The Williston basin covers parts of Montana, North Dakota, South Dakota, and Canada. It is a large intracratonic basin where sedimentation occurred throughout much of the Phanerozoic period and is 15,128 ft thick at its deepest known point according to a well located in McKenzie County in Western North Dakota. Paleozoic strata consist mainly of cyclic carbonate deposits, and the Mesozoic and Cenozoic strata consist mainly of siliciclastics. Prominent structural features include the Nesson, Billings, TR, Little Knife, and Cedar Creek anticlines. The Bakken petroleum system within the Williston basin consists of the Bakken formation, lower Lodgepole, Pronghorn, and upper Three Forks. 
Tectonic and Sedimentary History
The Williston structural basin began with a thin clastic sequence in Middle Ordovician time followed by mainly carbonate deposition. Carbonates were then deposited through the Lower and Middle Silurian periods and then followed by a period of erosion which produced a large unconformity. At the time of the Middle and Upper Devonian Epochs, the Williston basin was a part of the larger western Canada basin which is recognized as having carbonate deposition with a thick evaporite in the lower part and cyclical carbonates with some clastic and evaporite beds in the upper part. Deposition of carbonates and evaporites continued into Mississippian as the middle of the Madison depositional basin had merged with the present Williston basin. The next layer of rock is made up of mainly clastic sediment, and then was followed by a period of erosion which produced another unconformity. The Pennsylvanian and Permian periods can be summarized as producing mainly clastic deposition with the minor presence of carbonates and evaporites. At this time the basin underwent a period of subsidence being part of a larger depositional area that extended from the south to the west. Similar conditions persisted through the Triassic period while the sedimentation of the Jurassic and Cretaceous periods extended the basin eastward. Finally, the Tertiary period led to the deposition of non-marine beds thickening the basin westward.
Oil and gas exploration in the Williston basin has gone through four cycles of conventional exploratory drilling beginning in 1951. The first commercial oil well was drilled in 1951 in Williams county in North Dakota, and was followed by exploration of the Nesson Anticline later in the decade. The two major zones that fostered production along the 75 mile long anticline were the Mississippian subcrop in the northern part of ND, and the Mississippian/Pennsylvanian play in the southwestern part of ND.
The second cycle of drilling began in 1968 with oil being discovered in the Red River Formation in Billings county. The Red River discovery, along with the discovery at Bell Creek Field were what fueled the highly productive third cycle.
The third drilling cycle began in the mid-1970’s and was the leading cycle in terms of development. Many new fields were discovered in this cycle with most of the discoveries being located in west-central North Dakota. The cycle was led by two highly intensive discoveries. The first discovery came in 1972 in the Red Wing Creek Field. The Red Wing Creek Field was a complex geological structure with a pay section of 1,000 feet thick. The second discovery of cycle three happened in 1973 with the Arab Oil Embargo. Because of this, the Billings anticline complex, Mondak field, and the Knife field were discovered. Finally, cycle three saw the beginning of production in the Bakken.
Cycle four saw the discovery of the Dickinson Lodgepole Field along with the discovery of the horizontally drilled Red River “B” Pool in Bowman County.
Due to the advancement of U.S. unconventional oil and gas produciton, the Williston basin has experienced another uptick in activity. The Bakken petroleum system is an unconventional shale play that must be fracked to produce oil and gas in paying quantities. EOG was one of the pioneers to deploy this new technology in the Parshall field of the Bakken. After this discovery, operators were eager to obtain acreage to explore the rest of the Bakken petroleum system. The early unconventional development of the play was targeted in the center of the basin. Since this period of early devolpment, operators have expanded drilling to the outer regions of the petroleum system boosting production over time. The graph to the right shows the growth in daily production from 2009 through 2018, when the Bakken reached record production figures of 1,491 Mbo/d and 1.87 MMcfg/d.
Reservoir Rocks and Source Rocks
In the Bakken petroleum system there was short distance migration of hydrocarbons from the source rock into adjacent low permeable reservoirs. The Mississippian-Devonian Bakken petroleum system of the Williston Basin consists of source beds in the Bakken shales and lower Lodgepole, with an average total organic content of 11%. The shales were deposited in poorly circulated, anaerobic waters with low levels of oxygen present, helping preserve the organic material deposited. The source beds are so rich mainly because they contain high amounts of algal kerogen, an outstanding source material for the generation of oil.
There are reservoir rocks in the lower Lodgepole, Bakken shales, middle Bakken silty dolostones, Pronghorn dolomites and sandstones, and upper Three Forks silty dolostones. The primary oil producing zone that operators have historically targeted is the middle Bakken member which consists of dolostones/dolomite. This layer was deposited in a period of low sea levels with a higher oxygen content, leading to the build up of a carbonate bank. The petroleum system is an unconventional play due to the low permeability of its reservoirs and pervasive hydrocarbon saturation.
