Structural, Stratigraphic, and Diagenetic Factors Influencing Hydrocarbon Accumulations in the Bakken Petroleum System at the Elm Coulee Field, Williston Basin, MT

Henriette V. Eidsnes and Ellen R. Fehrs
Colorado School of Mines

The Bakken Petroleum System at the Elm Coulee field is an upper Devonian to lower Mississippian system consisting of the upper Three Forks, Bakken, and lower Lodgepole formations. Six distinct facies have previously been identified in the middle Bakken: the A, B, C, D, E, and F facies. These facies have been re-examined in six representative cores from the Elm Coulee field. All facies were identified and described except for the C and D facies. The main purpose of this research is to identify factors influencing hydrocarbon accumulation at Elm Coulee field. A study focusing on the structural and stratigraphic components of the Bakken Formation and Prairie Salt Formation at Elm Coulee addresses the basement structural trends and their influence on the Bakken pay interval and possible salt dissolution in the Prairie Salt. The latter is significant in explaining whether the anomalous thickness of the middle Bakken member is due to salt dissolution during Bakken deposition. Salt dissolution could have been initiated by basement structural faults and fractures acting as migration pathways for liquids, which could have provided an inflow of subsurface water to the Prairie Formation causing collapse of overlying beds and compensation infill. 3-D seismic has been used to map basement faults, create isochron maps to identify thickness anomalies, and to identify the seismic characteristics of the Bakken Petroleum System. Further, Elm Coulee has higher dolomite content than other fields. Because of this Elm Coulee features a different reservoir rock than other fields: the 'B facies' in the Middle Bakken Member. Characterizing the diagenetic history of the dolomite present should afford a better understanding of the secondary porosity and permeability, both of which control the reservoir rock at Elm Coulee. Diagenesis has produced pore space that acts as microfractures, which has led to increased production. A better understanding of this diagenetic evolution would provide an analog for similar unconventional reservoirs. Petrographic work, XRD and XRF analysis, isotope data, and SEM work will be used to constrain major diagenetic events and characterize the Elm Coulee dolomite as the product of one or more dolomite models.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013