GRC Transactions, Vol. 39, 2015
Comparisons of Geothermal Systems in Central Nevada:
Evidence for Deep Regional Geothermal Potential
Based on Heat Flow, Geology, and Fluid Chemistry
Stefan Kirby1, Stuart Simmons2, Mark Gwynn1, Rick Allis1, and Joseph Moore2
1
Utah Geological Survey, Salt Lake City, UT
Energy and Geoscience Institute, University of Utah, Salt Lake City, UT
[email protected] • [email protected] • [email protected]
[email protected] • [email protected]
2
Keywords
Deep geothermal, Central Nevada, carbonates, heat flow, geochemistry
Abstract
Central Nevada may have significant untapped deep geothermal potential. Existing geologic and heat flow data
indicate the potential for carbonate aquifers at temperatures greater than 180°C at depths below 3 kilometers. Compiled
fluid chemistry from producing geothermal fields at Tuscarora and Beowawe, produced water from the Blackburn oil field,
and thermal springs and wells (with temperatures greater than 30°C) all have unique and remarkably consistent major
ion chemistry. Nearly all compiled samples are Na-HCO3 type water with generally low total dissolved solids and high
concentrations of dissolved silica. These chemistries may result from equilibrium in warm carbonate aquifers and ion
exchange and mineral precipitation as thermal fluids move toward the surface. Similarities in chemistry may result from
similar thermal reservoir conditions beneath large areas of central Nevada. Simple geothermometry yields the highest
temperatures for samples from the Beowawe geothermal field. Other areas with elevated geothermometry and Na-HCO3
waters include the Tuscarora geothermal field, the Mary’s River area, and Ruby Valley. Samples from the Blackburn
oilfield are similar to other thermal samples except for higher Na, Cl, and HCO3 which may result from active petroleum
generation in this system. Limited stable isotope data indicate a significant geothermal signal for several samples with
lower geothermometry. Similarities in heat flow and geology between central Nevada and Utah may allow for analogous
geothermal conditions to exist in adjoining parts of western Utah.
Introduction
Central Nevada hosts several developed geothermal systems (Beowawe, Tuscarora) (Pilkington et al., 1980; Watt
et al., 2007), localized oil accumulations possibly related to geothermal upflow (Pine Valley, Grant Canyon) (Hulen et al.,
1990; Goff et al., 1994), and a number of high temperature springs (White, 1992; Penfield et al., 2010). These relatively
shallow thermal systems exist in a region of high heat flow underlain by structurally complicated but largely contiguous
carbonate rocks at depth. The purpose of this paper is to explore the possibility that deep carbonate rocks beneath the
region may be a large geothermal resource that is presently largely untapped.
Eastern Nevada and western Utah overlie extensive Paleozoic carbonate units that facilitate interbasinal groundwater
flow beneath the region (Heilweil and Brooks, 2011; Masbruch et al., 2012). These same carbonate units may also be
geothermal reservoirs where temperature and depth are optimal (Allis et al., 2012). Mines et al. (2014) have shown that if
temperatures of 150 to 200°C can be found in the depth range of 3 to 4 km, then geothermal power with a levelized cost
of about 10 c/kWh is possible with a high permeability reservoir. Allis et al. (2012) and Allis and Moore (2014) suggest
that 100 MWe-scale power plants sourced from stratigraphic reservoirs are possible.
The potential for future geothermal development of deep carbonate reservoirs is significant throughout the eastern
Great Basin including areas of western Utah and Nevada. A comparison of existing heat flow, geologic, and geochemical
25
Kirby, et al.
data for select geothermal systems in central Nevada may provide insight to the deep geothermal potential of carbonates
both in this area and similar geologic settings including westren Utah. This paper reviews the general geologic, heat flow,
and fluid geochemistry of central Nevada and then examines data from several producing geothermal and oil fields in the
area to assess the viability of a deep regional geothermal resource in the area.
Geologic and Heat Flow Background
Geochemistry of Thermal Waters
To examine the characteristics of thermal
fluids across central Nevada, a dataset was
115°0'0"W
’s Riv
er Va
Tuscarora
M ar y
Geologic Unit
Unconsolidated sediments
Tertiary sediments and volcanic rocks
Tertiary volcanic rocks
Mesozoic intrusive rocks
Mesozoic sedimentary rocks
Paleozoic carbonate and other rocks
Metamorphosed Paleozoic rocks
Precambrian quartzite
lley
Explanation
Select temperature
gradient sites (see figure 2)
Federal Interstate
U.S. Highway
State Highway
Secondary Roads
N
41°0'0"N
41°0'0"N
Elko
Rub
yM
ts
Carlin
Ru
ey
Va
ll
Pin
e
rlw
in
dV
all
ey
by
Va
ll
ey
Beowawe
W
hi
North-central Nevada has a complicated
geologic history resulting from an extended period
of marine deposition followed by significant
crustal shortening, volcanism, and more recent
extension, uplift and erosion (Coney, 1978; Speed,
1983; Dickinson, 2006) (figure 1). The major rock
types within the area include 1) Precambrian
quartzite and shale, 2) Paleozoic carbonate, shale
and siliciclastic rocks, 3) Mesozoic to Tertiary
igneous, metamorphic, and volcanic rocks, and
4) various younger alluvial and lacustrine basin
fill (Stewart and Carlson, 1978; Crafford, 2007).
