Julie LeFever explains, in a paper called Oil Production from the Bakken Formation: A Short History (available as a pdf from North Dakota’s Department of Mineral Resources), how the first horizontal well in the Bakken, dug in 1987, changed things for the state:
Drilling activity along the “Bakken Fairway” changed greatly after Meridian Oil, Inc. drilled and completed the first horizontal well in the Bakken Formation. The #33-11 MOI, was initially drilled vertically. It was cored, logged, and drill stem tested, all of which indicated that the formation was tight. Meridian then backed up the hole and kicked off at 9,782 ft. Horizontal drilling was attained at 10,737 ft (measured depth) with a resulting radius of 630 ft. The well was completed on September 25, 1987 for 258 BOPD and 299 thousand cubic feet (MCF) of gas. The well had a horizontal displacement of 2,603 ft and is now producing in the upper Bakken shale that is 8 ft thick. The decline curve for the #33-11 was remarkably stable for the first two years until additional nearby wells came online. The well has produced 357,671 BO and 6,381 barrels of water (BW) through December 2003.
The success of this well set off the previous notable play dealing with the upper Bakken shale. Operators were eager to use horizontal well technology to encounter more fractures and thus produce more Bakken oil. As in any play, there was a learning curve. The #33-11 MOI took 57 days to drill and complete; 27 days to drill the vertical borehole and 12 days to drill the horizontal section at a cost of $2 million. The third set of ten wells drilled by Meridian, further down the learning curve, had an average cost of $1.08 million and took 35 days to drill. By the end of the play, Meridian Oil, Inc. was touting the fact that they could drill and complete a horizontal Bakken well for essentially the same price as the drilling and completion of a vertical well, around $900,000. Successful wells were capable of producing high volumes of oil (IP’s in excess of 1900 BOPD).
Here is a follow-up report from the Associated Press in June 1989 on how horizontal drilling had already changed the landscape:
Horizontal drilling an oil recovery method pioneered in western North Dakota two years ago, is helping to spur activity in the state’s oil patch.
During the past week, Pacific Enterprises Inc. of Denver was granted 16 permits to drill horizontal wells in the shale-like Bakken formation in McKenzie County.
It is the highest number of drilling permits taken by one company in a single week since 1981, said Jack Wilborn of the oil and gas division of the state Industrial Commission.
More oil companies are expected to adopt horizontal drilling when attempting to recover oil in the Bakken formation, which is found in several oil-producing counties, including McKenzie, Billings, Dunn and Golden Valley, said Wes Norton, also of the oil and gas division.
“It will be interesting to see if they get drilled,” Norton said.
About 1 1/2 years ago, Meridian Oil of Billings, Mont., completed the first successful horizontal well in the state in northern Billings County. Through April, that well had produced more than 146,000 barrels of oil.
Meridian Oil has since drilled 10 horizontal holes that produce 30 to 300 barrels a day.
“None of them have been plugged as dry hole,” Norton said.
Five of the 15 active rigs in the state are drilling horizontal wells.
To date, Conoco Oil Co., is the only other company to try horizontal drilling.
But state officials say interest in the drilling method is increasing rapidly and interest played a key role in the success of the state’s spring oil lease auction.
The Land Department held its most successful sale in five years this spring, when it leased 65,000 acres of $3.5 million. A year ago, the state only leased 15,000 acres for about $400,000.
In February 1990, Greg Booth of the Grand Forks Herald added:
In western North Dakota, billions of barrels of oil lie some 10,000 feet underground, held in fractured shale known as the Bakken Fairway or Bakken Formation.
The formation is only about 10 feet thick. The oil lies in vertical planes, separated by 20 to 200 feet of rock that doesn’t produce or contain oil.
In conventional, vertical drilling, a well must hit one of these pockets to produce oil. If it misses, drillers are stuck with a dry well or one that doesn’t produce enough to warrant leaving it open. In horizontal drilling, the well passes through many fractures, allowing oil otherwise separated, by impenetrable rock to flow into a single pipe. The odds of a dry well are greatly reduced.
Consider this: In vertical drilling, as many as three quarters of the wells drilled in North Dakota are unsuccessful, and that’s considered a good rate. On the other hand, of 42 horizontal wells completed since 1987, only two have been dry, according to John Bluemle, assistant state geologist in Bismarck. That’s a 95 percent success rate, which he characterizes as “unusually high.” . . .
To go from vertical to horizontal, drillers use an angled motor in the well hole, said Bruce Hicks, staff petroleum engineer for the state Geological Survey in Bismarck. The motor is lowered into the vertical hole, and starts drilling at a predetermined angle to reach the oil reserves. When the desired angle is reached, an “angle hold assembly” is put in the hole so the rest of the well can be drilled horizontally along the oil-producing formation.
In a typical North Dakota horizontal well, it takes about 800 feet to go from vertical to horizontal, Hicks said.
In the Bakken Formation, there are two types of fractures, [Julie] LeFever said. Regional fractures are widely spaced. Within the regional fractures are smaller fractures, called microfractures.
Temperature and pressure combine to form oil from the organic material in rock, called “source rock.” The resulting liquid occupies more volume, forcing the rock to break up. The high pressure can force oil up to the surface when a well is first drilled, resulting in a flowing well that doesn’t require a pump in its initial stages.
Oil companies look for the fractured shale as a sign of a promising well. “The more busted up the rock is, the more oil you’re going to get,” LeFever said. . . .
The Bakken Formation is capable of forming as much as 92.3 billion barrels of oil, according to a paper written by geologist and UND graduate Rick Webster. Oil produced in the Bakken also can migrate to other rock formations underground. In all, about 10 billion barrels might be recoverable through horizontal and conventional techniques, some geologists estimate.
A Brief History of North Dakota Oil Production 
From http://www.bakkenprofits.com/?src=005
Although the USGS has not yet confirmed it, geologists at major, big-oil companies are betting that, when released, the new estimates will indicate there may be as much as 458 % more recoverable oil there than previously reported by the USGS in its last 2008 survey.
from 2006-
The Energy Information Administration (EIA) an agency within the U.S. Department of Energy reported and stated already in November 2006 that:
“With new horizontal drilling and completion technology taken into account, the technically recoverable resource base for the entire Bakken Formation is potentially much larger. A draft study by the late organic geochemist Leigh Price provides estimates ranging from 271 to 503 billion barrels (mean of 413 billion) of potential resource in place. This certain knowledge that 413 billion barrels of in-place oil exists in the Bakken source rocks in the Williston Basin presents the oil industry with an unparalleled exploration opportunity,” said Geologist J.W. Price, the Denver-based geologist who conducted the field assessment for the USGS (United States Geological Society).