2014 Rocky Mountain Section AAPG Annual Meeting

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


Data from six lowermost Mississippian (Kinderhookian) upper Bakken “shale“ cores along a 52-mile-long transect in the Williston Basin in North Dakota provide a rich suite of stratigraphic and analytical information for detailed comparisons. Thickness of this silty, organic-rich mudrock varies from 14 to 21 feet with no consistent regional trend evident along the transect. A visual display alone of three slabbed cores reveals some color variations but fails to show the subtle compositional and textural components of this interval. Analytical results from all six cores, however, serve to characterize and contrast these world class source rocks as they change from peak oil generation to thermally immature. Detrital silt, disseminated throughout the interval, 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 the stratified water column and accumulated on a mostly anoxic seafloor, although there were some minor periods of dysoxia indicated by burrowing. Mineral assemblages identified in thin sections and by SEM EDS are surprisingly diverse. X-ray diffraction results show that the major components that are relatively constant along the transect include quartz (30–50%, detrital, biogenic, and authigenic), kerogen (10–20 wt. %, mostly Type II), illite/mica (15–25%), and K-feldspar (4–8%). The constituents that vary the most include dolomite (2–13%, both detrital and authigenic), albite (2–9%), mixed-layer illite/smectite (1–17%, increasing eastward), pyrite (3–14%, all authigenic), and calcite (1–7%, mostly as skeletal fragments). Present-day as well as estimated original TOC values highlight the well-known westward increase in thermal maturity of the Upper Bakken from east to west with a few interesting exceptions close to the Nesson anticline. Corresponding organic pore development is also related to increasing thermal maturity. Elemental data from hand-held X-ray fluorescence show considerable vertical and lateral variability. The resulting chemostratigraphic interpretation of key elements has defined recognizable, fairly geographically continuous sequences that offer new insights into depositional processes and environments.