Faulting and folding of these rock units is locally
significant and the youngest extensional faults
are important conduits for fluid movement. The
north-south-trending Carlin mineral belt extends
along the western part of the area.
The Paleozoic limestone and dolomite
rocks are important regional aquifers (and
geothermal or petroleum reservoirs) (Hulen
et al., 1990; Goff et al., 1994; Heilwiel and
Brooks, 2011; Masbruch et al., 2012). These
carbonates are widely exposured in the mountain
ranges and lie in the subsurface across most of
the area. Carbonate rocks near the Beowawe
and Tuscarora geothermal fields are covered
by younger Tertiary volcanic rocks. Tertiary
volcanic rocks and sediments cover much of
the northern part of the area.
Central Nevada is an area of high
heat flow, with background heat flux greater
than 85 (mW/m2) and localized areas with
significantly higher heat flux (Blackwell et
al., 1991; Blackwell et al., 2011). A recent
examination of deep temperature profiles from
petroleum boreholes by Gwynn et al. (2014)
showed gradients across the study area are
broadly consistent with temperatures greater
than 180°C at depths greater than 3 km (figure
2). This depth and temperature range likely
represents the approximate reservoir conditions
for current geothermal up flow encountered at
thermal springs and the geothermal power plants
at Beowawe and Tuscarora.
116°0'0"W
0
116°0'0"W
5
10
20
Miles
115°0'0"W
Figure 1. Simplified geologic map of central Nevada modified from Crafford (2007).
Selected temperature gradient sites correspond with a subset of those in figure 2.
Figure 2. Summary of temperature gradient information for central Nevada modified
from Gwynn et al. (2014). Temperature gradients for the Tuscarora and Beowawe
geothermal systems are shown as dashed red lines. This figure is based on oil well
bottom-hole temperatures that generally underestimate in-situ temperatures because
of the effects of drilling disturbance. SV 74-23 is a geothermal well in North Steptoe
Valley, logged 90 days after drilling, that is a good example of a geotherm in thermal
equilibrium. The depth and temperature of prospective stratigraphic reservoirs is
shown as the red shaded zone.
26
Kirby, et al.
compiled, for samples with temperatures greater than 30° C, that includes (1) samples of geothermal production fluids from
Beowawe and Tuscarora, (2) thermal produced water from the Blackburn oil field, and (3) select thermal springs and wells
in the study area. All samples include the major ions of Na, K, Mg, Ca, Cl, SO4, and HCO3, as well as dissolved silica,
temperature, and pH for each sample (table 1). Other constituents include B, F, Li and the stable isotopes of deuterium
and oxygen-18. These data were used to calculate a simple, major ion water type for each sample (Kehew, 2000), a Na-K
geothermometer (Giggenbach, 1988), and a quartz-silica geothermometer (Fournier, 1991). Total dissolved solids (TDS)
was calculated as the sum of dissolved constituents for each sample.
Table 1. Select geochemical samples from central Nevada. Latitude and longitude are in GCS NAD 83. All concentrations are mg/Kg. ND indicates no data. Geothermometers are in degrees °C, Na/K is the Giggenbach (1988) geothermometer and Quartz is the quartz silica adiabatic geothermometer of Fournier (1991).
