Assessment of Undiscovered Resources in the Three Forks Formation of the U.S. Williston Basin, North Dakota and Montana

Kristen R. Marra, Troy Cook, and Stephanie B. Gaswirth
U.S. Geological Survey

In 2008, the U.S. Geological Survey (USGS) used a geology-based assessment methodology to estimate a mean of 3.65 billion barrels of oil of undiscovered resources in the Bakken Formation in North Dakota and Montana. This assessment was limited to the Bakken Formation and 50 feet of strata below the lowermost shale, which includes the informal Pronghorn member of the Bakken Formation and/or the uppermost Three Forks Formation where the Pronghorn is thin or absent. Since 2008, significant drilling and production have occurred within multiple intervals of the Three Forks, warranting the first USGS assessment of the Three Forks Formation. The Three Forks includes supratidal to intertidal units consisting of interbedded gray-green dolomitic mudstones, pink to tan silty dolostones, and variable anhydrite. The Three Forks is stratigraphically divided into lower and upper units, or four distinct 'benches', and is sourced from the overlying, organic-rich Bakken shales. Production is primarily from horizontal drilling into the upper Three Forks, although variable oil saturations have been identified throughout the formation to expand present-day drilling in both the upper and lower units. The 2013 assessment utilizes the geographic extent of the Three Forks Formation and overlying Bakken shales to define continuous and conventional assessment units (AUs). Within the continuous AUs, sweet spots are identified based on areas of maximum hydrocarbon generation (as determined by hydrogen index (HI) values), maximum formation pressures, and structural enhancements. The Pronghorn member is assessed with the Three Forks Formation as it is in fluid communication with the underlying Three Forks reservoirs. The newly revised assessment of technically recoverable, undiscovered oil and gas resources of the Bakken and Three Forks Formations will be available in 2013.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013