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The Williston Basin is a large, oval-shaped intracratonic sedimentary basin that extends through parts of Montana, North Dakota, South Dakota, Saskatchewan, and Manitoba. The total area of the basin extends approximately 375,000 square kilometers (140,000 square miles) with the thickest portion towards the center, reaching a depth of approximately 4900 meters (16000 feet). Accommodation was created within the basin due to crustal weakening in the region caused by the Trans-Hudson Orogeny that occurred approximately 2-1.8 billion years ago.

Map showing the approximate extent of the Williston Basin

Sedimentation in the Williston Basin began around Cambrian time (~540-480 ma), and persisted throughout the Tertiary (~65-2.5 ma). During the majority of the Paleozoic and Mesozoic, the basin was covered by epheiric (inland) seas and was in close proximity to the Paleo-Equator. This allowed for the formation of very fine-grained sandstones, shales, limestones, dolostones, and evaporites indicative of quiescent, shallow marine environments. Throughout time, reactivation of faults and dissolution of evaporites led to varied thickness of units within the basin. Additionally, faulting also created structural anticlines that were the primary target for oil extraction in many of the earliest fields.

Bakken Formation

Overview

Main article: Bakken Formation

The Bakken Formation represents the rock units within the Williston Basin spanning from late Devonian (~380-360 ma) to early Mississippian (~360-345 ma). It is composed of a lower and upper organic-rich shale unit and a middle unit composed of fossil-bearing, calcareous silty-sand units that vary lithologically throughout the basin. Deposited units within the formation were the product of fluctuations in the sea-level that varied the concentration of oxygen and terrigenous sediment input during Bakken deposition. The formation reaches a maximum thickness of 40 meters (140 feet) east of the Nesson Anticline in North Dakota. Furthermore, the formation is known to vary in thickness near its margins due to groundwater recharge causing dissolution of the underlying Prairie Salt member.

Depositional history and stratigraphy:

Stratigraphic column of the Bakken Formation, with the underlying Pronghorn member and the base of the overlying Lodgepole Formation shown as well. Modified from LeFever 1992.

The Bakken Formation is able to be deciphered stratigraphically by understanding the fluctuations of sea-level during deposition. First, the lower Bakken shale member uncomformably overlies a unit known as the Pronghorn member (formerly known as the "Sanish sand"). This unconformable surface represents the beginning of a phase of sea-level rise, which is referred to as a transgressive systems tract (TST) by sequence stratigraphers. Transgression and deposition of the lower member was initiated by tectonic activity in the Williston Basin. The generation of this tectonic activity is associated with movement of both the Antler and Acadian Orogenic belts. Deposition of the lower Bakken shale persisted until the maximum flooding surface (MFS) was reached. At this time, sea-level fall began simultaneously with increased terrigenous sediment input, represented as a lowstand systems tract (LST) The point of maximum sea-level regression represents an unconformable surface in the middle Bakken member. This unconformable surface is inferred to represent the lowstand surface of erosion that formed due to sub-aerial exposure. In addition, the composition at this time shifts from siltstone to sandstone, representing a increase in grain size. From this unconformable surface, the sea-level began a TST once again. This time period represents the transition from middle Bakken member to upper Bakken shale. Finally, the upper Bakken shale is conformably overlain by the Lodgepole formation, which represents highstand systems tract (HST) deposition.

Lithologic variations

Cross-section of the Bakken Formation (USGS 2008)

Throughout the Bakken Formation, the lithology varies from the center towards the depositional edges. In particular, the lower member tends to become more silty towards the margins of the basin. This is caused by increased terrigenous sediment that is associated with marginal marine environments. Additionally, the middle member varies as well. Towards the center of the basin, the siltstone and sandstone is more calcium-rich. Near the edges of the basin, where groundwater-recharge is common, the calcium-bearing units tend to be dolotimized.

Variations in aerobic conditions:

The fluctuations in sea-level that occurred throughout deposition of the Bakken Formation were in tandem with changes to aerobic conditions. During the transgressive phase of deposition in the lower and upper shale members, the oxygen conditions were anaerobic. As sea-level began to fall during the beginning of middle Bakken deposition, the conditions shifted from dysaerobic to aerobic. This is seen in core samples because fossils, such as crinoids and brachiopods, are found throughout the middle member.