Name
Data Source
Latitude
Longitude
Temp
(°C)
pH
TDS
(mg/L)
Water
Type
Na
K
Beowawe 85-18
Beowawe Frying Pan Geyser
Beowawe Ginn 1-13
Beowawe Rossi 21-19
Tuscarora 7A
Tuscarora 7C
Tuscarora 8a
Tuscarora 8B
Tuscarora DH 66-5
Blackburn #3 Well
Bruffrey’s Hot Springs
BW2
BW3
Cress Ranch Hot Springs
Elko Heat Company Well
Ellison Ranch Spring
GQDW-16
Hot Spring, Devils Punch Bowl
Hot Spring, Hot Springs Point
Hot Springs Creek north of Deeth, NV
Sulfur Hot Springs at Elko Day Care Center
Sulphur Hot Springs
Unnamed Spring NE of Carlin
Unnamed Springs Near Carlin
Well Dee 3
Cove Fort 91-4
Roosevelt 14-2
Cole and Ravinsky, 1984
White, 1992
Cole and Ravinsky, 1984
Cole and Ravinsky, 1984
Bowman and Cole, 1982
Bowman and Cole, 1982
Bowman and Cole, 1982
Bowman and Cole, 1982
Bowman and Cole, 1982
Goff and others, 1995
Penfield et al., 2010
Penfield et al., 2010
Penfield et al., 2010
Penfield et al., 2010
Penfield et al., 2010
Penfield et al., 2010
Penfield et al., 2010
Penfield et al., 2010
Clark et al., 2010
Penfield et al., 2010
Penfield et al., 2010
Clark et al., 2010
Penfield et al., 2010
Penfield et al., 2010
Penfield et al., 2010
Moore et al., 2000
Capuano and Cole, 1982
40.55824
40.56667
40.55833
40.55833
41.46530
41.46530
41.46530
41.46530
41.46530
40.17085
40.22005
40.98297
40.97963
41.19492
40.82492
41.46667
40.79964
41.26150
40.40659
41.25992
40.81325
40.58667
40.76409
40.69712
41.03491
38.56030
38.49357
-116.59759
-116.56667
-116.59667
-116.58333
-116.15100
-116.15100
-116.15100
-116.15100
-116.15100
-116.17121
-116.06850
-116.37480
-116.38730
-115.28590
-115.77590
-116.15330
-116.19920
-115.30710
-116.51760
-115.31090
-115.77920
-115.28470
-116.04170
-116.13420
-116.43430
-112.58125
-112.84134
160.0
98.0
211.0
198.0
89.0
56.0
73.0
95.0
110.0
91.3
57.2
51.5
49.0
30.0
80.0
93.0
30.5
50.0
50.0
51.0
72.0
93.0
64.0
79.0
45.0
163.0
265.0
9.1
9.0
8.4
8.1
6.9
6.9
7.6
7.4
8.4
8.2
6.9
6.2
6.4
7.7
7.1
8.1
6.8
6.0
6.5
6.5
7.1
8.5
6.9
7.6
6.5
6.0
6.2
1147
1166
956
818
739
871
744
689
792
1978
514
893
874
1516
867
785
544
1161
712
1285
874
680
1056
605
861
3939
7534
Na-HCO3
Na-HCO3
Na-HCO3
Na-HCO3
Na-HCO3
Na-HCO3
Na-HCO3
Na-HCO3
Na-HCO3
Na-Cl
Ca-HCO3
Ca-HCO3
Ca-HCO3
Na-HCO3
Na-HCO3
Na-HCO3
Ca-HCO3
Na-HCO3
Na-HCO3
Na-HCO3
Na-HCO3
Na-HCO3
Na-HCO3
Ca-HCO3
Ca-HCO3
Na-Cl
Na-Cl
277.0
230.0
203.0
143.0
151.0
169.0
145.0
148.0
163.0
558.0
40.0
80.0
81.0
358.0
110.0
157.0
35.3
231.0
136.0
230.0
110.0
135.0
231.0
45.0
77.0
1143.0
2200.0
35.0
16.0
30.0
14.0
15.0
11.0
19.0
20.0
25.0
42.0
7.3
23.0
23.0
33.0
35.0
16.0
10.8
44.6
17.0
40.0
36.0
8.9
27.0
16.0
22.0
220.0
410.0
Name
Ca
Beowawe 85-18
2.5
Beowawe Frying Pan Geyser
1.0
Beowawe Ginn 1-13
11.0
Beowawe Rossi 21-19
24.0
Tuscarora 7A
10.0
Tuscarora 7C
19.0
Tuscarora 8a
17.0
Tuscarora 8B
1.0
Tuscarora DH 66-5
14.0
Blackburn #3 Well
25.1
Bruffrey’s Hot Springs
50.0
BW2
96.0
BW3
94.0
Cress Ranch Hot Springs
6.5
Elko Heat Company Well
63.0
Ellison Ranch Spring
13.0
GQDW-16
59.0
Hot Spring, Devils Punch Bowl
33.3
Hot Spring, Hot Springs Point
22.0
Hot Springs Creek north of Deeth, NV
41.0
Sulfur Hot Springs at Elko Day Care Center 66.0
Sulphur Hot Springs
1.0
Unnamed Spring NE of Carlin
15.0
Unnamed Springs Near Carlin
60.0
Well Dee 3
100.0
Cove Fort 91-4
96.0
Roosevelt 14-2
6.9
Mg
Cl
SO4
0.3
31.0 76.0
0.1
69.0 130.0
0.3
59.0 47.0
7.1
25.0 28.0
0.5
18.0 52.0
3.0
19.0 34.0
2.0
16.0 50.0
0.5
6.0 55.0
2.0
26.0 47.0
9.3 423.0 229.0
19.0
14.0 41.0
22.0
14.0 67.0
23.0
16.0 73.0
0.8
25.0
2.0
12.3
15.0 70.0
0.2
9.0 75.0
20.0
12.8 57.4
14.4
31.4 43.2
5.8
27.0 62.0
14.0
21.0 11.0
14.0
15.0 70.0
0.0
23.0 40.0
