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Dolomitization of the Middle Bakken Member at Elm Coulee Field, Williston Basin


The Elm Coulee field in the Bakken Petroleum System is one of the most productive fields in the Williston Basin. The Middle Bakken Member at Elm Coulee has higher dolomite content than other fields, and this has been associated with higher TOC. This also means that Elm Coulee features a different reservoir rock than other fields: specifically, the “B facies” in the Middle Bakken Member. The main purpose of this study was to address dolomitization in this field. Characterizing the diagenetic history of the dolomite present afforded a better understanding of the secondary porosity and permeability, both of which control reservoir rock properties at Elm Coulee. This work has provided an analog for similar unconventional reservoirs. Petrographic work, XRD and XRF analysis, and stable isotope data were used to constrain major diagenetic events and characterize the Elm Coulee dolomite as the product of one or more dolomite models. Planar-s to planar-e textural fabrics were observed in thin sections taken from six different cores in the field. These textures have previously been associated with both the seepage reflux and mixing zone models of dolomitization. Isotope data suggested that diagenesis did not take place in a mixing zone environment. The results of this work show that that the dolomite in the Middle Bakken Member is the result of a seepage-reflux process. The reflux model is one in which the dolomitizing solution is an evaporative brine with a high Mg/Ca ratio due to the removal of Ca through aragonite, gypsum, or anhydrite precipitation. Model interpretation was based on low fossil content, low bioturbation index, chemostratigraphic data, and stable isotope analysis.