Bakken Shale Oil in the Eastern Williston Basin: “It's a Mystery Wrapped in a Riddle Inside from Enigma!”

Jay E. Leonard, Robert J. Coskey, and Veit J. Matt
Platte River Associates, Inc., Boulder, CO

The Bakken shale is a “world class” source rock that has been studied over the past several decades. Significant mud gas shows have been encountered during vertical drilling. Now with the application of horizontal drilling, the Bakken has become a major shale resource play.

In the North Dakota portion of the Williston basin, Bakken thermal maturity decreases eastward. In fact, very low transformation ratios are present just east of the Parshall field (Mountrail County, North Dakota (T53N-R90W)).

At the time of this abstract, seven wells in Parshall field produce over 115,000 barrels of oil per month. This is unexpected in such a low maturity play. Extensive geochemical analysis and modeling shows that the generation rate provides clues to a model that explains this anomaly.

In this portion of the basin, thermal maturity decreases from west to east. In those areas of rapidly increasing maturity, hydrocarbon generation rates increase and due to the high organic matter content, the thermo-mechanical properties of the Bakken shale change rapidly. The result is that very large volumes of oil are made available for local release. The production along the eastern flank of the Williston basin is within this transition zone.

Hydrocarbon migration generally plays little or no role in unconventional shale resource plays. This is not the case for the Eastern Williston basin although the expulsion and migration aspects are as unconventional as the play itself.

This model when applied through both space and time identifies “sweet spots” throughout the basin.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas