Heat Flow Data and Calculations for
Thermal Gradient Wells B2, B3 and B4
John Welhan, Idaho Geological Survey
Final Report, prepared for
NGDS Supplemental Drilling Project
(DOE Project ID-EE0002850)
April 30, 2014
Introduction and Scope
Four geothermal exploration borings were drilled within a 30 km (18 mile) radius of China Hat in the
Blackfoot volcanic field (BVF) during 2012 and three were completed as temporary thermal gradient
wells. A companion report summarizes information on well drilling, construction and well abandonment
procedures. This report describes the methods used to calculate heat flow and how the new heat flow
estimates inform on and, in at least one instance, help place into context pre-existing heat flow
information in Southern Methodist University’s database (SMU, 2008). This report is referenced as
supplemental information in the heat flow entries for these three wells in the NGDS heat flow template
that has been submitted as a deliverable under DOE contract ID-EE0002850.
Pre-existing Heat flow Information
Because of its geologic history, the region’s heat flow reflects the response of the continental
lithosphere to the passage of the Yellowstone hotspot as well as to crustal extension and upwelling of
mantle heat that are characteristic of the Basin and Range (B&R) province as a whole (Blackwell, 1989).
Blackwell estimated that average heat flow in the B&R is of the order of 100 ± 10 mW/m2, significantly
higher than mean crustal heat flow of 40-80 mW/m2. Lachenbruch and others (1994) reported mean
heat flow values in the northern B&R of 92 ±9 mW/m2 and mean heat flow in the southern B&R of 82 ±
3 mW/m2. The best estimate of expected background heat flow in the B&R of southeast Idaho is from
Henrikson and Chapman (2002) who reported a mean heat flow value of 91 mW/m2 based on 181 wells
in Utah’s Basin and Range. In that study, the northern-most wells, some 100 km south of the BVF, have
heat flow values ranging from 100 to 110 mW/m2.
However, reliable heat flow measurements have been difficult to come by in this geologic setting
because advective overprinting by ground-water flow in the area’s fractured basalts and county rocks
leads to anomalously low thermal gradients and lower than expected conductive heat flow estimates in
many shallow wells (SMU, 2008). Of 38 wells located in and around the BVF, 14 are classified in SMU’s
heat flow database as having reliable information, but only seven of those have associated heat flow
estimates (Table 1, Figure 1). Heat flows at four locations are greater than 120 mW/m2, including a
2900-meter deep wildcat well east of the BVF. That well’s estimated heat flow ranges from 126 to 184
mW/m2, depending on the effective thermal conductivity assumed to characterize its 2400 meter length
(Table 1). Surprisingly high heat flows are also reported in several wells (SEI-12, -15 and -16) of
unknown but presumably shallow depth (Table 3). In contrast, heat flow estimated in a 2400 meterdeep geothermal exploration well drilled 1.4 km south of the largest rhyolite dome in the BVF (SunHub
25-1) is surprisingly low (55 mW/m2), similar to estimates in several shallow wells whose bottom hole
temperatures are no greater than 16 oC, in which heat flows range from 28 to 51 mW/m2.
Well Siting Objectives
Three boreholes were completed as thermal gradient wells to better understand the heat flow
variations documented in SMU’s database and as a test of an hypothesis first posed by Autenreith et al.
(2011) that heat from the China Hat magma does not manifest within the graben but is diverted
eastward along the Mead and Absaroka thrust faults into shallower levels in the thrust blocks of the
Idaho thrust belt.
A fourth well, B1, drilled 7.5 km south-southwest of China Hat to confirm SunHub 25-1’s heat flow
estimate, failed to penetrate basalt at a depth of 166 meters and was abandoned as a thermal gradient
site because advective overprinting by rapid ground water flow in the basalt section (Dion, 1974)
precludes collection of useful conductive heat flow information in this rock unit. The thickness of basalt
penetrated at this location (137 m), together with the 230 meters encountered in SunHub 25-1 4 km to
the NNE suggests that this area is part of a deep volcanic rift graben that trends almost N-S rather than
the NNW-SSE trending rift system documented by Polun (2011) and extends from the southern China
hat graben at Soda Springs to the south end of Reservoir Mountain. If so, it would be the deepest
volcanic rift in the BVF and one that may extend for at least 19 km from the deepest part of the
Blackfoot Reservoir’s southern basin (34 meters of bathymetric relief; Wood et al., 2012) SSW to the
trace of the Paris thrust.
Well B2 was drilled close to the East Gem Valley fault, a late Quaternary active fault, at a location not far
from where high heat flows were reported for wells SEI-15 and SEI-16 (Table 1). Well B3 was drilled as a
test of whether elevated gradients exist in an area of the western overthrust belt where high heat flows
has been reported in a deep wildcat well (SMU, 2008). Well B4 was sited to confirm or refute the high
conductive heat flow reported in well SEI-12 in an area near where the warmest thermal springs in the
entire valley are found (35 oC). More information can be found in Welhan et al. (2013; 2014).
Data Collected
A summary of lithologic data, thermal conductivity measurements on chip samples, and thermal
gradient data for wells B2, B3 and B4 is presented in the companion report's Figure 6. Representative
chip samples of formation lithology encountered during drilling were collected at 3-meter intervals.