5.9
10.0 25.0
15.0
12.0 52.0
25.0
14.0 62.0
9.0 1691.0 393.0
0.1 3650.0 60.0
HCO3
SiO2
B
267.0 436.0 2.00
383.0 320.0
ND
260.0 335.0 1.70
145.0 427.0 0.90
352.0 129.0 0.80
484.0 122.0 0.90
382.0 103.0 0.90
345.0 104.0 0.90
397.0 109.0 0.80
559.0 122.0 6.70
282.0
60.0 0.29
548.0
41.0
ND
525.0
37.0
ND
959.0 132.0
ND
493.0
65.0 0.90
338.0 166.0 0.81
322.0
25.4 0.25
718.0
43.2 1.49
378.0
58.0 0.81
888.0
36.0 1.20
498.0
63.0
ND
244.0 210.0 0.20
690.0
52.0
ND
335.0
70.0
ND
537.0
23.0
ND
201.0 165.0 10.00
170.0 1002.0 28.00
27
F
Li
Balance
δ2H
(‰)
18.00
17.00
7.90
2.80
11.00
8.90
8.70
8.20
8.00
3.05
0.60
1.20
1.30
ND
2.00
9.40
0.70
0.70
5.00
1.90
1.90
17.70
ND
ND
1.00
6.00
4.80
1.90
ND
1.40
0.90
ND
ND
ND
ND
ND
1.14
ND
0.35
0.35
ND
0.30
0.60
0.19
0.09
ND
0.80
ND
0.46
ND
ND
0.29
5.00
2.30
25.4%
-6.1%
16.3%
37.2%
-3.4%
-4.9%
-3.0%
-3.1%
-0.7%
1.7%
0.5%
-0.5%
0.5%
1.0%
-1.3%
0.5%
-3.5%
1.2%
-4.0%
-4.1%
0.0%
-1.6%
-0.6%
-2.3%
2.2%
1.0%
-1.4%
ND
-130.0
ND
ND
-137.0
-133.0
-128.0
-137.0
ND
-132.4
ND
-130.0
-133.0
ND
ND
-135.0
ND
ND
-132.0
ND
-145.0
-130.0
ND
-133.0
ND
ND
ND
δ18O
(‰)
ND
-14.8
ND
ND
-16.1
-16.6
-16.1
-14.0
ND
-15.2
ND
-16.5
-16.2
ND
ND
-16.8
ND
ND
-15.8
ND
-15.3
-16.1
ND
-16.4
ND
ND
ND
T-Na/K
TQuartz
252
205
266
231
232
200
255
258
269
211
285
333
332
226
345
234
341
291
251
281
349
201
245
359
333
291
287
216
196
199
215
145
142
134
135
137
142
111
93
89
154
115
168
73
96
109
88
113
184
104
118
69
158
280
Kirby, et al.
g
+M
Cl
Ca
<=
+S
O4
=>
We assume that many of these samples (particularly spring samples) represent geochemical conditions that are in partial
equilibrium or are mixes of warm and cool fractions, and geothermomtery is not necessarily indicative of actual reservoir
conditions. Nonetheless, geothermometers, especially the quartz of Fournier (1991), are useful to subdivide the dataset into
general thermal categories (figure 3). Quartz geothermometers range from 69 to 216°C; the warmest temperatures, greater than
200°C, are calculated for samples at the Beowawe geothermal field. Na-K geothermometers range from 200 to over 300°C;
these temperatures may be much less indicative of actual reservoir conditions in this area because of the potential effects of ion
exchange and mineral precipitation and dissolution in these
Explanation
settings. Na-K temperatures often overestimate reservoir
80
80
T (°C) Quartz (Fournier, 1991)
temperatures; they are included here to show possible end>155
60
60
member reservoir temperatures.
155-135
135-105
The relative major ion chemistry of the thermal
40
40
<105
samples is quite consistent across all samples in table 1
BW Beowawe samples
TR Tuscarora samples
20
20
(figure 3). Nearly all of the compiled samples are of NaHCO3 water type with a smaller number of Ca-HCO3 and
Mg
SO4
Na-Cl type waters. Much of the variation in chemistry
among these samples is driven by Na, Ca, and HCO3
80
80
concentrations. Concentrations of Cl and SO4 are generally
low and account for a smaller degree in the chemical
60
60
variance among the samples. The sample of produced water
40
40
from the Blackburn oil field is a Na-Cl type water and differs
from the other samples primarily due to higher concentration
20
20
of Na, Cl, and HCO3. Samples with thermometer values
greater than 155°C, including samples from Tuscarora,
Ca
Cl
Beowawe, and Sulphur Hot Springs in Ruby Valley, share
Na+K HCO3+CO3
Figure 3. Piper plot of compiled geochemical samples. Samples are
a similar Na-HCO3 major ion chemistry.
A map plot of stiff diagrams shows the spatial colored based on the Quartz geothermometer of Fournier (1991).
