The Mountain Geologist

• Year: 2004
• Title: The Mountain Geologist
• Author: Rocky Mountain Association of Geologists
• Description: October 2004, Volume 41, Number 4
• Publications - Doc Type: Other

VERTICAL HYDRAULIC CONDUCTIVITYMEASURE.MENTS IN THE DENVER BASIN, COLORADO
those shale layers can exceed 2000 ft can be impractical.
Measurements can also be made by installing piezometers in
individual permeable layers and measuring head differences
across the separating confining layers while stressing one of
the layers; however, this can be costly.
From a practical perspective, vertical hydraulic conduc-
tivity of the confining shale layers is currently best mea-
sured by performing hydraulic tests on samples of the
shale layers. It is also important to collect the data where
the resource is being the most intensely exploited, i.e., the
center of the Basin. Samples from outcrops very likely will
not be representative of conditions within the basin due to
weathering effects and the absence of overburden pres-
sures. Therefore, core samples provide the best representa-
tive data; however core samples from within the Denver
Basin are rare.
To date, direct measurements from cores have been
published for only five locations (Fig. 1), three of which
are located in close proximity to each other near the deep-
est part of the Basin. The other two are at the north end of
the Basin near the South Platte River. The core hole pro-
jects and the hydraulic conductivity data collected from
fine- grained sediments encountered in the core holes are
summarized below.
Saint Vrain Core
In the late 1970s the USGS, in cooperation with the Col-
orado Department of Natural Resources, drilled a core hole
near Saint Wain, Weld County (Fig. 1) to evaluate the phys-
ical characteristics of the Laramie -Fox Hills aquifer (Major
et al., 1983). The core hole penetrated only the Laramie -
Fox Hills aquifer as the synorogenic sediments comprising
the upper Denver Basin aquifers are absent in this area.
Core samples were obtained to a depth of 890.5 ft for
geologic description, along with petrographic, hydraulic
conductivity, and porosity analyses. Sixteen core samples
were analyzed for both horizontal and vertical hydraulic
conductivity using air and water (Major et al., 1983); and
five of these were from shale intervals. Core Laboratories,
Inc. performed intrinsic permeability analyses on plug
samples taken from the cores and the results were con-
verted to hydraulic conductivity as listed in Table 3. The
results of analysis for vertical hydraulic conductivity
through the shale samples collected from the Laramie -Fox
Hills aquifer at this northern location in the Denver Basin
range between 8.0x10 -4 and 9.4x10 -8 cm /sec with a median
value of 7.7x10-5 cm /sec.
Castle Pines Core
In 1987 Castle Pines Metropolitan District and Castle
Pines North Metropolitan District, through their water
resources consultant Jehn and Wood, Inc., and in coopera-
tion with the USGS, drilled two core holes near the town
of Sedalia, Douglas County (Robson and Banta, 1993).
Located just 7.5 miles from the western edge of the Denver
Basin (Fig. 1), these core holes are relatively proximal to
the source area of the synorogenic Denver Basin sediments.
Core hole C1 penetrated approximately 1,895 ft through
the Dawson and Denver aquifers and into the Arapahoe
aquifer where a core barrel was lost, forcing abandonment
of the hole. A second core hole, C1A, approximately 28 ft
away from C1, continued coring through the Laramie -Fox
Hills to a depth of 3110 ft. Data collected from the core
included lithologic descriptions of about 2400 ft of core
and laboratory analysis of mineralogy, grain size, bulk and
grain density, porosity, specific yield, and specific retention
for selected core samples (Robson and Banta, 1993). From
the recovered core, 33 individual samples were selected
for permeability analysis. Many of the samples analyzed for
permeability were collected of coarser grained- sediments
in the aquifers, however, a number were reportedly col-
lected from finer- grained intervals described as consisting
of mudstone.
Permeability of the samples was measured using gas as
the saturating medium. A specific method is not referenced
in Robson and Banta (1993); however the source of the
data is a written communication from Anthony Garcia at
the Porous Media Laboratory at Colorado State University.
Table 3 lists the results as hydraulic conductivity in cm /sec
from 15 of the finer- grained or more poorly sorted litholo-
gies. The results of analysis for vertical hydraulic conduc-
tivity through the finer -grain and poorly sorted lithologies
collected from the Denver Basin aquifers at this western
location in the Denver Basin range between 7.3x10 -4 and
5.1x10-6 cm /sec with a median value of 8.6x10-5 cm/sec.
Kiowa Core
In 1999 the Kiowa core was obtained from a site within
the town of Kiowa, Elbert County (Fig. 1), as a component
of the Denver Basin Project, which is a cooperative and
multidisciplinary research effort by the Denver Museum of
Nature & Science to study the evolution of the Denver Basin
(Raynolds et al., 2001). The site, approximately 26 miles
from the western edge of the Denver Basin, was selected to
be farther away than the Castle Pines core location.
The core hole penetrated 2256 ft through the Dawson,
Denver, Arapahoe, and Laramie -Fox Hills aquifers and was
terminated in the Pierre Shale. Approximately 93% of the
rock penetrated was recovered. Core samples were han-
dled and stored to minimize dehydration and physical dis-
turbance ( Lapey, 2001). Fifty -five samples were collected
from the core for laboratory measurement of hydraulic
conductivity (Raynolds et al., 2001; Lapey, 2001) and
177 The Rocky Mountain Association of Geologists