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Journal of the Geological Society, London, Vol. 164, 2007, pp. 371–382. Printed in Great Britain.
371
A deep geothermal exploration well at Eastgate, Weardale, UK: a novel exploration
concept for low-enthalpy resources
D. A. C. MANNING 1,2, P. L. YOUNGER 2, F. W. SMITH 3, J. M. JONES 4, D. J. DUFTON 5 &
S. DISKIN 6
1School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
(e-mail: [email protected])2Institute for Research on the Environment and Sustainability, Newcastle University,
Newcastle upon Tyne NE1 7RU, UK3FWS Consultants Ltd, Merrington House, PO Box 11, Merrington Lane Trading Estate, Spennymoor DL16 7UU, UK
4March House, Horsley NE15 0HZ, UK5PB Power, William Armstrong Drive, Newcastle upon Tyne NE4 7YQ, UK
6Foraco–Boniface SA, ZI des Fournels, BP 173, 34401 Lunel, France
Abstract: The first deep geothermal exploration borehole (995 m) to be drilled in the UK for over 20 years
was completed at Eastgate (Weardale, Co. Durham) in December 2004. It penetrated 4 m of sandy till
(Quaternary), 267.5 m of Lower Carboniferous strata (including the Whin Sill), and 723.5 m of the Weardale
Granite (Devonian), with vein mineralization occurring to 913 m. Unlike previous geothermal investigations
of UK radiothermal granites that focused on the hot dry rock concept, the Eastgate Borehole was designed to
intercept deep fracture-hosted brines associated with the major, geologically ancient, hydrothermal vein
systems. Abundant brine (<46 8C) was encountered within natural fracture networks of very high permeability
(transmissivity c. 2000 darcy m) within granite. Evidence for the thermal history of the Carboniferous rocks
from phytoclast reflectance measurements shows very high values (>3.3%) indicating maximum temperatures
of 130 8C prior to intrusion of the Whin Sill. Geochemical analysis of cuttings samples from the Eastgate
Borehole suggests radiothermal heat production rates for unaltered Weardale Granite averaging 4.1 �W m�3,
with a mean geothermal gradient of 38 8C km�1. The Eastgate Borehole has significant exploitation potential
for direct heat uses; it demonstrates the potential for seeking hydrothermal vein systems within radiothermal
granites as targets for geothermal resources.
Geothermal energy is used globally in applications ranging from
electricity generation (using steam to drive turbines) to space
heating using ground source heat pumps (GSHP), which extract
energy from shallow groundwater and/or soil atmospheres. In
Europe, geothermal energy use depends on the balance of
availability of differing energy sources. Thus Iceland, with read-
ily available volcanogenic geothermal resources, is one of the
world’s major users for space heating. The UK has been very
slow to develop its more modest geothermal resources, with only
one significant scheme (<1.4 MW; a single geothermal well
contributing to Southampton’s district heating scheme), and a
growing number of GSHP applications for individual buildings
(mainly new-build domestic premises). The slow uptake in the
UK partly reflects the ready availability of indigenous oil and
gas over the last three decades. Norway and Sweden show the
contrast in geothermal uptake between countries with and with-
out hydrocarbon resources: Norway makes almost no use of
geothermal energy, whereas Sweden is one of Europe’s greatest
users, entirely in the form of GSHP (Sanner et al. 2003).
As energy costs rise, especially for fossil fuels, interest in
alternative sources is increasing. Globally, nuclear energy and
coal (with varying degrees of ‘cleanliness’ in burning) will be
used increasingly for electricity generation during the 21st
century. Interest in renewable sources of energy, such as wind
power, is also increasing. But geothermal energy can play a
much greater role within a portfolio of energy supply options
than is currently considered. There are potentially many locations
where geothermal energy is insufficient to generate electricity,
but where it can contribute to space heating and other low-grade
uses currently met by distributed fossil fuel combustion. Dis-
placement of the domestic gas boiler from tens of millions of
homes would contribute more to the UK’s greenhouse gas
emission reduction obligations than almost any other techno-
logical change (cf. Caneta Research 1999).
In Weardale, County Durham, UK, the recent decommission-
ing of the Lafarge cement works has led to a major redevelop-
ment opportunity at a rural site. The Weardale Task Force
anticipates a mixed-use redevelopment, making use of indigen-
ous renewable energy sources, including wind and hydroelectric
power generation, and biomass combined heat and power.
Geothermal energy is being considered for space heating and for
use in a spa tourist attraction.
The concepts underlying the geothermal exploration activity
undertaken at the Eastgate site are novel, and represent a
significant development from those previously applied to the
inferred geothermal resources of the UK. Initiated in 1976 at
the time of the Middle East oil crisis, geothermal exploration in
the UK focused until its end in 1990 on the potential for
generating electricity. The cost-effective generation of electricity
from geothermal resources (DiPippo 2005) generally requires
that waters yielded by production wells have temperatures con-
siderably in excess of 100 8C, so that on exposure to atmospheric
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(or near-atmospheric) pressure, they will ‘flash’, i.e. liberate
steam at rates high enough to drive conventional turbines.