Samples with geothermometer values greater than 155°C share a very
variation in chemistry for the compiled samples (figure 4). similar major ion chemistry. All samples have a very different chemical
Most samples show a characteristic major ion chemistry that signature compared to examples of Na-Cl dominated geothermal water
is high in Na and HCO3 relative to other ions. Small variation from Cove Fort and Roosevelt in Utah.
in chemistry is apparent and different areas appear to have
minor variance in chemistry driven primarily by differences
Explanation
in Ca, Na, and HCO3. The Blackburn oil field sample has
higher HCO3 and much higher Na and Cl than the other
thermal samples. Samples from the Tuscarora geothermal
Tuscarora
field have higher HCO3 than the Beowawe samples, but
are otherwise very similar to Beowawe samples. Samples
in the Mary’s River area contain greater concentrations of
all ions but share a similar Na-HCO3 profile to the other
Elko
samples. Hot Sulphur Springs in Ruby Valley is very similar
to other samples with geothermometry greater than 155°C
Beowawe
at Tuscarora and Beowawe.
Carlin
Plots of TDS versus the major ions of HCO3 and Na
show trends in geochemistry among samples with otherwise
similar major ion chemistry (figure 5). Samples from
Beowawe and Tuscarora define overlapping zones based
on Na and TDS, while the samples from these two areas
are more distinct based on HCO3 concentrations. Nearly all
Blackburn
samples are dilute with TDS less than 1200 mg/L. A plot
of HCO3 versus TDS show a possible linear trend between Figure 4. Map of stiff diagrams of compiled chemistry. Color indicates
samples from the Tuscarora area and the Mary’s River the Quartz geothermometry (see table 1) of the samples; red is greater
area. A second HCO3 mixing trend, which includes many than 155°C, orange is between 155 and 135°C, yellow is between 135
of the hottest samples, may exist between the Ruby Valley and 105°C, and green is less than 105°C.
sample and the Blackburn field produced water. Lower
HCO3 concentrations along this second mixing trend may be driven by the inverse solubility of carbonates with increased
temperature in the high-temperature samples. The plot of Na versus TDS shows a linear mixing trend between high TDS
samples of Blackburn field produced water and Cress Ranch Springs in Mary’s River Valley, and lower TDS samples.
Cove Fort
Roosevelt
Blackburn
Ellison Ranch
Spring (TR)
BW
BW
Sulphur Hot Springs
80
60
20
20
40
60
80
40
TR
116°0'0"W
Na
115°0'0"W
’s Riv
er Va
HCO3
SO4
lley
Cl
Ca
Mg
25 meq/L 0 25 meq/L
Mary
Federal Interstate
U.S. Highway
State Highway
Secondary Roads
41°0'0"N
ey
Va
ll
Pin
e
W
hi
rlw
in
dV
all
ey
Ru
by
Va
ll
ey
Rub
yM
ts
41°0'0"N
0
116°0'0"W
28
5
10
20
Miles
115°0'0"W
Kirby, et al.
HCO3 (mg/kg)
A subset of the compiled geochemical samples contains stable isotope data (table 1). Stable isotopes of deuterium
(δ2H) and oxygen (δ18O) can provide information about both the source of recharge and the degree of high-temperature
water-rock interaction of geothermal fluids (Clark and Fritz, 1997). Nearly half of the samples with stable isotope data
plot near (< 1‰ shift in δ18O) the meteoric water line of Craig (1961) (figure 6). Thermal samples near the meteoric
water line represent fluids recharged by precipitation and or surface water and show little isotopic effect of residence
in a geothermal reservoir. Other samples, including Beowawe, Blackburn, and Sulfur Hot Springs at Elko, are shifted
away from the meteoric water line. These samples show evidence for isotopic fractionation of 18O likely resulting from
water-rock interaction at high temperatures. The samples
1000
for Tuscarora 8B and Sulfur Hot Springs at Elko both
(a)
show significant 18O excess, this may imply geothermal
900
Cress Ranch Springs, Mary’s River
reservoir temperatures greater than that implied by Quartz
geothermometry. Samples from the Tuscarora area define a
800
zone of isotopic values that indicate variation in the degree
of geothermal isotopic fractionation for the Tuscarora
700
geothermal system.
600
-90
Blackburn
T (°C) Quartz-Silica (Fournier, 1991)
500
>155
TR 7C
TR 8a
400
155-135
TR DH 66-5
135-105
BW Frying Pan Geyser
TR 8B
<105
TR 7A
Ellison Ranch (TR)
300
BW 21-19
100
400
800
1200
TDS (mg/l)
1600
M
δ2H (‰)
200
W
-110
BW 85-18
BW Ginn 1-13
Sulphur Hot Springs Ruby Valley
2000
L
r
(C
1)
96
,1
g
ai
Ne
L
W
aM
d
va
Geothermal shift 18O excess
Tuscarora 8a
-130
Beowawe
Hot Springs Point
Explanation
Blackburn
Ellison Ranch Tuscarora
T (°C) Quartz (Fournier, 1991)
Tuscarora 7A
Tuscarora 8B
>155
155-135
Sulfur Hot Springs at Elko
135-105
-150
-18.0
<105
BW Beowawe samples
TR Tuscarora samples
-16.0
δ18O (‰)
-14.0
-12.0
Figure 6. Stable isotope plot for thermal samples in table 1.
