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Underlying an area approximately the size of France, the Bakken Formation oil reservoir keeps expanding into what has become North America’s hottest onshore oil play.
Thomas Smith
While the recent U.S. Geological Survey’s 2008 assessment of the U.S. portion of the Williston Basin did not start this oil boom, they certainly added fuel to the fire. Their report estimated mean undiscovered resources of 3.844 Bbo and 3.705 Tcfg (0.68 BBoe) for the Williston Basin Province. This estimate does not even account for a large portion of the play that lies to the north in Canada.
While the Bakken has long been viewed as an excellent hydrocarbon source rock, it was slow to become a legitimate drilling target. In the 1980s and 1990s, wells drilled into the upper shale member produced some marginal results. The real breakthrough came ten years ago, in May 2000, when a horizontal well was completed in Montana’s Elm Coulee Field, first discovered in 1996. The play has since been expanding rapidly into North Dakota, Saskatchewan, and Manitoba as well as into the underlying and overlying formations.
Prior to discovery of the Elm Coulee Field, the primary Bakken target was the upper shale member of the Bakken Formation. Richard Findley, an independent geologist in Billings, studied the Bakken Formation in detail. While many wells had penetrated the formation, only minor amounts of oil had been produced. Reviewing the old drilling records, Mr. Findley found that good porosity had developed in the middle member that had largely been ignored by the oil industry. He found this porosity across a large area in Richfield County, Montana, and concluded a sizeable field of high-quality crude resided in this middle layer and started leasing. With the prospect simply too big for his one man company, he had to seek out a partnership.
“I sold the original idea to Lyco as a completely defined development opportunity,” says Richard Findley. “I mapped what I originally called Sleeping Giant (later named Elm Coulee) as a possible continuous accumulation over 64 km long and 7 km wide. As it turns out, I was actually conservative in my cutoffs as the field has expanded beyond my original limits.”
“Originally, independent geologist Dick Findley came to Lyco Energy Corporation in Dallas, Texas, selling an idea to reenter old plugged and abandoned Red River wells (the Ordovician Red River Formation is one of the primary producing horizons in the Williston Basin) and proposed a simple frac job in the middle Bakken horizon. He estimated a cost of about $85,000 per well and the company would benefit in new and relatively cheap oil production,” recalls Bob Larson, formerly with Lyco at the start of Bakken project, now a Senior Geologic Engineer with Enerplus in Denver, Colorado.
“The reentries were an inexpensive and prudent way to begin to prove my concept of a very large oil field,” explains Mr. Findley. “This was the beginning of the very long learning curve which we are still on today.”
“We entered 9 wells. Three had junk in the way, two came out pretty good, two were ok, and two were not so good,” says Bob. “We were not overwhelmed with the results and felt the program was not working the way it was designed to work. We knew there was oil in the middle Bakken but the low rates were not economic at the time. We needed a way to extract this oil commercially.”
“We made an agreement with Halliburton to support some of the costs and design a frac program for a horizontal well in the middle Bakken,” says Bob. “We all agreed that with Halliburton’s expertise in horizontal technology, coupled with being able to frac more of the reservoir, the program may become commercial.”
Their first horizontal well, Burning Tree State well, spudded in March 2000, drilled 500 m laterally along the middle Bakken member. Then the Halliburton trucks moved in with sand, water, and a lot of diesel horsepower and the fracing process began. The well, which had an initial production of 196 bopd, 85 mcfpd gas, and only 7 bwpd, started a boom that is still going strong.
“The Bakken is the most complicated system I have encountered in my 42 years in this business,” says Bob Larson.
The Elm Coulee Field that started this play may be an exception rather than the rule for Bakken production. The producing field is in a relatively confined area (800 to 1,300 km²) when compared to the formation’s areal extent in Montana and North Dakota of over 65,000 km².
The Bakken Formation is fairly homogeneous across the field where the middle member is a clean dolomite 1.5 to 5 m thick with an average porosity of about 8.5%. Down dip from the accumulation, the middle member becomes thinner and matrix porosity is nearly absent. The accumulation is bound up dip to the southwest where the formation pinches out against older strata.