Lower and upper member source rock potential:

Since the 1960s, many studies have proven that the lower and upper members of the Bakken Formation are a world-class source rock. Source-rock quality is quantified using the total organic carbon (TOC) of source rock samples in weight percent (wt-%). The American Association of Petroleum Geologists (AAPG) define a excellent source rock as those with TOC>2.0% (via AAPG wiki TOC). On average, the lower and upper Bakken members have an average TOC of 12% and 16%, respectively. Early studies of Bakken-sourced hydrocarbons were difficult to distinguish from Madison group hydrocarbons. More recent studies have been able to distinguish Bakken oil from Madison oil based on sulfur content. Additionally, Van Krevelen diagrams for the lower and upper Bakken from different locations within the basin show that the majority of oil originated from algal sources.

Middle member reservoir rock potential

The middle member of the Bakken Formation is favorable as a reservoir rock due to its increased porosity and permeability as well as its position relative to the upper and lower member. Oil was able to migrate from the lower and upper members into the middle member due to fractures created during hydrocarbon generation, fault reactivation, and evaporite dissolution. Additionally, the slow burial rate allowed for the oil within the middle member to not become overmatured.

Upper member seal

The upper Bakken member serves as the seal for the majority of reserves within the Williston Basin. For Bakken-sourced oil that is produced from reservoirs in the underlying Three Forks and Pronghorn members, the lower Bakken member acts as a seal.

Stratigraphic trap

The majority of oil within the Bakken Formation is stratigraphically trapped. The formations pinches out at its margins. For many of the first conventional wells, anticline domes were the primary target for conventional wells. This was due to the fact that these domes correlated with increased thickness in a specific area, rather than the domes acting as a structural trap.

Continuous hydrocarbon accumulation:

Due to its continuity throughout Williston Basin, the Bakken Formation is considered to be a continuous type of petroleum accumulation. There are many different attributes of continuous accumulations, the most significant being low recovery factors, little distance between the reservoir rock and source rock, and large volumes of oil in close proximity to where it was generated.

History of Bakken-sourced hydrocarbon production within Williston Basin

Historical background

Conventional reserves

Chart showing oil production in the Williston Basin for North Dakota, South Dakota, and Montana. Note that this is production for all oil, not just Bakken-sourced reserves.

Recovery of conventional reserves of Bakken-sourced hydrocarbons has occurred within Williston Basin since the 1950s, when production started in Antelope Field in North Dakota. Other significant fields that followed Antelope Field include Elkhorn Ranch and Buckhorn. After conventional oil production began, it steadily increased until it peaked around 1986. At this time, the majority of conventional reserves had been extracted from Bakken-Sourced fields within the Williston Basin. In North Dakota, the total recovered reserves of Bakken-sourced oil as of 1990 was estimated at approximately 21 million barrels of oil.

Unconventional reserves

Horizontal drilling and fracture stimulation led to the discovery of giant oil fields within the Bakken Formation since 2000. Due to the discovery of these fields, hydrocarbon production within the Williston Basin has begun to increase substantially in tandem with technological advances. The first field within the Williston Basin to use Horizontal Drilling to produce oil was the Elm Coulee Oil Field in Northeast Montana. As of 2009, Elm Coulee is estimated to have around 280 million barrels of oil that is recoverable. Horizontal drilling has also led to new discoveries in other areas of the basin, such as the 2006 discovery of Parshall Oil Field located in North Dakota.

Estimated undiscovered reserves for the United States:

USGS Assessment 2013

Technological improvements in methods of oil recovery have caused estimated undiscovered reserves within the Bakken Formation to increase greatly. In 2013, the United State Geologic Survey conducted an assessment of the undiscovered reserves of the Bakken and Three Forks formations within the United States. Estimated undiscovered reserves of oil was approximately 7.4 billion barrels of oil within 6 assessment units (AUs). This was a great increase from estimates by the USGS in 2008, which stated that approximately 4 billion barrels of oil had yet to be discovered. Horizontal drilling and fracture stimulation are the main production methods for Bakken-sourced oil production. Since the prior assessment in 2008 by the USGS, 450 million barrels of oil was produced from Bakken and Three Forks- sourced petroleum systems. Current exploration focuses on finding "sweet spot" areas with higher porosity and permeability.