600
(b)
Blackburn
Discussion of Beowawe, Tuscarora, and
Blackburn Fields
500
Na (mg/kg)
400
Three sites (Beowawe, Tuscarora and Blackburn
fields) in central Nevada contain a combination of thermal
fluids and Paleozoic carbonate rocks at depth. These
sites are examined in more detail to better understand
the specific characteristics of deep carbonate geothermal
systems that may be typical of potential deep carbonate
geothermal systems in central Nevada.
Gwynn et al. (2014) reviewed the deep temperature
information reported in oil exploration wells around
northeast Nevada including the Beowawe, Tuscarora
and Blackburn fields. Significant exploration drilling
and modest oil production has occurred in Pine Valley
(Blackburn field), some 50 km southeast of the Beowawe
geothermal field. Here, the well-determined production
temperature of Blackburn oil field is 120°C at 2.2 km
depth, and the deep gradient in bedrock is 35°C/km.
Cress Ranch Springs, Mary’s River
300
BW 85-18
BW Frying Pan Geyser
TR DH 66-5
TR 7C
TR 7A
200
Ellison Ranch Spring (TR)
TR 8B
100
BW Ginn 1-13
TR 8a
BW Rossi 21-19
Sulphur Springs Ruby Valley
Burffreys Hot Spring
Cool Groundwater
0
400
800
1200
TDS (mg/l)
1600
2000
Figure 5(a),(b). Plots of the major ions of HCO3 and Na versus TDS.
29
Kirby, et al.
The heat flow for the valley is 90 ± 10 mW/
m2. The oil in Blackburn field has accumulated
near the top of Devonian dolomite units which
immediately underly the inferred source rock
of Mississippian Chainman Shale (Hulen et al.,
1990).
When the temperature data from Beowawe
and Pine Valley are merged, the one deep well at
Beowawe located in the footwall of the Malpais
fault zone (Collins 76-17, 1 km from the fault
zone) has identical temperatures and gradient to
the best-fit geotherm from Pine Valley (figures 2
and 7). This is consistent with the regional
conductive thermal regime found by Gwynn et
al. (2014) in many valleys of northeast Nevada.
There are slightly higher temperatures at less than
1.5 km depth in the Collins well, but these can
be attributed to two-dimensional thermal effects
from hot upflow feeding the original Beowawe
geyser field on the west-dipping Malpais fault
zone.
Figure 7. Compilation of temperature data from Pine Valley (PV), Blackburn oil field
(BB), and Beowawe geothermal field. The regional heat flow away from the effects
of the hot upflow zone at Beowawe is 90 ± 10 mW/m2.
Figure 8. Upper cross section is through the
Beowawe geothermal field, modified from Layman
(1984). The wellhead of the Collins well is about 1
km from the surface intersection of the Malpais fault
zone and the original geyser field. The middle cross
section is through part of the Tuscarora geothermal
prospect, Paleozoic carbonate rocks are shown in
blue, modified from Goranson (2005). The lower
cross section is through the Blackburn oil field,
modified from Hulen et al. (1990). Abbreviations
are: QTs = Quaternary-late Tertiary sediments,
Mb = Miocene basalts, Tv = undivided Miocene
to Miocene ignimbrite and siliciclastic rocks,
Ti = Tertiary(?) granodiorite, Mc = Mississippian
Chainman Shale, D = Devonian siliciclastic and
carbonate rocks, undivided, and Dn = Devonian
Nevada Group (dolomite). The dashed line beneath
the Mississipian and Devonian units is a low-angle
detachment(?) fault. The green zone highlights the
main oil reservoir near the top of the Devonian
dolomite.
30
Kirby, et al.
Beowawe, Tuscarora, and Blackburn fields share similar geology at depth (figure 8). Each of these fields is
characterized by a combination of Tertiary volcanic rocks and younger basin fill overlying Paleozoic rocks at depth. Drilled
depth at these fields is generally less than 2.5 km (Zoback, 1979; Pilkington et al., 1980; Hulen et al., 1990; Goranson,
2005). Potential geothermal carbonate reservoirs exist beneath these areas at depths greater than 3 km.
Beowawe
Beowawe was notable for intense surface thermal activity consisting of boiling hot springs and geysers, that produced a prominent silica sinter terrace that extends about 850 m along the edge of the Malpais fault scarp. Most of the
hot water discharge ceased due to fluid production, but vestiges of surface thermal activity exists today (White, 1992).
Exploration drilling began in 1959 and the first wells were located near the thermal ground, where shallow temperature profiles followed hydrostatic boiling temperature to a depth of about 150 to 200 m. The deepest well, Ginn 1-13,
encountered the hottest temperature of 214°C at >2 km depth. This well and two others (Ginn 2-13, 77-13) are the main
producers, and they are confined to an area of <1km2, that is located ~2.5 km southwest of the surface thermal activity.
