Characterization of the Bakken System of the Williston Basin from Pores to Production; The Power of a Source Rock/Unconventional Reservoir Couplet

Grau, Anne ¹; Sterling, Robert H.²
(1) Independent Geologist, Denver, CO. (2) Cirque Resources, Denver, CO.

The Williston Basin Bakken system development in the last Five years has become the largest field in the Continental USA with 3.8 billion barrels recoverable (USGS). The Devonian aged Middle Bakken Carbonate interval and the Three Forks dolomites comprise the reservoirs of this highly economic sequence, and the world class source rocks include the upper and lower Bakken shales.

The reservoir rocks of both the Middle Bakken and the Three Forks Formations are considered tight and unconventional, with average porosities of 4-8% and permeability in the microdarcy range. It is the close vertical juxtaposition of these reservoirs with the world class source rock shales that create an ideal target for stratified oil-saturated reservoir targets perfect for horizontal drilling. Multistage stimulation techniques bring the state-of-the-art completion technology necessary for effectively stimulating these tight reservoirs and producing highly economic volumes of oil.

The Bakken reservoir rocks are highly complicated and variable. There are many stratigraphic targets and sweet spots for lateral drilling around the basin. Variables such as thermal maturity and facies distribution are primary controls on the distribution of the overall play. Natural fracturing of the reservoir is also key to success, and ranges from microfracturing, diagenetically-enhanced fracturing, hydraulic fracturing due to hydrocarbon generation, and tectonic fracturing of brittle rock types. Facies controlled lithologies and subsequent diagenesis also play a role in reservoir quality. Finally, reservoir pressure and water saturation play a role in the ultimate recoveries. Understandably, these variables yield a wide range of reservoir targets and production characteristics around the Basin. Case studies from several of these areas will be presented. The Bakken system at Elm Coulee, Parshall Field, and the Nesson anticline will be presented, showing how each of these areas varies in terms of reservoir specifics and recoveries.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.