Hydrostratigraphy and distribution of secondary permeability in the Brule Formation, Cheyenne County, Nebraska

Abstract

This study defines the hydrostratigraphy of a portion of the upper Orella Member (and perhaps lowermost Whitney Member) of the Brule Formation, an Oligocene siltstone that is used as an aquifer where wells intercept zones of secondary permeability. Drilling experience, lithologic logs of core, and borehole geophysical logs indicate that the portion of the Brule Formation which we studied (1) consists of siltstone with thin claystone and sandstone layers, (2) includes numerous paleosols, (3) includes distributed secondary permeability features such as pedotubules and parting planes, and (4) includes a subhorizontal zone (average thickness, 2.3 m) of intensely fractured siltstone that can be traced as a hydrostratigraphic unit across the western portion of the study site. Horizontal and vertical hydraulic conductivity (K r , K z ) measurements on core samples which do not exhibit secondary permeability features generally are similar and fall within expected ranges. Measurements of horizontal hydraulic conductivity from single-well injection tests in relatively unfractured intervals within the Brule Formation have a narrow range. Log mean K r of predominantly silt-stone intervals from single-well tests (K r ∼10 −6 m/s) is an order of magnitude greater than log mean K r measurements on core silt-stones (K r ∼10 −7 m/s); this difference is attributed to distributed secondary permeability features noted in core. The subhorizontal zone of fractured siltstone is interpreted to be a highly conductive hydrostratigraphic unit within the relatively impermeable Brule Formation.