The Beowawe power plant (16.7 MWe gross) was commissioned in 1985, but initial production induced a pressure
drop of ~12 bars as observed in Vulcan 2, an early well used for monitoring located on top of the sinter terrace (Benoit
and Stock, 1993). A temperature drop from 180 to <100°C accompanied the pressure drop, possibly as a result of inflow
of cool groundwater. These changes occurred in spite of injection into the nearby Batz well, indicating poor pressure
communication between the deep (>500 m) and shallow parts of the system, across the Malpais fault. Importantly, there
was an overall decline in power production to ~12.5 MW (Butler et al., 2001), which was subsequently rejuvenated by
drilling and new production from 76-17 in 1991 (Butler et al., 2001).
Rocks hosting the geothermal system consist of volcanic deposits (~1000 m thick) produced by mid-Miocene
magmatism associated with the Northern Nevada Rift, and underlying Ordivician Valmy Formation, which consists of
meta-sedimentary siltstone, quartzite, chert, and shale (Sibbet, 1983). Beneath the Valmy Formation lie Paleozoic carbonate
rocks (Zoback, 1979; Watt et al., 2007). Quaternary alluvium, sinter, and playa sediments form a thin veneer that caps
the stratigraphy.
The Malpais fault is the principle structural element that serves to localize fluid flow (Sibbett, 1983; Layman,
1984; Hoang et al., 1987; Faulder et al., 1997; Butler et al., 2001; Garg et al., 2007). This basin-bounding fault strikes
east-northeast and dips ~ 65-70° west (Layman, 1984). Most of the deep permeability is found in a narrow zone (<500
m wide) of subsidiary faults and fractures that occur in the hanging wall. Fluid rises along the Malpais fault zone in a
subvertical, northeasterly direction, and this explains the 2.5 km horizontal offset between the location of the production
wells and the sinter terrace.
Olmsted and Rush (1987) estimate the natural convective heat flow to be ~17 MW, equivalent to upflow of 18 kg/s
at 229°C. Of the total, 14 MW is conductive heat transfer covering an area close to 70 km2, plus 2MW in convective fluid
discharge in the geysers area, and 1 MW attributed to outflowing hot groundwater. The shallow conductive heat flow
ranges from 50 to 4000 mW/m2, and increases in proximity to the sinter terrace and surface thermal activity. Olmsted and
Rush (1987) used thermal conductivity values of 1.69±.20 W/m °K based on lab measurements of volcanic rocks. In an
earlier study Smith (1983) obtained a similar range of thermal conductivity values (1.20 to 2.26 W/m °K), but slightly
lower value for conductive heat flow of about 5 MW because the area of anomalous heat flow was <15 km2.
Modern production figures are unavailable, but judging from the data reported in Faulder et al. (1997) and the
Nevada Bureau of Mines and Geology website, production of 250-260 kg/s of a deep liquid at ~215°C is required to
sustain generation at full plant capacity of 16.7 MWe.
The chemical composition of thermal water at Beowawe is best described as a weakly alkaline (near neutral
pH), bicarbonate water, with minor and subequal concentrations of sulfate and chloride (table 1, figures, 3,4, and 5). It
resembles thermal waters at Tuscarora. The aqueous silica concentrations range from 160 to 480 ppm, corresponding
to quartz equilibration temperatures of 160 to 220°C, and the Na/K values correspond to equilibration temperatures
of 205 to 266°C. Tuscarora
Surface thermal activity (Ellison Ranch Springs) consists of warm to near boiling hot springs that extend for ~1 mile
along Hot Creek and have produced significant deposits of silica sinter and travertine (Sibbett, 1982). Springs cluster in
two groups: one near the site of the modern production wells (66-5) and one near the first deep Amax well (65-8). Both
groups have discharge temperatures that are >90°C.
Amax evaluated the geothermal resource potential of the area in the 1970s using geological, geochemical, and
geophysical methods. They drilled 38 thermal gradient holes (32 shallow holes 40-100 m deep; 6 holes 310-530 m deep)
in addition to one deep exploration well (66-5) drilled to 5454 feet (1662 m) depth (Pilkington, 1981). Temperature
profiles from the Amax wells are not reported, but spot temperature data are supplied. The shallow to intermediate depth
31
Kirby, et al.
wells mostly encountered temperatures in the range of 50 to 100°C, and the hottest temperature of 115°C was measured
at 522 m. The deep well had a maximum temperature of only 110°C on fluids produced during a flow test of the deep
well (Pilkington, 1981). Temperature gradients range from 13 to 2558°C/km (Pilkington, 1981), and the maximum value
is too high to be realistic. Nonetheless, it is apparent that a shallow thermal anomaly extends over a broad area of >10
km2 (Pilkington et al., 1980).
