--> X-Ray Fluorescence and Rock Hardness Relationships Within the Devonian-Mississippian Bakken and Three Forks Formations, Williston Basin, North Dakota

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X-Ray Fluorescence and Rock Hardness Relationships Within the Devonian-Mississippian Bakken and Three Forks Formations, Williston Basin, North Dakota

Abstract

Detailed investigation of the chemical nature and the hardness of the Devonian-Mississippian Bakken Formation as well as the underlying Devonian Three Forks Formation, Williston Basin of North Dakota, were undertaken to both evaluate the relationships of these rock properties within a stratigraphic and paleontological framework and to assess their utility in correlating intervals within these formations across a broad areal extent. Hand-held X-ray fluorescence (XRF) data and rock hardness analyses were acquired on the same intervals at a high density (generally every 3”) throughout the entire Bakken and into the Three Forks in two widely spaced wells (~30 miles apart) and these data were augmented with XRF data from a third well (~40 mi away). Chemofacies established from the XRF data were primarily defined using calcium, silicon and aluminum and secondarily by other major elements as well as the trace metals iron and molybdenum. A strong, negative relationship exists between rock hardness results and intervals enriched in aluminum and potassium (associated with clay minerals), resulting in particular hardness zones and chemofacies co-occurring at the same stratigraphic intervals. These intervals also appear to co-occur with distinct lithofacies and biofacies that facilitate the subdivision of the Bakken Formation and the upper Three Forks into at least 9 zones that can be correlated across this transect in the Williston Basin. Of the 9 zones, a total of 7 are apparent in the Bakken, with 2 each in the upper and lower shale members, and 3 in the middle member. The remaining 2 zones were identified in the upper part of the Three Forks. Some facies show minor or significant lateral variations in chemical, faunal, hardness, or sedimentological character whereas others appear continuous across the studied transect. The 9 stratigraphic intervals can be confidently distinguished in the Bakken and the Three Forks by combining different facies/chemofacies characteristics. Because many of these facies also have identifiable well log signatures, the integration of facies evaluations from Bakken/Three Forks cores with XRF and rock hardness data may improve interpretations of log data from locations where core is not readily available, especially as it pertains to predicting rock properties.