2014 Rocky Mountain Section AAPG Annual Meeting

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Visual and analytical comparisons of lower Bakken “shale” cores from a west-to-east transect, McKenzie and McLean Counties, North Dakota


The upper Devonian lower Bakken member of the Bakken Formation is classified as a silty, organic-rich mudrock, and is a major hydrocarbon source rock in the Williston Basin of North Dakota and Montana. Data collected from six cores along a 52-mile east-west transect provide a rich suite of stratigraphic and analytical information for regional comparisons. Six slabbed cores of the lower Bakken, as defined by the authors, reveal visual similarities and differences of this world-class source rock. Additionally, chemostratigraphic interpretation of key elements from handheld XRF data clearly demonstrates trends within the mudrock that help better characterize the depositional and diagenetic variations of the lower Bakken. The lower Bakken in this transect varies in thickness from 10 to 38 feet. Detrital silt is disseminated throughout the interval and occurs as scattered grains and thin, locally discontinuous laminae that are visible in the slabbed core. These eolian silt grains, along with a variety of pelagic fossil fragments and fecal pellets, settled through a stratified water column and accumulated on a mostly anoxic seafloor. At the top and base of the lower Bakken, thin, in-situ bioturbated intervals have been recognized by whole core CT scans documenting minor periods of dysoxic conditions. X-ray diffraction results show that components varying along the transect include quartz (30–45%, detritral, biogenic and authigenic), kerogen (11–18 wt. %, mostly type II), illite/mica (13–29%), K-feldspar (5–11%), dolomite (2–11%, both detrital and authigenic), albite (1–8%), mixed layer illite/smectite (3–15%, increasing eastward), pyrite (3–9%, all authigenic) and calcite (0–7%, mostly as skeletal fragments). The boundary between the base of the Lower Bakken “shale” and upper Pronghorn interval is defined by: 1) lower TOC, 2) lower silicon, 3) more bioturbation and 4) coarser quartz grains (vf sand) in the upper Pronghorn mudrock. Although this boundary is subtle in the core, it is easily recognized on wireline logs and in analytical analyses. Integration of analytical data from cores and wireline logs aids in characterizing and understanding the lower Bakken “shale.”