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Bakken Oil Accumulations - What's the Trap?

Coskey, Robert J.1; Leonard, Jay E.2
1 Rose Exploration, Inc., Denver, CO.
2 Platte River Associates, Inc., Boulder, CO.

A major Bakken hydrocarbon accumulation has been discovered in west-central North Dakota, near the town of Parshall. At end of 2008, the productive area extended over 115 miles north-south and 15-20 miles east-west.

This productive fairway is located in a thermal transition zone between immature Bakken to the east and highly mature Bakken to the west. In this zone, the physical properties of the Bakken shales change rapidly. Due to its high original organic carbon content, immature Bakken is in-part kerogen supported. As kerogen matures in this zone it becomes pliable, loses strength and begins to convert to liquid hydrocarbons. A volumetric increase associated with this conversion causes locally elevated pressure cells to develop. The increased pressure induces the formation of microfractures. Oil is expelled from the thermally weakened kerogen through microfractures and allows the shale to collapse. The measurable result of this process is the increase of shale bulk density through the maturity window.

Burial history modeling suggests that this process began in the central part of the Williston basin approximately 50 Ma, spread laterally in a ‘wave-like’ manner, and reached the current Bakken play area at approximately 5 Ma.

The trap mechanism is likely due to the physical characteristics of the Bakken and its position within both the thermal window and the thermal transition zone. Those areas that have recently entered the oil window are micro-fractured and retain overpressure, whereas those areas where the Bakken is above the oil window the system is pliable, normally pressured, unfractured and acts as a barrier to migration.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009