Additionally, binary cycles (including the Kalina cycle) are used
for power generation in which geothermal waters at ,140 8C
vaporize other ‘working fluids’ to produce gas streams sufficient
to turn turbines (DiPippo 2005). Outside active volcanic areas,
there are few parts of the world in which economically accessible
geothermal waters are hot enough for flash or binary cycle
electricity generation. However, there are many direct uses that
can exploit the thermal energy in sub-100 8C geothermal waters
(e.g. space or district heating; greenhouse heating; aquaculture;
health spa tourism developments, etc.; Lund et al. 1998; Dickson
& Fanelli 2005). These options were not a priority during the
1976–1990 investigations in the UK. Rather, two principal
options for deriving supra-100 8C waters from the UK continen-
tal crust were examined (Barker et al. 2000).
(1) Hydrothermal aquifers: corresponding principally to Meso-
zoic basins, in which permeable formations are likely to be
present at depths that will ensure that they contain hot water.
(2) Hot dry rock (HDR): in which the heat generated by
radiothermal granites was to be exploited by drilling deep bore-
holes, between which fractures would be developed artificially (e.g.
hydraulic fracturing; explosives). Cool water would be pumped
down into the fractured granite, left to equilibrate thermally, and
then pumped out again at much higher temperatures.
In many ways, both of these exploration concepts were decades
ahead of their time. Although the hydrothermal aquifer investiga-
tions did not result in the identification of flash or binary grade
energy, they did identify resources suitable for direct-use applica-
tions, as used to the present day by the Southampton Geothermal
Heating Company. Many other similar resources await exploita-
tion. Although the HDR experiments undertaken in the Carnme-
nellis Granite (Cornwall), using boreholes sunk at Rosemanowes
Quarry to depths of 1700–2600 m, did not result in an operational
geothermal exploitation scheme (Richards et al. 1994), they did
yield a dataset used widely to test numerical simulation codes
(Kolditz & Clauser 1998). They provided much of the conceptual
basis upon which the European Union pilot project at Soultz-sous-
Forets (Rhine Graben) has successfully built (e.g. Bachler & Kohl
2005), using artificial fracturing to enhance natural structures in
granite to obtain an exploitable resource, which is now generating
electricity using a binary power plant.
Given their focus on resources suitable for electricity genera-
tion, the 1976–1990 investigations did not fully consider the
direct use for space heating of deep groundwater or GHSP
resources. They were also too early to investigate the possibilities
of man-made hydrothermal aquifers now associated with satu-
rated mine workings at great depth (.1000 m), which have
become flooded only since 1990 (see Younger & Adams 1999;
Banks et al. 2004).
In this paper, we present a further exploration possibility: the
concept that ancient hydrothermal vein structures associated with
the radiothermal granites of the UK still function as geothermal
plumbing systems, and may be economically viable. In testing this
concept, we have sunk the first deep geothermal exploration
borehole to be drilled in the UK for 20 years, and only the second
borehole ever to penetrate the Weardale Granite, a key component
of the classic ‘block-and-basin’ geology of the Carboniferous of
northern England (e.g. Fraser & Gawthorpe 2003).
Geological background
Weardale lies within the UK’s first Geopark, designated in
recognition of the importance of the North Pennines in shaping
views on the origin of Mississippi-Valley Type mineral deposits.
The North Pennines Orefield is famous (Dunham 1990) for its
zoned fluorite–sphalerite–galena–barite mineralization, which is
principally developed in Lower Carboniferous limestones. The
zoning of the orefield stimulated geophysical investigations that
demonstrated a gravity ‘low’ coinciding with the fluorite zone of
the mineralization (Bott & Masson-Smith 1953, 1957), which
was interpreted as being due to the presence of a buried granite.
Drilling at Rookhope in 1960 (Dunham et al. 1965) proved the
existence of the Weardale Granite, which unexpectedly proved to
be Early Devonian in age, hence older than the mineralized host
rocks, a finding that required a complete revision of ore deposit
models for the North Pennines (Dunham 1990).
Interest in the Weardale Granite as a ‘radiothermal’ granite is
based on its comparatively high concentrations of uranium,
thorium and potassium. It is one of a family of similar granites
in the UK with high heat production rates (Webb et al. 1985,
1987; Downing & Gray 1986), including the Carnmenellis
Granite (the former HDR prospect in Cornwall; Richards et al.
1994).
