The Bakken shale, a tight oil resource play covering 64,750 sq km (25,000 sq miles) in South Dakota, North Dakota, and Montana in the US and in the Canadian provinces of Saskatchewan and Manitoba, is ranked as the largest continuous onshore oil accumulation in the Lower 48.

Early Mississippian-Late Devonian in age, the Bakken-Three Forks petroleum system in the Williston basin was discovered in the 1950s and has since evolved into a world-class hydrocarbon resource play. The US Geological Survey has estimated North Dakota and Montana could hold 5 Bbbl to 10 Bbbl of technically recoverable oil in the Bakken-Three Forks complex alone.

Encountered at depths of approximately 2,590 m to 3,200 m (8,500 ft to 10,500 ft), the Bakken comprises the upper shale, middle dolomite (Middle Bakken), and lower shale, with the Middle Bakken targeted as the primary oil reservoir. The Middle Bakken is characterized by average porosity of 5%, low permeability of 0.04 md, and thickness up to 43 m (140 ft).

Directly below the Bakken lies its sandy counterpart, the Three Forks formation, and together these form a petroleum system with oil-producing rock situated between shale layers that can be found approximately 3 km (2 miles) below the surface. Also a focus of Bakken operators, the Sanish sandstone occurs between the Three Forks and Lower Bakken shale, but the formation is less prevalent in the Williston basin.

Since US production from the Bakken shale began in earnest with the 2000 discovery of the Elm Coulee field in Richland County, Montana, the play is rapidly transforming from early-stage exploration into development. Horizontal drilling and multistage fracturing campaigns continue to prove the region’s significant unconventional potential, adding to long-term development in the region.

Most active onshore Lower 48 oil play

Since 2008, the Bakken has been coming of age predominantly in North Dakota, where Williston basin shale potential has attracted significant international attention. The first international operator to enter the basin, Statoil ASA announced in October 2011 it planned to acquire Bakken-focused Brigham Exploration Co. for US $4.7 billion. The deal, which closed at year-end 2011, grants the Norwegian company access to more than 375,000 net acres (60% derisked) in the tight oil play where the Austin-based independent is producing 21,000 boe/d gross and has an existing risked resource base of 300 MMboe to 500 MMboe. Within five years, Statoil estimates its Brigham output has the potential to escalate to 60,000 boe/d to 100,000 boe/d.

The transaction also provides Statoil with approximately 430 miles of oil, natural gas, and water transportation systems centrally located in the basin.

Twenty-four hour initial production (IP) rates have averaged 2,800 boe/d from 88 North Dakota wells on production, according to Brigham, which reported a record-setting Bakken well in March 2011. The #2-H Sorenson 29-32 in the Alger field flowed 5,330 boe/d (4,661 bbl; 4.01 MMcf/d).

Brigham operates 12 rigs in the Bakken shale play, where it has drilled six of the 10 highest IP rate Bakken wells to date, and has 140 wells planned for 2012.

Meanwhile, leading Williston oil producer and Bakken play founder Continental Resources is investing in its ECO-Pad technology to increase per-well recoveries, reduce drilling and completion costs, and improve its environmental footprint in the play. The company has deployed this technology to centralize its Bakken operations while drilling from a single pad on two adjoining 1,280-acre spacing units.

Continental also has maintained long-term growth expectations for the play. The company estimates the Bakken/Three Forks play holds 24 Bboe of potentially recoverable reserves, which it says could double US oil reserves. Continental is the largest acreage holder in the Bakken with a lease position of more than 901,000 net acres as of September 2011; 72% of the company’s acreage is in the North Dakota portion of the play.

Infrastructure takeaway is expected to triple from 540,000 b/d in 1H 2011 to more than 1.4 MMb/d through year-end 2013 as the Bakken expands, according to information published by Kodiak Oil & Gas in December 2011.

Meanwhile, Bakken economics are profitable in the current oil-pricing environment. At year-end 2011, Williams spinoff WPX Energy listed the play pre-tax IRR at 65% based on $95/bbl oil, among other factors. The company also anticipates Bakken takeaway will increase to more than 1 MMb/d by early 2013.

Montana Bakken still evolving

While North Dakota has remained the focal point for many Williston basin operators, Montana Bakken activity is accelerating, and production has improved since the initial #1-4H Piranha (630 boe/d) discovery by Slawson Exploration in 2008.

Top-producing Montana Bakken wells include the Brigham-operated #1-H Gobbs 17-8 A well in Roosevelt County (24-hour peak flow rate of 1,817 boe/d), the XTO Energy-operated #11X-12 Thiel well in Richland County (1,201 boe/d), and the EOG-operated #5-1003H Stateline in Roosevelt County (409 boe/d).

EOG recently completed two top Montana Bakken discoveries – the #1-2423H and #10-1211H Stateline – which are the first Bakken producers in their respective townships in Roosevelt County. According to IHS Inc., the wells are near the Elm Coulee Northeast field and produce from fracture-stimulated, openhole Middle Bakken intervals.

The #1-2423H Stateline produced 5,375 bbl of oil, 3.62 MMcf of gas, and 12,960 bbl of water (5,979 boe/d) from the Middle Bakken in 15 days during August 2011, according to IHS data. The horizontal completion is in Section 24-28n-59e and was drilled to 4,868 m (15,972 ft), with a true vertical depth (TVD) of 3,148 m (10,327 ft) and bottomhole location in Section 23-28n-59e.

The #10-1211H Stateline produced 3,385 bbl of oil, 2.19 MMcf of gas, and 11,239 bbl of water (3,750 boe/d) in 10 days during August 2011, according to IHS. The horizontal completion is in Section 12-28n-59e and was drilled to 4,752 m (15,590 ft), with a TVD of 3,154 m (10,347 ft).

EOG has scheduled four additional horizontal Bakken wells on its leases in Roosevelt County township 28n-59e in Sections 13, 15, 24, and 26.

Continental completed the first commercially viable horizontal well in the North Dakota Bakken. The Robert Heuer 1-17R was horizontally drilled and fracture-stimulated in Divide County in 2004. (Image courtesy of Continental Resources)

Advancing the shales

The Bakken stands at the forefront of R&D. Bakken wells are consistently drilled to around 6,100 m (20,000 ft) with 2,740-m (9,000-ft) laterals, and new technology has created 20- to 30-stage fracturing and multiple laterals from common pads, making it one of the cleanest and most efficient plays in the world, according to the Hart Energy 2012 North American Unconventional Yearbook.

In 2007, the Bakken Research Consortium was formed to develop best practices for developing Bakken wells. The consortium’s 31 members include prominent Bakken operators such as Chesapeake Energy Corp.,

EOG Resources Inc., Hess Corp., Newfield Exploration Co., and Exxon Mobil Corp. via XTO Energy Inc., as well as agencies such as the Department of Energy and North Dakota Geologic Survey.

As operators continue to favor North American shale development, premium oil shales like the Bakken will redefine domestic oil production, attracting both local and international attention for the long term.