Traps and Seals
The traps and seals in the basin vary with specific location. Most of the traps in the Winnipeg - Deadwood parts of the basin are structural. Seals associated with some parts of the basin, like the Winnipeg-Deadwood reservoirs transition from a shale section to a limestone one. Because there is a lack of stratigraphic traps, the lateral seal potential is not very clear. In the Red River system of the basin, again most of the traps are structural, however there are some traces of stratigraphic traps that lead to better lateral seals. In the Winnipegosis petroleum system, the majority of traps are again structural, the seals in this part of the basin can be tight dolomite, but are usually anhydrite. Lateral seals could be either tight dolomite, or tight limestone. In the Duperow system there are large anticline traps, as well as rare unconformity stratigraphic traps, because of these traps, it has some of the best production in the whole basin. There are many fault traps and anticlines in the Cedar Creek location of the Williston Basin. Finally, unlike the majority of the basin in the Tyler Total reservoir, there are mainly stratigraphic traps, this is because of a reduction of porosity and permeability and by sandstones trapped in mudstones and shales.
Primary Geological Risks
This basin is different than many other basins in the country in that, economic drilling here is relatively new. The risks associated with the conventional plays that are more devloped across the Williston basin are relatively basic. The main concern when targeting these plays are related the structure and seal of the reservoir. For example, a concern when drilling into the Mississippian formation of the Nesson Anticline would be that the spill point is deep enough to prevent hydrocarbons from leaking outside the trap. If it was too shallow, there would not be enough oil and gas in economic quantities to produce.
Another important geologic risk relating to the more recent, unconventional plays across the Williston basin is the increase of sulphur content in crude oil. The souring of the crude oil produced from Bakken reservoirs is an important concern due to its impact on commodity prices, health effects, and corrosive nature. Crude oil with a higher sulfur content compared with the traditional light, sweet grade associated with shale plays can trade at a price that results in 10% less profit to the operator. The presence of hydrogen sulfide can be lethal to employees who inhale it. It also erodes the well bore, leading to constant work overs/repairs. The general cause of this phenomenon is related to certain operational practices that include geomechanical forces (intrusion into another formation), biogenic, or thermochemical.
The basin's remaining reserves are significant, as there is an estimated 270-500 billion barrels of oil in place contained in the Bakken shale portion of the basin according to a USGS study, and an estimated 7.4 billion barrels of undiscovered oil, 530 MMbbl of NGLs, and 6.7 Tcf of gas are recoverable. An important note to make is that the improvements in well productivity have been in most prolific areas of the Bakken. As the prime drilling locations are reduced, lower well productivity is expected. Another important emerging trend related to unconventional plays across the United States is the Parent-Child well problem. The debate surrounding this issue is that operators project type curves based off parent wells. Operators made assumptions that the subsequent wells spaced tightly together would produce as efficiently as the parent wells (first wells drilled into reservoir). This has proved to be incorrect, resulting in lower production figures from wells, overstated recoverable reserves. The Bakken play has not been immune to this phenomenon, and it an important issue to follow.
Petroleum and Facility Engineering Aspects
The Williston basin is not in an ideal geographic location compared to other basins in the United States. Unlike the Permian basin or Eagle Ford basin being close to a Houston hub, the Los Angeles basin being obviously close to LA, or the Appalachian basin being close to Northeastern populations, the Williston basin is further away from densely populated areas that need energy. Early in the development of the unconventional plays in the Williston basin, rail cars were used to transport oil and gas to refineries across the United States. This proved to be a dangerous way to transport fuel as several railway accidents occurred across the United States and Canada. The deadliest accident occurred in 2013 in Lac Meantic, Quebec as 47 people were killed when a train ignited carrying Bakken crude.
Another result from the lack of pipeline capacity due to the rapid production growth in the Bakken was a large pricing differential between the WTI spot price and local spot price. Pressure on local prices has eased as midstream assets have been developed in the region. This is shown in the graph depicting Continental Resource's differential in the Bakken improving since since 2015 when the use of railways was at its peak.
Finally, the most significant engineering achievement in the Bakken has been the optimization of drilling and completion techniques. This has been the main driver of production growth in the play along with a modest increase in rig count. From 2015 to 2017 new well oil production has tripled from 500 bo/d to 1,500 bo/d in the heart of the Bakken. This has led to tremendous improvements in single well economics, with an average rate of return of 140% at $65/bo WTI.
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