The stratigraphy of Tuscarora is flat lying and consists of Miocene-Oligocene volcanic rocks of intermediate to
felsic composition that overlie Paleozoic sedimentary units (Sibbett, 1982). At Tuscarora the Ordivican Valmy Formation
consists of quartzite, chert, and shale, overlain by Mississippian-Permian conglomerate, sandstone, mudstone and shale
of the Schoonover Formation. Paleozoic carbonate rocks were intersected beneath the Valmy Formation in the deep drill
hole around 4500 feet. Carbonate rocks probably form host the main geothermal reservoir.
Although some degree of folding affects most rocks, Paleozoic thrust faults and Tertiary normal faults represent
important deformation products (Sibbett, 1982) that could influence geothermal fluids. The thrust faults occur at the base
of the Valmy Formation and at the base of the Schoonover Formation. North-south-trending normal faults, which are
products of basin-and-range extension, represent the youngest deformation. The influence of these faults in hydrothermal
fluid flow is poorly known.
The chemical composition of deep thermal water is near neutral pH, bicarbonate water with minor and subequal
concentrations of sulfate and chloride (table 1; figures, 3, 4, and 5). The aqueous silica concentrations range from 100 to
130 ppm and correspond to quartz-silica equilibration temperatures of 132 to 155°C.
Blackburn
There is some published information about the production characteristics of the Blackburn oil reservoir. The porosity
in the Nevada Group dolomite is dominantly fracture-controlled with fractures enhanced by dissolution (Hulen et al.,
1990). Minor intercrystalline porosity is present. The average porosity is 5%, and the permeability is about 20 mD. Hulen
et al. (1990) suggest that much of the fracturing may be related to earlier, high-temperature hydrothermal events, perhaps
at the time of emplacement of nearby intrusives. Assuming this permeability is uniformly dispersed through the rock, a
500 m horizontal leg to a production well should have a transmissivity of about 10 Darcy-meters. Such wells should be
good geothermal production wells.
Conclusions
The geology in central Nevada is complicated and much of this area is underlain by structurally contiguous Paleozoic
carbonate rocks (Heilweil and Brooks, 2011; Dickinson, 2006). These carbonates are exposed in the ranges of central
Nevada and lie in the subsurface beneath Tertiary volcanic rocks and younger basin fill. Paleozoic carbonate rocks form
important regional scale aquifers (Heilwiel and Brooks, 2011), and where temperature conditions are optimal they may also
represent an untapped regional-scale geothermal resource. Heat-flow across the area is generally greater than 85 mW/m2,
and localized areas near geothermal upflow zones may be significantly higher than the background heat flow. Heat-flow
and geologic data indicate potential for carbonate reservoirs at temperatures greater than 180°C exist at depths below 3
kilometers across much of central Nevada (Gwynn et al., 2014).
Compiled fluid geochemistry of thermal waters from producing geothermal fields at Tuscarora and Beowawe, produced
water from the Blackburn oil field, and thermal springs and wells with temperatures greater than 30°C provide additional
support for a regional-scale geothermal resource. Nearly all thermal samples share a Na-HCO3 major ion chemistry, low
TDS, and high concentrations of dissolved silica. Variations in chemistry are driven by differing concentrations of Ca, Na,
and HCO3. Quartz geothermometry yields the highest estimated temperatures from the Beowawe field. Other areas with
elevated geothermometry and Na-HCO3 waters include the Tuscarora geothermal field, the Mary’s River area, and Hot
Sulfur Springs in Ruby Valley. The produced water from the Blackburn oil field may represent a high Na, Cl, and HCO3
end member of the thermal samples (Goff et al., 1994).
To account for the bicarbonate water compositions at Beowawe and Tuscarora, the deep thermal water was likely
subject to considerable water-rock interaction with carbonate host rocks. The implication is that the main carbonate reservoir
that feeds thermal systems at Beowawe, Tuscarora, and elsewhere across central Nevada consists of a stratabound aquifer
hosted in a carbonate units, and that the permeability structure guiding thermal water to the surface is via extensional faults.
These chemistries may result from equilibrium in warm carbonate aquifers and ion exchange and mineral precipitation
as thermal fluids move toward the surface. We infer that similarities in chemistry result from similar thermal reservoir
conditions beneath large areas central Nevada. Limited stable isotope data may indicate a geothermal signal for several
samples with lower geothermometry. Other samples show little isotopic shift and likely represent water that was heated
with little isotopic fractionation.
32
Kirby, et al.
Detailed examination of the Beowawe, Tuscarora, and Blackburn fields shows similar high heat flow and geologic
settings characterized by Paleozoic units overlain by Tertiary volcanic rocks and basin fill. Fluid chemistry between Tuscarora and Beowawe are similar implying comparable reservoir conditions at depth.
Acknowledgements
This paper is part of a project titled: “Novel Geothermal Development of Sedimentary Basins in the United States,”
Moore, J.N. and Allis, R.G. (co-Principal Investigators), which is partially funded by the Geothermal Technologies Program
of the U.S. Dept. of Energy (Award DE-EE0005128).
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