In 2010, Whiting Oil and Gas (WLL) drilled exploratory wells in the area of Belfield, ND, just west of Dickinson. These wells were considered "wildcats" or wells drilled a distance away from known horizontal oil production. The sentiment in the industry was that if these wells were economic producers, thousands upon thousands of acres would open up to oil production south of the current exploration area and the game would be significantly changed. Although The Bakken formation was not the target formation in these tests, the Three Forks formation proved to be economically viable. (When people reference the Bakken Oil Field the Three Forks Formation is usually implied). Fast forward to today and there are 6 active drilling rigs in Stark County, ND, with no signs of slowing down.
Soon after thousands of additional acres were opened up, another company, Chesapeake (CHK), decided to come into the play and drill a little farther out from the known production. Chesapeake, which usually is a front runner in shale plays but came to the Bakken very late in the game, decided to try for economic Three Forks production South of Dickinson. Unfortunately, early reports are that most of these wildcats were not successful and have been plugged. There have even been reports that Chesapeake sent letters to mineral owners in the area saying they did not intend to honor their leases.
On the Southern end, it appears the edges of economic production from the Bakken and Three Forks formations have been found. This is not to say that new technology will not be able to exploit oil from this area in the future or that there are no other formations able to produce horizontal oil. For instance, the Tyler formation's horizontal potential is only starting to be developed in this same area.
On the Northern end of the Bakken region, we know they are producing economic Bakken wells up to and well past the Canadian border.
On the Eastern end, the jury is still out. Other formations, such as the Spearfish formation North of Minot, have been hit and miss with economically producing wells.
The western border of the field, however, seems to be the grayest. The Bakken formation, which lies inside the Williston Basin, extends part way into Eastern Montana. The adjacent basin to the west, the Alberta Basin, has a formation with characteristics similar to the Williston Basin Bakken Formation and has been dubbed the "Alberta Bakken" by many. Operators such as Anschutz Exploration, Newfield Exploration (NFX), and Primary Petroleum (PETEF) have been exploring this formation in Glacier County Montana. Most of these wells are still in confidential status but some have been rumored to be economic producers while others are said to have been plugged.
The economic Williston Basin Bakken formation seems to have fairly clear defined edges while the underlying Three Forks formation is slowly being defined as well. Although these prolific formations are being more defined, there are a host of other formations with increased oil potential in ND. With 208+ rigs running in Western North Dakota and Eastern Montana, "unconventional" wisdom would say E&P's will exploit all the formations they can within their lease holdings due to the advancing drilling efficiencies and reliable infrastructure (pipelines, rail, service companies) that is slowly but surely being built to assist with resource development. That being said, the resource potential of the current operators could be severely understated. The largest leaseholders in Western North Dakota are Conoco Phillips (COP), Continenial Resources (CLR), EOG, Whiting Oil and Gas and Hess (HES).