In the late 1980s, investigations were carried out on a tepid
saline water found at depth in Cambokeels Mine at Eastgate
(Manning & Strutt 1990), issuing from the eastern forehead of
the fluorite-bearing Slitt Vein where it cuts Dinantian limestones
and clastic sediments. Manning & Strutt (1990) compared the
Cambokeels mine water with saline waters, originating from
fractures in the Carnmenellis Granite, reported by Edmunds et
al. (1984). They suggested that the Eastgate mine water was
derived from deep within the Weardale Granite, and that it had
precipitated silica minerals during its ascent through the Slitt
Vein fracture system. Use of geochemical thermometers sug-
gested that the water had equilibrated with a granitic host at
temperatures up to 150 8C.
There is further evidence of a high geothermal gradient in the
Eastgate–Rookhope area. Downing & Gray (1986) reported a
temperature gradient of 30 8C km�1 for the Rookhope Borehole,
and Younger (2000) reported a gradient of over 60 8C km�1 from
the Frazer’s Grove Mine (Greencleugh Vein, similar in structure
and filling to the Slitt Vein), several kilometres west of the
Rookhope Borehole and directly above a mineralization spread-
ing centre (Dunham 1990).
On the basis of the geological information summarized above,
it was decided that the geothermal exploration well at Eastgate
should commence within the Slitt Vein as a suspected pathway
for deep water upflow, and attempt to follow the vein and
associated splays vertically downwards for up to 1 km (Fig. 1),
the total depth being limited by the available budget. A starting
position above the sub-crop of the Slitt Vein against the base of
the Quaternary deposits was identified by trial-pitting and drilling
five inclined boreholes (at 45–608) to depths of up to 60 m. The
final borehole site was precisely surveyed by global positioning
system (GPS) with reference to the National Grid, and lies at
393890.932 E 538200.147 N, commencing at a surface elevation
of 250.867 m above Ordnance Datum (AOD). In the interests of
speed and economy, the deep borehole was drilled ‘open-hole’
(by Foraco, Lunel, France), recovering cuttings at 1 m intervals
(within sedimentary rocks and granite) down to 615 m, and then
at 5 m intervals (granite). From surface to 93 m the well was
cased and grouted to 1338
inches, from 93 m to 403 m cased and
grouted to 958
inches (with this casing being continued to
surface). From 403 m to full depth at 995 m, the borehole was
completed without casing, at a drilled diameter of 812
inches.
Cuttings taken from the well were washed and their lithologies
logged on site. A complete suite of cutting samples was
D. A. C. MANNING ET AL.372
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deposited with the British Geological Survey. Selected samples
of cuttings from the sedimentary sequence were taken for
vitrinite reflectance determination, and samples of cuttings from
the granite were taken at intervals of c. 50 m for analysis using
X-ray fluorescence (XRF; fused beads for major elements;
powder pellets for trace elements) at the University of Leicester,
Department of Geology. Water samples were taken for analysis
every 10–20 m down to 300 m, and then at 50 m intervals.
Specific electrical conductance (‘conductivity’) and temperature
of the water arising from the borehole were monitored continu-
ously at the cuttings separator at the wellhead.
Summary of findings: geology
The Eastgate Borehole penetrated 271.5 m of recent and Lower
Carboniferous cover rocks then nearly 723 m of basement
granite. Overall, the sequence closely resembled that penetrated
by the Rookhope Borehole (Dunham et al. 1965; Fig. 2).
Superficial deposits of Recent and Quaternary age were encoun-
tered down to rockhead at about 4 m (all depths in this section
are below drilling table, which was at 252.75 m AOD), and
consisted mostly of sand, gravel and boulders. The borehole wasFig. 1. Schematic cross-section to illustrate the design of the Eastgate
Borehole.
Fig. 2. Summary lithological log, comparing Rookhope and Eastgate boreholes. Lst, limestone; qtz, quartz; fluor, fluorite; gal, galena; py, pyrite; hm,
hematite.
GEOTHERMAL EXPLORATION WELL, WEARDALE, UK 373
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open-holed to 10 m without casing, so that there was consider-
able cross-contamination of rock cuttings with caved drift
material over that interval.
The Scar Limestone was present from rockhead to about 12 m
below drilling table, and was karstified (the shallow inclined
boreholes penetrated caves that were entirely filled with damp
mud). This was underlain by the ‘Alternating Beds’, a sequence
of thinly bedded limestones, mudstones and sandstones. The
Tynebottom Limestone was recognized between 39.5 and 51.5 m.
As with the other horizons below the Scar Limestone, this
formation was heavily mineralized (by a combination of quartz
replacements and veinlets); it was also the source of a significant
increase in groundwater influx to the borehole. The Jew Lime-
stone was encountered between 76 and 83 m, again mineralized.
Immediately beneath it (83–87 m) was a vein of coarse green
fluorite, interpreted as a branch of the Slitt Vein. The Great Whin
Sill, extensively carbonated and altered to ‘white whin’, was
intercepted at 92 m, and continued to 158.5 m.
