Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Middle Bakken Facies, Williston Basin, USA: A Key to Prolific Production
(1) Colorado School of Mines, Golden, CO.
The middle Bakken Formation of the Williston Basin is the focus of current horizontal drilling activity. The middle Bakken consists of six distinct facies over much of the basin. Each facies represents deposition in shallow to deep shelf environments (neritic). Facies are subdivided on the basis of lithologic and biologic aspects. Contacts between individual facies are generally transitional. Individual facies pinch out towards the basin margins.
The middle Bakken has poor reservoir qualities with low porosities (< 8%) and permeabilities (< 0.1 md). Diagenesis plays an important role in controlling reservoir quality. Horizontal drilling and multi-stage fracture stimulation are necessary for commercial production.
For mapping purposes the six facies are combined into three geophysical log facies. Thickness variations in these facies are one of the keys to “sweet spot” identification in the Bakken. Thickness variations are influenced by paleo-basin configuration, paleostructures located within the basin, and deeper evaporite dissolution (Prairie) that was concurrent with Bakken deposition.
The mineralogy of the middle Bakken ranges from a silty to very fine grained dolomitic sandstone to a silty dolostone. The source area for clastics appears to be the Canadian Shield area to the north. Overall carbonate content increases in the southern part of the Williston Basin. Ichnofacies within the middle Bakken range from skolithos to zoophycos.
Knowledge of middle Bakken facies distributions and thickness variations, lithologic variations, diagenesis and paleostructure identification along with upper and lower Bakken shale thickness (source beds), quality, and maturity are keys to finding future “sweet spot” areas in the Williston Basin.