes from the Three Forks is not known because as a formation within the Bakken petroleum system, the upper Three Forks bench is considered part of the Bakken pool, so most E&P companies simply identify production in reports to the appropriate government agencies as Bakken.
Production from the lower benches of the Three Forks is much more closely watched because it is essentially still a science project.
According to Continental Resources’ senior vice president of exploration, Jack Stark, as of the end of May 2013 there were only 11 producing wells in the lower benches of Three Forks in the U.S. Williston Basin, meaning second and third benches since none have yet been drilled into the fourth bench.
Five wells have been drilled by other oil and gas companies — EOG Resources, ConocoPhillips, XTO Energy and Zenergy — and six by Continental.
Of the 11 wells, eight are second bench producers with an average initial production, or IP, of 1,695 barrels of oil equivalent per day and a range of 1,031 to 3,150 boe per day.
Three of the 11 wells target the third bench of the Three Forks and produce an average IP of 797 boe a day and a daily range of 465 to 1,340 boe.
Stark talked about preliminary results from both private and state models designed to quantify the potential of the lower Three Forks benches in a late May presentation to DUG Bakken and Niobrara conference attendees in Denver, reported by Oil & Gas Investor on June 5.
Recalling a Bakken petroleum system estimate made by Continental in 2010 that predicted ultimate recoverable reserves of 24 billion boe, which excluded the lower Three Forks benches, Stark said an update to include the lower benches would increase the estimate to 32 billion barrels (see chart on page 21).
The revised estimate incorporates lower Three Forks core data and keeps unchanged the 3.5 percent recovery factor used with the previous estimate of reserves based on the middle Bakken and upper Three Forks formation members.
“What is the true recovery factor? We still don’t know; we’re still estimating it,” Stark was quoted as saying in Hart Energy’s Oil & Gas Investor. “However, for perspective, every 1 percent in incremental recovery factor translates into an additional 9 billion barrels of estimated ultimate recoverable reserves in the field. It’s a remarkably huge field.”
Stark said second and third bench performance was consistent with Continental’s geologic mode.
“The third bench doesn’t have as good reservoir characteristics as the second bench, but they still contain oil and they will deliver.”
With anhydrite nodules, the third bench tends to have a little less continuity and greater variability, Stark said, noting the fourth bench was “much more localized” and found more on the west side of the Bakken system versus the east.
Helms upbeatContinental is not alone in its optimism about the untapped potential of the lower benches of the Three Forks.
When asked what effect the lower benches could have on overall North Dakota oil production during the North Dakota Oil and Gas Division’s monthly press conference on June 14, 2013, Director Lynn Helms said that recent modeling suggests that long-term recoveries from the Three Forks could be much higher than initially thought.
Helms said that based on preliminary results from both private and state models, he believes that production curves for the lower benches of the Three Forks formation, which is part of the Bakken petroleum system, will extend out farther than originally estimated. “So in other words, we don’t think that the natural production is going to go a lot higher than what the current models say,” Helms said. “But we think that the lower benches of the Three Forks are going to result in that peak being really extended. So that instead of hitting a peak and it dropping off, starting to decline in a year or two, we could see a decade of those kind of production numbers.”
As Petroleum News Bakken reported earlier in June, the North Dakota Industrial Commission approved a grant of $8 million to the North Dakota Oil and Gas Research Council to help fund a $115 million, three-year project to evaluate exploration and production in the various benches of the Three Forks formation in the Williston Basin. The project, which will focus on exploring Three Forks reserves, determining optimal Bakken pool well densities and optimizing Bakken production, will be conducted in McKenzie County.
Helms said industry is picking up the additional $107 million needed to fund the project.
And while it is very early in terms of how many wells and what the overall potential is going to be, the preliminary results are exciting.
“I just met with a couple of operators last week and they’re both looking at deeper Three Forks benches, and industry has got a lot of plans ongoing with regards to infill drilling and development.”
According to the Oil & Gas Investor article, Continental is proving incremental reserves from the lower Three Forks in two ways, the first being drilling interference tests that offset exploratory wells.
In its North Dakota Barney unit, where initial wells have been producing from the second and third benches, wells testing for interference will be drilled in the adjacent upper zone, but offsetting by 660 feet.
Continental’s second approach, the publication reported, is with its Hawkinson 320-acre pilot density project, which involves testing full development of four zones on 320-acre spacing within a 1,240-acre unit. The pilot provides for drilling 11 new wells near three existing producers — two in the upper Three Forks and one in the middle Bakken — whose combined production is running at running at 800 barrels of oil a day.
Of the 11 wells, seven will be new wells targeting the first three Three Forks zones, as well as the middle Bakken.
The remaining four will be monitor wells, “and we’re going to have three of them live every time we do a frac,” said Stark.
Stark said the company hopes “to have the project basically all complete and turned on” by November 2013.
To date, Stark said Continental is “very encouraged” by results from the lower benches, which have shown performance characteristics suggesting separation.
“We want to get to the next step and to be able to say with proof positive that we are dealing with incremental reserves, and if there is some communication to be able to say what percent is truly incremental reserves from these zones. So stay tuned,” Stark said.