Outside the Elm Coulee area, the middle member thickens toward the basin center to over 22 m but is much more heterogeneous. Scattered porous dolomite stringers and natural fractures hold a lot of reserves. The challenge has been to connect these porosity stringers. Operators in North Dakota are using advanced geosteering with the aid of structural maps to keep the well bore near the top of the middle member. Some recent well production tests in North Dakota have shown the Bakken to be as productive there as wells drilled in the Elm Coulee Field.
According to Scott Norrid, SAS Region Asset Manager, Northern U.S. for Halliburton, “Each part of the basin is different but if the right technology is applied, commercial completions can be achieved.”
Further north in Saskatchewan, the Bakken has been fueling an exploration boom since 2005. Their first middle Bakken discovery was in southeastern Saskatchewan in 1956. The first modern commercially successful Bakken wells were drilled by Bison Resources Ltd. at the Viewfield in 2004.
According to Erik Nickel, the Senior Research Petroleum Geologist with the Saskatchewan Ministry of Energy and Resources, “There are over 1,506 wells currently producing from the Bakken, most drilled since 2005. Prior to 2005, there were between 10 and 50 Bakken wells active since 1956. Total Bakken production through May, 2009, is nearly 40 MMb. The boom has also affected the sale of Crown Mineral Leases, jumping nearly 3-fold in 2008 from the previous year which also showed record sales.”
Ed Dancsok, Director of the Geology and Petroleum Lands Branch for the Saskatchewan Ministry of Energy and Resources, says “there could be between 25 and 100 Bb of Bakken oil in place in the province.” Companies there expect to achieve a 15 to 19% recovery using infill drilling at eight wells per section. As much as 20 Bbo may be ultimately recovered and that is a lot of oil.
While the Bakken continuous reservoir play has spread from the Elm Coulee Field in Montana east into North Dakota and north into Saskatchewan, new discoveries fueled by Bakken sourced oil are springing up in adjacent horizons as well.
Most researchers believe that the oil sourced from the Bakken Formation did not travel very far, at least vertically. Being overpressured, Bakken oil has travelled into both the underlying and overlying formations as well as up dip out from the areas of oil generation. Erik Nickel and other Canadian geologists “believe that the oil in the Viewfield area migrated northward from the basin center in North Dakota, a distance of at least 200 km.”
The Devonian Three Forks/Sanish sand that underlies the Bakken Formation is seeing plenty of action in North Dakota and Manitoba as well as the equivalent Torquay Formation in southeastern Saskatchewan.
According to Michelle Nicolas, a Petroleum and Phanerozoic Geologist with the Manitoba Geological Survey, “While the Bakken play in the U.S. and Saskatchewan is a big one, in Manitoba it is more of a Three Forks play. Located on the eastern edge of the Williston Basin, the lower Bakken Shale is absent except for some local preservation in salt collapse features. This leaves the middle Bakken sand in direct contact with the underlying Three Forks siltstones. Manitoba has some production from the Bakken sand, most modern production is from the commingled middle Bakken and Three Forks formations. The Three Forks is the dominant oil producer.”
“As a new exploration target discovered in the Sinclair Field in 2004, future potential exists north and east of the field,” Nicolas adds. “Over 1,050 wells in four fields are now producing from the Three Forks. The largest, the Sinclair Field, has proven and probable reserves estimated at 6.8 MMm³ (43 MMb) with the current production at 11,067 bopd or about 48% of Manitoba’s total production.”
The Three Forks/Sanish sand play in North Dakota could “eventually equal the Bakken,” according to Lynn Helms, Director of the State Department of Mineral Resources.
In North Dakota, near the basin center, the Three Forks Formation is made up of sand and porous dolomites that are conformably overlain by the Bakken. While still very early in this play, researchers are still trying to find out if the Three Forks is a separate oil-producing formation from the Bakken. Recent production show these producing horizons can act as two separate reservoirs with Three Forks testing as good, or better, than some of the Bakken wells.
Running out of ideas?University of Tulsa Petroleum Geology Professor Parke A. Dickey possibly had the Bakken discovery in mind when, in September, 1958, he wrote “We usually find oil in a new place with old ideas. Sometimes, we find oil in an old place with an old idea. Several times in the past we have thought that we were running out of oil, when actually we were running out of ideas.” |
Oil flowing from the Bakken Formation has reversed declining production from an onshore basin that has seen exploration for nearly 100 years and 60 years of production.