Rocks beneath the Great Whin Sill were heavily silicified and
veined by quartz down to about 175.5 m. Beneath this the
sequence was essentially devoid of mineralization for almost
50 m, and the Smiddy, Upper and Lower Peghorn, and Birkdale
Limestones were easily recognized. The Robinson Limestone
(223.5–234.5 m) was fractured and heavily replaced by quartz
and calcite. Similar mineralization continued into the Melmerby
Scar Limestone (239.5–249.5 m), below which the Orton Group
and Basement Beds (sandstones with occasional thin limestones)
occurred between 249.5 and 272.5 m, cut by veinlets of miner-
alization.
The granite surface at 272.5 m below table (271.5 m below
surface) was marked by the occurrence of cuttings of a sticky
white clay, presumably kaolinitic, over an interval of 0.5 m. The
first few metres of granite were relatively soft, then became
harder and more coherent. The granite resembles the cored
material from the Rookhope Borehole, and is fairly uniformly
coarse grained (2–6 mm), consisting of feldspars, quartz, musco-
vite and biotite. It has a greenish hue, probably the result of
hydrothermal alteration of the feldspars. Mineralization was
found throughout the granite, usually as quartz veinlets with
sporadic occurrences of pyrite, chalcopyrite or galena.
Cuttings from 415 to 615 m were uniform, with sparse
indications of mineralization. Below 615 m cuttings were taken
at intervals of 5 m. Fluorite mineralization was intersected
between 620 and 650 m (recognized in waste cuttings, rather
than in the samples collected at 5 m intervals). Minor quartz–
pyrite veinlets were encountered at 655.5 m, associated with
sticky white clay (presumably kaolinite). White quartz veins were
encountered at 690 m and 720–721 m. Deeper, three fine-grained
quartz veins with pyrite and hematite were encountered (740–
742 m, 888.5 m, 912–913.5 m).
Comparing the depths at which different marker horizons have
been encountered in other boreholes and at outcrop, it appears
that the pre-Carboniferous surface was very flat (8 m or so relief
over the 10 km distance between Rookhope and Eastgate). This
suggests that future boreholes can be planned with greater
confidence using existing deep borehole data.
In summary, typical North Pennine Orefield mineralization
was found down to about 720 m, with more complex mineraliza-
tion beneath that depth. These observations suggest that the
borehole followed the Slitt Vein structure down to 720 m. Below
this depth the Slitt Vein may have died out, or more probably
deviated too far from the borehole azimuth to be recognized.
In addition to the lithological description, phytoclast reflec-
tance (individual macerals were not distinguished because of the
high maturity) was determined on cuttings from the sedimentary
sequence and compared with those obtained for material from
the Rookhope core (Table 1). Reflectance data are similar for
both boreholes, and clearly show the contact metamorphic effects
of the Great Whin Sill, with a regular profile above the sill (Fig.
3; cf. Creaney 1980), perturbed by the Little Whin Sill in the
Table 1. Phytoclast and vitrinite reflectance data for sediments from theEastgate and Rookhope boreholes
Eastgate Borehole Rookhope Borehole
Depth (m) Phytoclastreflectance (%)
Depth (m) Vitrinitereflectance (%)
19 4.56 26 2.5623 4.50 63 3.1027 4.77 94 3.7530 4.85 112 3.0638 5.48 122 2.9452 5.80 146 3.4662 5.56 159 4.6975 7.36 183 6.70164 9.31 195 7.91167 8.05 284 6.04172 5.79 293 6.76179 4.84 337 4.53190 5.07 373 3.73203 5.25 381 3.54215 5.13223 4.75236 4.35239 3.76251 4.42256 4.47264 4.50268 3.52271 3.37
Fig. 3. Phytoclast reflectance profiles for Carboniferous sediments from
the Eastgate and Rookhope boreholes.
D. A. C. MANNING ET AL.374
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Rookhope Borehole (the Eastgate Borehole started lower in the
succession than this). Below the Great Whin Sill, Figure 3 shows
considerable scatter in the profile of reflectance measurements.
This may be due to caving from higher in the borehole prior to
the installation of casing. Immediately above and below the
Whin Sill, reflectance values reach 9% or more, consistent with
contact temperatures in excess of 400 8C (Karweil 1955).
Furthest below the Great Whin Sill, reflectance values for the
lowest two samples are remarkably similar: 3.52% and 3.37% for
Eastgate and 3.73% and 3.54% for Rookhope. The similarity
between these values and the profiles shown in Figure 3 suggests
that the heat flow from the granite to the overlying sediments
was very similar for the two locations, which, although separated
by 10 km, have identical intervals between the base of the Great
Whin Sill and the top of the Weardale Granite (114 m).
