Effects of depositional environments and thermal maturity on source
and reservoir characteristics –
A case study of Upper and Lower Bakken “shale” cores, Williston Basin, North Dakota
Laura L. Wray
The upper Devonian lower Bakken “shale” and lowermost Mississippian (Kinderhookian) upper Bakken “shale” members of the Bakken Formation are major hydrocarbon source rocks in the Williston Basin of North Dakota and Montana. The term “mudstone” is a better descriptor than “shale” because these source rocks are not clay-rich, laminated, fissile, detrital sedimentary rocks. Rather, they are better described in this study as massive, dark-brown to black, fine-grained, organic-rich, silty, fossiliferous mudstones. Data from six cores analyzed along a 52-mile east-west transect in McKenzie and Mountrail County, ND, provide a rich suite of information for regional comparisons of stratigraphy, reservoir characteristics, depositional environments, and thermal maturity.
Overall thicknesses in both intervals are variable across the transect. Visual inspections of these mudstones fail to show many subtle compositional and textural components. Both the upper and lower Bakken contain detrital silt that is disseminated throughout, seen only in thin sections and SEM images, as well as occasional thin, locally discontinuous silt laminae that are visible in the slabbed cores. These eolian silt grains, along with a variety of pelagic, biogenetic quartz fossil fragments and fecal pellets, settled through a stratified water column and accumulated on a mostly anoxic seafloor.
Mineral assemblages identified in thin sections and by SEM images are surprisingly diverse. X-ray diffraction results show that the major components that are relatively constant along the transect include: detrital, biogenic, and authigenic quartz (30-50% for upper Bakken, 30-45% for lower Bakken); predominantly Type II kerogen (10-20 wt. % for upper, 11-18 wt. % for lower); illite/mica (15-25% for upper, 13-29% for lower); and K-feldspar (4-8% for upper, 5-11% for lower). The constituents that vary the most include: detrital and authigenic dolomite (2-13% for upper, 2-11% for lower); eastward-increasing mixed-layer illite/smectite (1-17% for upper, 3-15% for lower); authigenic pyrite (3-14% for upper, 3-9% for lower); and calcite composed mostly of skeletal fragments (1-7% for upper, 0-7% for lower).
Present-day as well as estimated original TOC values highlight the well-documented westward increase in thermal maturity parameters for both source rocks with a few interesting exceptions close to the Nesson Anticline. Corresponding organic pore development appears to be related in increasing thermal maturity.
Chemostratigraphic interpretation of key elements from hand-held X-ray fluorescence demonstrates trends within both mudstones that characterize depositional and diagenetic variations, allowing several sequences to be defined that otherwise would not be viewed visually or with other analyses. These sequences can be related directly to sea level changes and accompanying water column chemistry, upwelling, and detrital input.