Bakken and Three Forks: USGS Estimates 4.3 Billion Barrels of Undiscovered Oil, 2021

The nine Bakken Formation assessment units, North Dakota and Montana. U.S. Geological Survey.

The estimate

Using a geology-based method, the U.S. Geological Survey (USGS) estimates a mean of 4.3 billion barrels of oil and 4.9 trillion cubic feet of associated gas, undiscovered but technically recoverable, in the Bakken and Three Forks Formations of the Williston Basin Province in North Dakota and Montana. These are continuous (unconventional) resources.

Why assess again

USGS assessed the Bakken in 2008 and 2013, and the Three Forks first in 2013. Since then, more than 6,400 more wells have been drilled into the Bakken and about 4,100 into the Three Forks. In all, more than 17,500 wells have reached the two formations, which have produced about 4 billion barrels of oil.

The rocks

Both belong to the Bakken Total Petroleum System, which spans the Three Forks (Upper Devonian), the Bakken (Upper Devonian to Lower Mississippian) and the base of the Lodgepole Formation (Lower Mississippian). The Williston Basin is an intracratonic basin reaching across Montana, South Dakota and North Dakota and into Saskatchewan and Manitoba.

• Three Forks: sits on the carbonates of the Birdbear Formation and is capped, across an unconformity, by the Bakken. Its dolomitic shale, siltstone and dolostone suggest very salty marine, subtidal and intertidal settings, and anhydrite nodules are common low in it. Industry often splits it into four benches.
• Bakken, in four units:
• • the Pronghorn Member — sandstone to shale and limestone, fining upward, patchy across the basin, and assessed with the Three Forks because their reservoirs connect;
  • the lower and upper shale members — organic-rich and the source of the oil; the upper shale reaches farthest and sets the outer edge of the assessment units;
  • the middle member — sandstone, siltstone, dolomite and mudstone from shallow water as the sea retreated, and the main horizontal drilling target.

Oil from the shales has moved locally into the middle member, the Pronghorn and dolomitized parts of the Three Forks. The Three Forks is charged mostly from the lower shale above it, where the shale is thicker, more mature and overpressured. Drillers aim mainly at its upper interval, closest to the source and usually richest in oil; the middle and lower parts have given mixed results.

The seven Three Forks Formation assessment units. U.S. Geological Survey.

How it was divided

USGS defined nine Bakken and seven Three Forks assessment units, bounded by thermal maturity, structure, where the formations reach and thin, and differences in water saturation.

Results

Fifteen units were assessed — all but the Eastern Three Forks unit. The fully risked mean totals, with the range from a 95% chance (F95) to a 5% chance (F5):

By unit (means):

The Northwestern Three Forks unit was given a 0.5 probability; every other unit, 1. Assessment methods: the USGS Energy Resources Program.

Sources

Based on Assessment of Undiscovered Continuous Oil Resources in the Bakken and Three Forks Formations of the Williston Basin Province, North Dakota and Montana, 2021, U.S. Geological Survey Fact Sheet 2021–3058, by the USGS Bakken and Three Forks Formations Assessment Team, USGS Publications Warehouse; a work of the United States government in the public domain. Maps and the results table recovered from the fact sheet's PDF.