Although it is difficult to recast vitrinite reflectance data into
palaeotemperatures, it can be assumed that the very high
reflectance values reported here (not less than 3.3%) correspond
to maximum temperatures of the order of 130 8C close to the
contact between the overlying sediments and the granite. These
temperatures may have been reached prior to intrusion of the
Great Whin Sill (305 Ma; Fitch & Miller 1967; recalculated by
Cann & Banks 2001), which, according to the sill emplacement
model of Goulty (2005), was into sediments with an approximate
depth of 1.4 km (total sediment thickness on top of the granite of
1.5 km). This is consistent with the conclusion of Dunham
(1987) that the Weardale area had exceptionally high geothermal
gradients prior to the emplacement of the Whin Sills.
Geochemistry of the Weardale Granite: implicationsfor heat production
Limited geochemical analyses of selected samples from the
granite were made to evaluate its heat production potential, and
to provide additional geological information and indicate the
degree of uniformity of the rock. Sixteen samples of cuttings
were selected at c. 50 m intervals, and analysed for major and
trace elements using XRF (Table 2). Originating from a borehole
drilled in conditions that were aggressive towards the drilling
equipment, the cuttings were contaminated with tungsten, molyb-
denum and chromium (metals associated with hardened tools), as
well as iron. Despite this contamination, compositional data are
similar to those reported by Brown et al. (1987) for core samples
of the Weardale Granite from the Rookhope Borehole.
Downhole homogeneity is readily assessed from data for
Na2O, K2O and CaO (Fig. 4). For both the Rookhope and
Eastgate samples, the top 200 m of the granite show considerable
variation. Below this depth, these oxides are relatively constant.
Similarly, the trace elements Rb and Sr, and the naturally
radioactive elements Th and U show variable behaviour above,
and greatest values below, 200 m depth within the granite, in
both boreholes (Figs 5 and 6).
Variation within the top 200 m of the granite may partly be
due to the presence of fluorite mineralization (consistent with
observed F contents; Table 2) as well as weathering prior to the
deposition of the overlying sediments. Similarly Dunham et al.
(1965) showed considerable enrichment in Al2O3 (and K2O)
within the upper 25 m of the granite, consistent with the
development of an illitic or micaceous palaeosol.
The heat production capacity of the granite can be calculated
from the chemical analysis (Downing & Gray 1986), using the
equation (P. C. Webb, pers. comm.)
A ¼ 0:1326r(0:718U þ 0:193Th þ 0:262K)
where A is heat production in �W m�3, r is density in g cm�3, U
is uranium content in mg kg�1, Th is thorium content in mg kg�1
and K is potassium content in element percent.
For the purpose of calculation, the specific gravity of the
granite was assumed to be 2.63 (Dunham et al. 1965); estimation
of specific gravity for each sample by measuring the displace-
ment of 100 ml of water by 100 g of cuttings to obtain their
volume, agitating ultrasonically to remove occluded air, gave a
value of 2.58.
Figure 7 shows downhole variations in the calculated heat
production capability of the granite at Eastgate, compared with
recalculated values for the Rookhope Borehole using geochem-
ical data from Brown et al. (1987). For the Eastgate samples in
general, heat production values rise from 3 �W m�3 to an
average value of 4.1 �W m�3 below 200 m depth from the
granite surface. Observed reductions in heat production values at
depths .400 m within the granite may be due to quartz veining,
and have been excluded in the calculation of the average heat
production value.
Overall, the geochemical data from the granite yield the
following results (Table 3).
(1) The heat production value for unaltered granite is
estimated to be 4.1�W m�3, excluding samples at borehole
depths of 400 m and shallower, and excluding the 740 and 950 m
samples, which contain quartz vein material. This value exceeds
that reported by Downing & Gray (1986) for the Weardale
Granite (3.7 �W m�3).
(2) Heat production appears to increase with depth, overall,
with local perturbations related to the occurrence of veins of
quartz and other discontinuities that are unlikely to affect the
bulk heat production capacity.
No attempt has been made to measure the thermal conductiv-
ity of the granite from the Eastgate Borehole. Heat flow has been
estimated on a preliminary basis with the following assumptions.
For the entire vertical interval, a mean value for thermal
conductivity of 2.99 W m�1 K�1 has been estimated using values
from Downing & Gray (1986) for different rock types (Weardale
granite from the Rookhope core and Carboniferous lithologies),
weighted according to thickness. Heat production within the
granite has been neglected, and a ground surface temperature of
8 8C has been used. With these assumptions, the heat flow at
Eastgate is estimated to be 115 mW m�2, well in excess of
values reported by Downing and Gray (1986) for Rookhope.
