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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Reservoir Characterization and Petrology of the Bakken Formation, Elm Coulee Field, Richland County, MT

Chloe S. Previous HitAlexandreTop1; Stephen Sonnenberg1; J Frederick Sarg1

(1) Geology and Geological Engineering, Colorado School of Mines, Golden, CO.

Elm Coulee Field, discovered in 2000 in Richland County, Montana, is the largest oil field in the Williston Basin. This field produces from the Devonian-Mississippian middle Bakken formation and has an estimated ultimate recovery of 200 million barrels of oil. Horizontal drilling and fracing lead to quick recoveries from this field due to the low permeability and porosity of the producing formation.

Characterization of the distribution of porosity, permeability, fractures, and TOC within these rocks lends insight into the exact reasons for the success of this field. These results can then be applied to the search for similar fields in this basin that may be good targets for future production.

Stratigraphic descriptions and depositional interpretations of four cores from vertical wells across the Elm Coulee field constrain the facies present in this field. Descriptions of thin sections from the cores include mineralogy, grain size measurements, porosity estimates, and characterization of diagenetic changes. Finally, source rock analysis gives insight into the maturation characteristics of this field.

The Bakken Formation in Elm Coulee Field is composed of three main members with an average total thickness of 40 feet. The lower Bakken is hard, black, marine shale. This member thins and becomes silty in the western portion of the field. The middle Bakken member contains five main facies ranging from a bioturbated, brachiopod-rich dolostone to a laminated silty-dolostone. The upper Bakken member has a composition similar to the lower shale. The upper and lower Bakken shales are the source beds for the oil found within the middle Bakken reservoir.

Based on the results of this study in conjunction with previous analyses of the Elm Coulee Field, the Bakken system in the Williston Basin has huge potential for future discoveries. Combined with an understanding of the facies distribution within the middle Bakken reservoir member, this data is key to determining future drilling locations.