Abstract

The Upper Devonian to Lower Mississippian middle Bakken Formation in the Williston Basin has become a prolific unconventional reservoir in the past ten years. The Bakken petroleum system is likely to become the largest oil field discovered in the United States in the last 40 years, and it is rapidly approaching production rates that rival Alaska’s North Slope. While it is commonly held that the Bakken is a shale play, it is a source rock/reservoir couplet system. The petroleum system consists of two world-class source rocks, the upper and lower Bakken shales, that sandwich and source the middle Bakken reservoir. In addition, the Bakken shales are in immediate contact with and locally source potential reservoir units below (Devonian Three Forks Formation), and above (Mississippian Lodgepole Formation). In places, the shales themselves also act as reservoirs. The close juxtaposition of these rich source rocks with low-porosity, low-permeability reservoirs on a regional-scale creates the huge unconventional resource play that is the Bakken petroleum system.

Within the Bakken play, many local variations in reservoir facies, stratigraphy, fracturing, and lithology are present that influence reservoir quality, oil saturation, and well productivity. One example of this variability is illustrated by the enhanced reservoir quality and prolific production found at Parshall Field. Three main lithofacies exist at Parshall Field: the lower middle Bakken bioturbated facies, the middle Bakken high energy “shoal facies, and the upper Bakken lagoonal facies. It is the distribution of these facies and local diagenesis that creates the unique reservoir quality found at the field. Enhanced reservoir quality and the position of the field with respect to the up-dip thermal maturity barrier are the main drivers in making the Parshall area the most prolific Bakken play type seen to date in the Williston Basin.