Water strikes and hydrogeological conditions
Significant water strikes were encountered during drilling, with
unusually high rates of groundwater ingress from the Carbonifer-
ous sequence (requiring a tri-cone roller bit instead of using
hammer drilling). At times, the water yield from the Carbonifer-
ous strata of this one borehole exceeded the entire former
dewatering rates of a number of local mines. These high water
yields reflect the high permeability of fractures associated with
the Slitt Vein structure.
Installation of casing isolated shallow-sourced groundwaters
from the borehole. The first casing sealed the hole off from water
associated with limestones above the Whin Sill, on the assump-
tion that the Sill itself is rarely a prolific aquifer. However, two
major water feeders were encountered within the Whin Sill,
bringing the borehole water yield back to levels found in the
overlying sedimentary strata (c. 60 m3 h�1).
Once the borehole had penetrated 130 m into the granite, the
installation of the second casing was intended to eliminate all
shallow feeders. Given the generally low permeability of granite,
GEOTHERMAL EXPLORATION WELL, WEARDALE, UK 375
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Table 2. Chemical composition of the Weardale Granite, Eastgate Borehole
Sample depth (m)
280 300 350 400 450 505 550 600 650 700 740 745 750 800 850 900 910 915 920 950 995
wt%
SiO2 78.89 76.66 70.14 79.46 70.56 74.15 75.87 74.29 69.99 73.62 89.79 75.44 74.39 72.74 71.11 72.66 74.23 72.47 71.81 76.08 72.33
TiO2 0.13 0.14 0.22 0.07 0.10 0.10 0.09 0.12 0.10 0.15 0.08 0.09 0.14 0.14 0.14 0.15 0.12 0.10 0.16 0.12 0.14
Al2O3 9.76 12.82 12.07 9.68 15.45 13.19 12.91 14.57 12.97 14.24 5.50 13.33 14.21 14.44 15.70 14.76 14.15 14.40 15.14 12.62 13.83
Fe2O3 0.83 0.67 2.50 0.62 0.80 0.79 1.02 0.85 5.66 0.95 0.56 1.01 1.03 1.26 1.11 1.20 1.00 1.11 1.24 1.15 1.18
MnO 0.04 0.02 0.08 0.02 0.03 0.02 0.01 0.03 0.06 0.03 0.01 0.04 0.03 0.04 0.04 0.04 0.03 0.04 0.05 0.04 0.04
MgO 0.74 0.61 0.94 0.32 0.28 0.30 0.29 0.33 0.31 0.28 0.22 0.25 0.29 0.33 0.30 0.38 0.27 0.28 0.33 0.49 0.37
CaO 1.21 0.89 2.87 2.13 0.92 0.82 0.56 0.67 0.51 0.53 0.41 0.63 0.60 0.81 0.77 0.65 0.52 0.59 0.66 0.52 0.65
Na2O 1.22 1.63 2.12 1.55 3.60 2.66 2.41 3.34 2.55 3.30 0.83 3.32 3.35 3.43 3.96 3.49 3.54 3.76 3.88 1.82 3.49
K2O 4.09 5.04 4.55 4.35 6.06 5.84 5.68 5.15 5.52 5.28 1.89 4.42 5.00 4.98 5.37 5.44 5.11 4.89 5.25 5.37 5.54
P2O5 0.13 0.14 0.22 0.18 0.27 0.22 0.21 0.26 0.24 0.28 0.12 0.24 0.26 0.26 0.29 0.29 0.25 0.27 0.32 0.24 0.26
SO3 0.33 0.12 0.29 0.09 0.06 0.18 0.15 0.02 0.10 0.02 0.05 0.05 0.03 0.02 0.01 0.01 0.03 0.01 0.02 0.07 0.01
LOI 1.95 1.44 3.49 1.21 1.09 0.97 0.71 0.85 1.29 0.81 0.63 1.04 0.87 0.92 0.87 0.93 0.95 1.01 0.99 1.14 0.91
Total 99.30 100.18 99.48 99.68 99.21 99.23 99.90 100.47 99.28 99.50 100.09 99.84 100.19 99.37 99.66 99.99 100.19 98.93 99.84 99.66 98.75
mg kg-1
As 19.9 24.3 23.4 18.2 9.9 31.8 14.3 13.9 21.5 4.8 ,2 ,2 2.1 5.1 6.6 1.0 3.9 2.2 4.5 9.9 3.1
Ba 395.6 303.8 271.2 256.8 245.5 299.3 371.2 223.7 342.7 190.0 83.5 186.5 200.0 186.0 196.6 194.0 173.5 172.3 186.9 174.3 176.0
Bi 3.1 ,1 2.1 ,1 3.3 3.4 1.4 1.7 1.9 ,1 30.7 2.7 1.3 ,1 1.3 2.0 ,2 2.1 3.0 2.3 1.4