Samples were washed and dried and subsamples were sent to the University of Utah’s Geothermal
Studies Laboratory for thermal conductivity analysis using a divided bar technique with an average
measurement precision (1 ) of 0.43 W/m/oK as determined in 47 replicate measurements on 21 chip
samples. The thermal conductivity data and measurement protocols are reported by Chapman (2013)
and are reported here. More than six months after drilling, equilibrium thermal profiles were measured
by the Utah Geological Survey on the equilibrated gradient wells using a resistance-temperature probe
with a temperature measurement precision of ±0.01°C (M. Gwynn, written comm., 2013).
Heat Flow Calculations
Computations were carried out in four steps: (1) an assessment of the thermal conductivity
measurements to determine the appropriate data averaging approach; (2) an evaluation of thermal
Figure 1 – Available heat flow data within the study area, from SMU’s (2008) database and locations of shallow
thermal gradient wells (B2, B3, B4) that were completed under this project. Abbreviations used: (RM) Reservoir
Mountain; (GV) Gem Valley; (BRS) Blackfoot Reservoir; (CH) China Hat; (SS) Soda Springs.
Table 1 - Available heat flow estimates in the study area, from Southern Methodist University Geothermal
Laboratory (SMU, 2008).
gradient data to determine whether and over what vertical interval a conductive thermal gradient could
be estimated; (3) correction of the conductive thermal gradient estimate for the effects of local
topography; and (4) computation of conductive heat flow using Fourier’s Law, the corrected thermal
gradient, and the appropriate harmonically averaged thermal conductivity.
The heat flow calculation procedure is described below:
(1) The measured thermal conductivity (TC) values were plotted vs. depth to ascertain overall
trends and patterns of variation. Thermal conductivity values showed systematic shifts with
depth that did not always correlate with changes in lithology, so two different methods of
assigning interval-average TC estimates were evaluated: (1) grouping by lithology and averaging
measured TC values within each lithologic interval; and (2) grouping according to changes in TC.
The analysis is summarized in Figure 1 and analysis details of the TC grouping method are
provided here. The effective interval TC was calculated as the harmonic mean of the grouped
averages over the interval in which heat flow was calculated.
Figure 1 – Variation of measured TC values in Wells B2, B3 and B4. Dashed lines represent interval averages and
vertical solid lines represent two sample standard deviations about the mean.
As shown in Figure 1, the group average TC values are statistically distinct at a 95% or higher
confidence level in Wells B2 and B3, whereas the group averages in well B4 are not, even at 85%
confidence. Because Well B2’s thermal profile was deemed to be conductive only in the 22-35
m interval, only TC measurements in this interval were used to define the average TC for the
heat flow calculation. In Well B4, the effective TC was the average of all TC values and heat flow
was the same whether TC values were grouped by lithology or not.
Only in Well B3 did grouping by TC values or lithology make a difference in calculated heat flow,
albeit very slight (85.6 vs. 86.8 mW/m2).
(2) Figure 2 shows temperature profile data collected by the Utah Geological Survey (M. Gwynn,
written comm., 2013) in Wells B2, B3 and B4. Two-point thermal gradients were calculated for
each well across an interval deemed to be the most representative of that well’s conductive
regime. Well B2’s thermal profile does not define a simple conductive gradient; rather, it
appears to reflect upwelling of deeper, warmer water along the East Gem Valley fault, where a
warm spring is known to issue from the fault some 10 km north of Well B2’s drill site. Well B3
displayed a simple, apparently purely conductive thermal profile, whereas Well B4’s thermal
profile showed minor deviations from linearity.
Figure 2 – Thermal profiles measured on Wells B2, B3 and B4 more than 180 days after drilling and completion (M.
Gwynn, written comm., 2013).
(3) The thermal gradients calculated in (2) were corrected for near-surface topographic
perturbations of the thermal field using Lees’ (1910) 2-dimensional approximation. Figure 3
summarizes the topographic context of the boreholes and the topographic approximations used
to correct the thermal gradient in each case.
(4) Finally, the corrected thermal gradients and weighted harmonic mean thermal conductivities
were used to calculate conductive heat flow using Fourier’s Law:
q = k ( T/Z ),
where q is heat flow (mW/m2), k is bulk thermal conductivity (W/m.oK), and T/Z is thermal
gradient (oC/km). Calculations are summarized here and in Table 2.
Figure 3 – Lees-type 2-dimensional corrections for topographic perturbation of the shallow thermal field. The
thermal gradient correction in each case is: Well B2: +0.24 oC/km; Well B3: +0.18 oC/km; Well B4: -0.08 oC/km.
Table 2 – Summary of results for heat flow calculations, Wells B2, B3 and B4.
