A new federal assessment estimates the recoverable oil resources for the continuous, unconventional formation at 3.6 billion bbl.
David Michael Cohen, Production Engineering Editor
In April 2008, the US Geological Survey completed a long-awaited re-assessment of the US portion of the Bakken Formation, a late Devonian-early Mississippian formation of the Williston Basin of Montana, North Dakota and Saskatchewan, Canada. The agency estimated the region’s undiscovered, technically recoverable oil resources to be between 3.0 billion and 4.3 billion bbl, making the Bakken play the largest oil accumulation it has ever assessed in the Lower 48.
Technically recoverable resources are those that can be recovered using existing industry technologies and practices. This number does not take into account economic factors, as reserve figures do; nor should it be confused with the total in situ resources. The latter was estimated by USGS in 2001 to be between 200 and 400 billion bbl.1
The mean-case estimate of 3.6 billion bbl represents a 25-fold increase compared with USGS’s 1995 estimate of 151 million barrels of oil. The increase is largely due to advances in drilling and production technologies, such as horizontal drilling and hydraulic fracturing. The new report coincides with a flurry of drilling activity during the last year in the North Dakota section of the formation.
FORMATION GEOLOGY
USGS describes the Bakken play as a continuous oil accumulation, meaning the oil is dispersed throughout the formation instead of located in discrete deposits. The principal oil reservoir in the 200,000-sq-mi Bakken Formation is a thin region of very finely grained sandstone and siltstone, and low-porosity, low-permeability dolomite. This oil-rich region is sandwiched between two organic-rich marine shales, about 2 mi below the surface. This geology has led many to refer to the area as the “Bakken Shale.”
The upper and lower shale regions have fairly consistent lithology and act as both seals for the sandy middle region and petroleum source rocks, forming part of the continuous hydrocarbon reservoir. The middle sandstone varies in thickness, lithology and petrophysical properties. Each overlying layer of the Bakken is of greater geographical area than the layer beneath it.
The Bakken is bounded below by the impermeable Devonian Three Forks Formation and above by the Lower Mississippian Lodgepole Formation. Oil produced from the Bakken is sweet and light, averaging 42°API.
According to a 2005 “best practices” review for the Bakken by Landmark (Halliburton), porosities in the Bakken Formation average between 8% and 12%, permeabilities range from 0.05 mD to 0.5 mD, and the highly pressurized productive area has a net thickness of 6-15 ft throughout most of the formation.2 These factors combine to make oil production from this unconventional play extremely challenging. However, natural horizontal fractures within the rock have led to great success in horizontal drilling programs there.
RESOURCE ASSESSMENT
USGS based its 2008 assessment on the following geologic elements of the Bakken-Lodgepole Total Petroleum System (TPS):
- Source-rock distribution, thickness, organic richness, maturation, petroleum generation and migration
- Reservoir-rock type (conventional vs. continuous), distribution and quality
- Character of traps and time of formation with respect to petroleum generation and migration.3
Historical exploration and production analyses and detailed framework studies in stratigraphy and structural geology were also consulted. Seven Assessment Units (AUs) were defined within the TPS, and six of these within the Bakken Formation were quantitatively estimated, five continuous and one conventional, Fig. 1.
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Fig. 1. The Bakken-Lodgepole TPS (in blue) is shown with its five continuous Assessment Units (AUs, in green), and one conventional AU (in yellow).
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USGS estimated total mean technically recoverable resources (signifying 50% probability of recoverable resource) of 3.65 billion bbl of oil, combining mean technically recoverable resources of 410 million bbl in the Elm Coulee-Billings Nose AU, 485 million bbl in the Central Basin-Poplar Dome AU, 909 million bbl in the Nesson-Little Knife Structural AU, 973 million bbl in the Eastern Expulsion Threshold AU, 868 million bbl in the Northwest Expulsion Threshold AU, and 4 million bbl in the conventional Middle Sandstone Member AU, Table 1. The mean case falls between a high value (5% probability) of 4.3 billion bbl and a low value (95% probability) of 3.0 billion bbl.
TABLE 1. USGS undiscovered, technically recoverable resource assessment for the Bakken Formation
Click Table to Enlarge. |
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The new assessment makes the Bakken the largest oil accumulation assessed by USGS in the contiguous US, representing 7.5% of the Lower 48’s 48.5 billion bbl of undiscovered, technically recoverable oil. The second-largest accumulation is Texas’s Western Gulf Basin, with 3.38 billion bbl.4
Mean values for the Bakken also include 1.85 Tcf of associated/dissolved gas and 148 million bbl of natural gas liquids.
DEVELOPMENT HISTORY
Companies first produced oil from the play in the 1950s, focusing on the upper and lower shales. These efforts achieved little success. Starting in 2001, advances in horizontal drilling and hydraulic fracturing enabled operators to target the more oil-rich middle sandstone layer, resulting in a flurry of new drilling activity.