Ce 23.1 40.9 36.8 30.4 37.5 31.3 22.1 30.6 25.3 32.5 22.6 34.3 37.2 32.8 34.7 34.4 32.8 40.4 37.6 30.3 35.4
Co ,1 ,1 3.5 ,1 ,1 ,1 ,1 ,1 15.3 ,1 ,2 ,2 ,1 ,1 ,1 ,1 ,2 ,2 ,2 ,1 ,1
Cr 198.5 214.2 171.5 205.8 152.6 173.3 225.3 149.4 584.7 147.3 157.8 113.2 171.5 171.0 197.5 145.2 117.3 137.9 121.3 217.2 153.8
Cs 31.1 32.9 38.8 26.8 29.7 23.6 31.1 33.8 45.8 33.5 31.1 37.3 37.6 31.4 37.8 44.6 43.0 38.9 40.9 61.9 54.2
Cu 23.9 15.6 23.7 7.5 19.9 1.8 2.3 1.2 27.8 ,1 ,2 ,2 ,1 3.7 1.4 2.5 ,2 ,2 ,2 5.6 ,1
Ga 13.7 15.6 17.3 15.3 22.5 17.5 18.4 20.4 21.6 22.5 10.0 21.7 20.7 21.6 22.3 23.7 23.2 23.0 24.3 20.0 23.1
Ge ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,2 ,2 ,1 ,1 ,1 ,1 ,2 ,2 ,2 ,1 ,1
Hf 3.2 4.1 4.2 2.8 3.7 2.2 2.5 3.5 1.9 3.8 2.4 2.5 3.4 2.3 2.6 4.2 3.3 3.4 4.4 2.3 2.9
La 19.1 25.6 25.1 17.5 21.7 23.0 18.2 20.0 20.2 23.7 12.5 18.6 20.2 20.8 20.7 20.6 17.9 20.7 21.8 22.5 23.2
Mo 2.7 2.3 3.4 2.7 ,1 1.3 7.1 ,1 68.3 1.9 3.3 2.6 2.8 2.7 1.5 1.5 ,2 2.0 ,2 3.5 2.3
Nb 8.5 9.4 11.5 10.6 9.6 11.5 10.2 12.4 10.7 15.2 9.0 12.1 13.7 13.0 15.5 16.0 14.6 15.8 17.3 14.2 16.4
Nd 11.8 17.6 17.4 15.3 19.3 16.6 14.1 15.1 13.0 19.4 12.0 13.3 18.4 13.8 17.3 18.1 15.6 17.7 15.5 15.8 18.1
Ni ,1 2.2 4.7 ,1 1.3 ,1 1.3 ,1 26.5 ,1 ,2 ,2 ,1 ,1 ,1 ,1 ,2 ,2 ,2 ,1 ,1
Pb 20.7 160.3 146.1 16.8 27.3 18.9 21.4 20.2 20.1 14.1 9.5 38.1 47.7 35.9 30.0 23.6 31.7 25.8 18.8 20.3 14.8
Rb 270.3 295.0 365.6 361.3 483.7 444.8 471.2 444.3 485.6 462.0 259.8 439.8 445.9 433.2 459.9 469.9 474.6 486.7 504.3 501.6 456.8
Sc 2.5 3.4 4.4 3.1 2.7 ,1 ,1 ,1 ,1 1.5 2.8 4.1 3.6 3.1 ,1 3.1 4.3 ,2 ,2 ,1 2.2
Sn 14.5 9.4 8.9 13.7 16.2 15.1 11.7 17.0 23.8 18.5 8.5 15.3 20.1 12.2 18.7 20.6 19.4 20.6 24.6 16.2 21.3
Sr 46.1 70.9 99.5 37.6 76.8 54.5 49.7 58.9 64.9 53.4 21.7 63.7 56.9 69.5 76.5 66.6 63.3 65.8 65.5 54.6 66.3
Ta ,1 ,1 6.3 4.8 ,1 ,1 ,1 2.7 ,1 ,1 3.6 2.7 1.4 6.3 6.4 ,1 4.3 4.7 4.6 5.3 3.2
Th 8.6 13.2 15.0 11.1 12.4 12.3 13.1 10.0 10.5 12.2 15.4 19.6 14.4 15.2 14.7 16.5 17.3 19.9 21.3 12.6 15.6
U 6.2 4.9 7.1 5.3 9.5 8.9 11.2 8.9 8.2 9.4 4.4 8.4 10.1 13.4 10.7 11.5 9.4 6.8 11.2 3.9 10.5
V 15.9 21.6 29.1 10.3 10.8 15.2 11.5 12.4 14.7 11.6 9.2 11.7 14.6 12.4 14.3 11.4 9.9 9.9 13.7 12.9 15.3
W ,1 1.6 ,1 ,1 ,1 ,1 2.5 1.9 46.2 1.9 34.4 4.6 3.7 2.7 1.1 ,1 2.7 5.4 3.3 1.5 2.3
Y 3.8 4.3 6.5 3.5 1.2 ,1 ,1 2.8 ,1 ,1 ,2 ,2 1.2 2.0 3.6 ,1 ,2 ,2 ,2 1.6 ,1
Zn 5.3 7.0 30.5 14.2 30.3 164.9 15.2 28.3 23.2 27.4 8.4 28.3 29.0 66.9 35.9 39.9 37.8 39.2 42.0 39.5 35.9
Zr 72.0 93.2 86.0 47.2 72.0 67.6 52.1 71.7 1.8 90.8 36.6 68.3 76.8 73.1 79.5 80.6 71.3 84.3 83.9 70.3 80.3
Cl 141.6 86.1 171.2 106.8 168.4 174.4 89.7 43.8 541.9 65.5 178.3 168.5 67.4 57.2 52.3 142.5 138.4 173.4 123.0 89.7 67.0
F 1147.6 2251.7 1591.7 11581.0 1464.8 2128.5 2826.3 1717.1 2029.4 2182.5 1083.4 1966.8 2366.7 2784.4 2453.7 2720.6 2091.3 2198.8 1706.3 1862.2 2147.9
S 1172.8 129.0 588.7 133.6 192.4 251.9 699.6 357.0 100.8 338.0 26.2 34.5 229.6 382.8 360.8 111.8 49.5 67.2 71.0 288.8 252.2
Heat
production
(�W m-3)
2.57 2.67 3.29 2.55 3.87 3.69 4.31 3.48 3.36 3.76 2.35 3.91 4.04 4.94 4.26 4.59 4.09 3.58 4.80 2.41 4.29
LOI, loss on ignition.
D.
A.
C.
MA
NN
ING
ET
AL
.3
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subsequent feeders would be expected to be minimal unless there
was unusually intense fracturing. As expected, after the second