To many explorationists, the basin had been almost completely explored. At any rate, the area was one of the most heavily explored in the U. S. Then, through one person’s ideas and the cooperation between a service company and an independent oil company, the basin hosts the highest-producing (Elm Coulee alone produced over 15 MMbo in 2008) onshore field found in the lower 48 states in the past 56 years, according to the U.S. Energy Department. North Dakota’s oil production is now at the highest levels in their history (62.8 MMbo in 2008 and over 50 MMbo through August, 2009) due to the success of the Bakken.
“The Bakken play is a great example of how, through research, technological advancements, and sound geological work, reserves can be increased,” says Erik Nickel. “This happened in an area thought to be ‘drilled out’. Many had written southeast Saskatchewan off as a mature Mississippian oil play before the Bakken was re-worked.”
“While I really appreciate the awards and recognition for the discovery of Elm Coulee, it is important to note the team effort of the scientists and engineers at Lyco and Halliburton that made the Bakken economic,” says Richard Findley, the Billings, Montana independent that started the Bakken play.
“More importantly, Elm Coulee appears to be a sweet spot within a much larger continuous oil accumulation that the USGS reevaluated. Since that evaluation, frac technology has advanced and, combined with the potential of the Three Forks Formation, I believe their assessment is greatly understated.”
“Now we need to look beyond the Williston Basin. Elm Coulee is a good analogy for other continuous oil plays throughout the world,” concludes Richard Findley, a true oil-finder.
The Williston Basin - playground for stratigraphersDeveloped on the North American Craton during the Ordovician, the Williston Basin is a roughly circular depression containing more than 4,800 m of sedimentary section. The thickest section is near the center of the basin at Williston, North Dakota. Strata generally thin and become shallower toward the basin margins. The basin covers about 780,000 km² across North and South Dakota, Montana, and the Canadian provinces of Manitoba and Saskatchewan. After 23 serious attempts starting in 1924, the 1951 Clarence Iverson Farm well struck oil. This would be the first major discovery in a new geologic basin in the continental U.S. since WWII. Now, the basin hosts what could become one of the largest onshore oil finds (Bakken Formation) in the continental U.S. The Department of Energy ranks the Elm Coulee Field as the 23rd largest in the entire U.S. The basin records a nearly continuous sedimentation separated by periods of erosion. Several cycles of marine transgressions that filled the basin are separated by marine regressions. In the 1940s, Northwestern University’s Lawrence Sloss used the Williston Basin as an example area when he first proposed his cratonic sequence concept demonstrating a series of rising and falling sea levels. This concept eventually grew into the depositional sequences on seismic data that Vail, Mitchum, and Thompson proposed, or better known as seismic stratigraphy, and is a major tool in hydrocarbon exploration. Sloss divided the Phanerozoic strata into 6 sequences, the oldest being the Cambrian to Lower Ordovician Sauk Sequence. The Bakken Formation and the reservoirs associated with this source fall in the third sequence, the Devonian to Mississippian Kaskaskia Sequence. The Williston Basin tilted northward during the Devonian, the marine communication from the Cordilleran shelf to the Elk Point Basin of western Canada. This shift caused more restricted conditions in the Williston Basin during the deposition of the organic-rich Bakken shales. The shift also explains the connection and stratigraphic similarities between the Bakken Formation and the Devonian rocks in Canada rather than the Devonian rocks deposited on the Cordilleran shelf. The USGS contends that, of the three Bakken Formation members, the middle sandstone is a major contributor to the composite continuous reservoir. Since the discovery of the Elm Coulee Field, it has also been the most sought after drilling target in the basin. This member varies considerably across the basin in thickness, lithology, and petrophyscial properties. The structure of the basin is relatively simple and probably resulted from basin subsidence initiated by two Archean shear systems. Folding and faulting are the main evidences of structural deformation. Some of these structures such as the Nesson, Cedar Creek, Little Knife, Antelope, and Billings anticlines produce oil. |
| Article from GEO ExPro Magazine NO2 - 2010 |
Updated: 02.06.2010 14:55 by Alf Kvassheim
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