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Interpreting depositional and diagenetic trends in the Bakken Formation based on handheld x-ray fluorescence analysis, Mclean, Dunn and Mountrail counties, North Dakota
Kocman, Katie Beth
Kocman, Katie Beth
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2014
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2014
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2015-04-01
Abstract
The Bakken Formation in the Williston Basin is an economically viable hydrocarbon play that is getting vast attention throughout the world. It is an especially interesting formation because it contains both world-class hydrocarbon source rocks and billions of barrels of producible oil. New technologies are continuously being applied to better understand the Bakken Formation and to determine why it has become such a commercial reservoir. This study examines elemental data collected by a handheld X-ray fluorescence (XRF) device and integrates these elemental data with core descriptions, petrography and helically computed tomography (CT scans) of both the source and reservoir facies to provide new insights into the depositional environments, depositional history and sequence stratigraphy of the Bakken Formation. A handheld Thermo Fisher Scientific Niton X-Ray Fluorescence Analyzer (Niton XL3t GOLDD+) was employed to sample slabbed cores and drill cuttings. This is a cost-effective and efficient way to obtain semi-quantitative elemental analysis. Abundances of up to 40 elements can be simultaneously collected for elements ranging in atomic number from magnesium (12) through uranium (92). A sequence stratigraphic framework was developed for the Upper, Middle and Lower Bakken Members based on their elemental trends. The Lower Bakken Member, which is dominated by organic-rich black mudstones, has been subdivided into the lower Bakken "silt" and four units within the lower Bakken shale. The Upper Member, another interval dominated by organic-rich black mudstones, is subdivided into two units. The Upper and Lower Members show significant enrichment of molybdenum, uranium and vanadium trace metals. Based on elemental enrichments, high preserved TOCs, lack of benthic fauna, burrowing and bottom current evidence, it is concluded that the Bakken black shales were deposited under mostly euxinic conditions with a few, short periods of dysoxia (represented by rare, very thin intervals of bioturbation and ripples). The six facies of the Middle Bakken Member, labeled A through F in ascending sequence, each have distinct elemental signatures. These elemental trends help to better define the boundaries between the facies in core, where some of the contacts are gradational. Elemental trends aid in the correlation of these facies across the study area. Cores straddling the "line of death", which is the commercial limit of middle Bakken production on the eastern, up-dip edge of the basin, were analyzed in search of a mineralogic change across the boundary between commercial and non-commercial production. Significant elemental differences were not observed in cores across this line. An exception to this is that Facies E and F have higher magnesium values west of the line, which may imply more dolomitization. Elemental analysis of cuttings from a lateral that crossed the "line of death" was inconclusive in terms of revealing any clearly defined mineralogic trends. However, the geometries of the grains before (equant), at the onset of (splintery), and after (equant) the gas shows within this lateral imply that a fault may be present. This interpretation is supported by an aeromagnetic anomaly that is roughly coincident with the "line of death".
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