Well
Longitude Latitude1
Top Interval, m
Bottom Interval, m
Ttop, oC
Tbottom, oC
Heat Flow, mW/m2
____________________________________________________________________________________________________________________________________________________________________________________________________________________ ___________________
B2
-111.70886 42.60186
22
35.0
9.94
10.61
106
B3
-111.27462 42.93268
28
105.9
7.45
9.89
86
B4
-111.69361 42.94141
13
83.8
7.61
8.52
45
Discussion of Results
Well B2 was drilled near the East Gem Valley fault (Figure 1) in order to determine whether the very
high heat flows documented in the SMU database (wells SEI-15, -16; Figure 1) represent an area of
localized high heat flow or are part of a broader trend extending into Gem Valley; basalts in this area
erupted as recently as 15 ka and several thermal springs and warm water wells are known on the east
side of the valley. This well’s thermal profile is the most complex of the three gradient holes and proved
to be unsuitable for estimating conductive heat flow. The upper, basalt-dominated portion of the well
may be influenced by infiltration of surface water from the nearby Bear River, whose temperature varies
seasonally from 4.5 to 14.2 oC at this location (ERI, 2006). The aquifer is artesian below 70 m, however,
so temperatures in the lower half of the well should be immune to river leakage. The well’s overall
elevated temperatures (11.7 oC at 50 m; 12.4 oC at 137 m) and temperature reversals at 70 m and 85 m
indicate a prevailing influence of thermal water over most of the well’s lower portion. A respectable
thermal gradient was found in the upper 30 meters of this well (106 mW/m2) but is unrepresentative of
background conductive heat flow in Gem Valley because of the effects of river infiltration and/or warmwater upwelling along the late Quaternary East Gem Valley fault (Figure 1).
Well B4, north of Reservoir Mountain, was drilled to a depth of 86 meters to confirm the high heat flow
reported in SMU’s database for a well of unknown depth (SEI-12) on the west side of Reservoir
Mountain. Well B4’s thermal gradient defined a conductive regime but its low heat flow (45 mW/m2)
suggests that the high heat flow reported west of Reservoir Mountain is not representative of
conductive heat flow the area. The presence of several small travertine terraces in the vicinity of SEI-12
and warm water (up to 40 oC) documented in the Corral Creek area just south of SEI-12 (Mitchell, 1976)
suggest that, like Well B2, the anomalously high heat flow at SEI-12 is also attributable to the presence
of warm water at shallow depth, and it is not indicative of elevated conductive heat flow in the
Reservoir Mountain area.
Well B3, drilled to a depth of 107 meters near the trace of the Mead thrust fault, was sited in that area
to determine if other indications of above-normal heat flow are to be found near the Gentile Valley 9-1
well. This well’s heat flow was computed as 86 mW/m2 which is at the low end of the range of heat
flows in the same area that were calculated using best BHT estimates and formation thermal
conductivities in deep oil and gas wells (Welhan and Gwynn, 2014).
Conclusions
Three boreholes were completed as thermal gradient wells during 2012 in the area of the Blackfoot
volcanic field. Chip samples collected during drilling were analyzed for thermal conductivity at the
University of Utah in early 2013 and equilibrium thermal profiles were measured by the Utah Geological
Survey more than six months after well completion.
Conductive thermal gradients were measured in Wells B3 and B4. Well B2’s thermal profile, however,
was affected by advective influences due to infiltration of warm river water and/or upwelling of warm
water along the East Gem Valley fault. Conductive intervals for the estimation of heat flow were
identified between 28 and 106 meters depth in Well B3 and between 13 and 84 meters depth in Well
B4. Only a very small interval (22 to 35 meters depth) in Well B2 could be used to estimate shallow
conductive heat flow.
Thermal conductivity variations in each well were evaluated in the context of the lithology encountered
and the data were grouped according to systematic shifts in thermal conductivity to estimate interval
averages that were then harmonically averaged over the conductive temperature intervals for
calculation of heat flow across the interval.
Two-point estimates of heat flow calculated in this manner ranged from 45 mW/m2 in Well B4 to 86
mW/m2 in Well B3; both estimates appear to reflect true conductive heat flow in the vicinity of these
wells. In contrast, the shallow heat flow estimate derived for Well B2 (106 mW/m2) is not
representative of conductive heat flow in the vicinity of this well and instead reflects the influence of
warm water entering the formations, either from river water infiltrating into the deeper units and/or
upwelling of warm water along the nearby East Gem Valley fault. The conductive heat flow measured at
Well B4 indicates that the Reservoir Mountain area is characterized by low heat flow, unlike the heat
flow recorded in SEI-12 on the west side of Reservoir Mountain (SMU, 2008). As observed in Well B2,
SEI-12’s heat flow may be unrepresentative of actual conductive heat flow in the area due to the
presence of warm water at shallow depth (20-40 oC), such as that encountered in exploration drill holes
on the southwest side of Reservoir Mountain (Mitchell, 1976).
References
Chapman, D.S., 2013, Thermal Conductivity Measurements for Idaho Geological Survey ARRA
Supplemental Data Project; Geothermal Studies Laboratory, 11 pp.
ERI, 2006, Bear River / Malad River Subbasin Assessment and Total Maximum Daily Load Plan;
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Idaho, 353 p.
Henrikson, A. and D.S. Chapman, 2002, terrestrial heat flow in Utah; unpubl. report, University of Utah;
http://geology.utah.gov/emp/geothermal/pdf/terrestrialhf.pdf
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