“Technology has finally caught up to the Bakken Formation,” wrote Julie A. LeFever of the North Dakota Geological Survey in 2005. “The ability to fracture stimulate these horizontal wells is what makes this play work.”5
Initially, this new Bakken activity focused on Montana and especially on Elm Coulee Field, discovered in 2000. In 2005, about 50,000 bopd was produced from Elm Coulee, equal to about half of Montana’s total production that year, and all of the state’s 2000 production.6 Of the 105 million bbl of oil produced from the Bakken by the end of 2007, 65 million bbl were produced at Elm Coulee.7
More recently operators have expanded into the North Dakota section of the Bakken, led by EOG Resources and Whiting Petroleum. The two companies established a strong presence in Parshall and Sanish Fields, the two most prolific zones identified. In 2006, 2.2 million bbl of oil was produced from 300 wells in the North Dakota section of the Bakken.8 The next year, that figure more than tripled to 7.4 million bbl, from 457 wells.9
EOG and Whiting still hold the wells with the highest initial production in the North Dakota Bakken: Whiting’s Liffrig 11-27H well flowed 2,247 bopd and 1.7 MMcfgd from its Robinson Lake prospect in Montrail County in late January 2008, taking the top spot from EOG, one of whose wells initially flowed 2,000 boe/day. As of January 2008, EOG had 16 of the 20 wells with the highest initial production.7 High output in the Bakken was a major driver for a 28% jump in EOG’s overall oil output to 54,400 bpd between the first quarters of 2007 and 2008.10 The company estimates its Bakken assets to hold 50-70 million bbl oil.11
In the last year or so, several other companies have invested heavily in the North Dakota Bakken. Brigham Exploration has increased its presence there to 287,000 net acres, and has completed its fifth horizontal well in Moutrail County, North Dakota.12 Hess is also increasing its role there, with six rigs running in the North Dakota Bakken and plans to increase that number to eight and produce 8,000 bopd by the end of 2008.13 Newfield Exploration plans to drill 10 wells this year in the Bakken, where it holds 170,000 acres.14 Other companies entering or increasing their role in the area include Northern Oil and Gas, XTO and Marathon.
Operators are finding that they must adjust their drilling technique when moving from the Montana section of the formation into North Dakota. Pressures and temperatures at the bottom of the well are higher in North Dakota, which raises the cost of fracturing but results in higher returns. For example, EOG’s average well cost in the Bakken has risen to $5.2 million from $4.2 million, but its net reserves per well have also gone up, to 700,000 bbl from 500,000 bbl.15
The Saskatchewan part of the Bakken has also seen increased activity in recent years, with production from just three of the big players there-Crescent Point, Petrobank and Tristar-reaching 56,000 bopd in 2007. This year, Tristar has 650 net drilling locations so far in the Saskatchewan Bakken, and Petrobank has drilled 135 wells there on its own and 38 wells with partners in 2008.16
The release of the new USGS assessment will doubtless drive even more investment in the Bakken Formation, giving oil companies the opportunities to put technologies developed in the Barnett and Fayetteville Shales to use developing what may become one of the US’s most important domestic oil plays. 
LITERATURE CITED
1 Pitman, J. K., Price, L. C. and J. A. LeFever, “Diagenesis and fracture development in the Bakken Formation, Williston Basin: Implications for reservoir quality in the middle member,” USGS Professional Paper 1653, November 2001, p. 1.
2 Landmark, “Bakken horizontal best practices review,” September 2005, http://www.pttc.org/workshop_presentations/rockies_092405/BakkenPract.pdf, accessed May 19, 2008.
3 US Department of the Interior, US Geological Survey, “Assessment of undiscovered oil resources in the Devonian-Mississippian Bakken Formation, Williston Basin Province, Montana and North Dakota, 2008,” Fact Sheet 2008-3021, April 2008.
4 US Department of the Interior, US Geological Survey, “2008 U.S. Geological Survey petroleum resource assessment of the Bakken Formation, Williston Basin Province, Montana & North Dakota, 2008,” slide presentation, April 2008, p. 17
5 LeFever, J. A., “Horizontal drilling potential of the middle member Bakken Formation, North Dakota,” 2005 Rocky Mountain Section, AAPG Annual Meeting Technical Program, Sept. 24-26, 2005, http://aapg.confex.com/aapg/rm2005/techprogram/A100165.htm, accessed May 19, 2008.
6 US Department of Energy, Energy Information Administration, Office of Oil and Gas, “Technology-based oil and natural gas plays: Shale shock! Could there be billions in the Bakken?” November 2006.
7 “EOG and others target North Dakota Bakken,” Oil Daily, April 14, 2008, p. 1.
8 North Dakota Department of Mineral Resources, Oil and Gas Division, “2006 North Dakota oil production by formation,” table, 2007, https://www.dmr.nd.gov/oilgas/stats/2006Formation.pdf, accessed May 20, 2008.
9 North Dakota Department of Mineral Resources, Oil and Gas Division, “2007 North Dakota oil production by formation,” table, 2008, https://www.dmr.nd.gov/oilgas/stats/2007Formation.pdf, accessed May 20, 2008.
10 “Chesapeake to exit Woodford Shale, EOG unveils new Texas play,” Oil Daily, May 5, 2008, p. 2.
11 “EOG aims to deploy capital more wisely than competitors,” Oil Daily, Sept. 12, 2007, p. 5.
12 “Brigham expands Bakken acreage,” Oil Daily, May 8, 2008, p. 8.
13 “Hess continues Pony work, may make year-end development decision,” Oil Daily, May 1, 2008, p. 6.
14 “Shale play looms large in Newfield budget,” Oil Daily, Feb. 5, 2008, p. 2.
15 “Bakken oil play spreads into North Dakota,” Oil Daily, Sept. 29, 2007, p. 1-2.
16 “Saskatchewan’s portion of the Bakken oilfield, US plus Canada Bakken production 131,000 bopd end of 2007,” Next Big Future blog, April 26, 2008, http://nextbigfuture.com/2008/04/saskatchewans-portion-of-bakken.html, accessed May 20, 2008.
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