casing had been grouted into the borehole, water yield dropped
to zero, and this continued for a further 7 m. However, at around
410 m below ground, the drill stem pressure gauge jumped to
23 bar, and the drill bit suddenly dropped by 0.5 m. At this point
the pressure gauge went off-scale (.30 bar), and water surged
into the hole, rapidly rising to within 10 m of the ground surface.
It is clear that a major open fissure had been encountered at this
point. The electrical conductivity of this water greatly exceeded
that of the waters previously encountered in the Carboniferous,
and it was also warm to the touch (around 26 8C). Air-lifting at
rates of up to 60 m3 h�1 (maximum capacity of the equipment)
failed to lower the water level by more than 1 m, indicating a
transmissivity in excess of 2000 darcy m. This is believed to be
an unequalled value for granites worldwide (E. Sudicky, pers.
comm.), although it clearly reflects the influence of the Slitt
Vein, rather than the permeability of more typical extensional
joints and faults typical of most plutons. Other fractures were
intersected at depths of 436, 464, 492–496, 654, 720–721, 739.5
and 813–814.5 m. These fracture intersections were not accom-
panied by dramatic events such as occurred at 410 m, and the
quantities of water that they introduced to the borehole were
difficult to discern given that the 410 m feeder had already
exceeded the air-lift capacity of the rig. A gradual increase in the
temperature of the water arriving at the well head (.27 8C
towards the end of drilling) indicated that a significant amount of
warmer deeper water was mixing with the 410 m feeder water.
After the end of drilling geophysical logs for fluid tempera-
ture, conductivity and flow rate (by impeller) were run twice:
first through the static water column, and then with a 100 mm
electric submersible pump stimulating the borehole water column
by pumping from just below the water surface at a rate of around
1.4 m3 h�1. Comparison of the two suites of logs indicates
Fig. 4. Distribution with depth of CaO, Na2O and K2O within the
Weardale Granite for the Eastgate Borehole (this study) and the
Rookhope Borehole (Brown et al. 1987).
Fig. 5. Distribution with depth of Rb and Sr within the Weardale Granite
for the Eastgate Borehole (this study) and the Rookhope Borehole
(Brown et al. 1987).
Fig. 6. Distribution with depth of Th, U and K within the Weardale
Granite for the Eastgate Borehole (this study) and the Rookhope
Borehole (Brown et al. 1987).
GEOTHERMAL EXPLORATION WELL, WEARDALE, UK 377
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significant water feeders associated with fractures at c. 730 and
756 m depth. (Other feeders were also indicated by the geophy-
sical logs at 420, 434, 447, 485, 497, 527, 540, 557, 670 and
686 m.) Although none of these were as prolific as the 410 m
feeder, they demonstrate the occurrence of permeable fractures at
depths approaching those that could be considered for a long-
term production borehole.
Groundwater quality
The electrical conductivity and temperature of water from the
borehole were measured continuously at the cuttings separator.
Water samples were taken initially every 10–20 m, and every
50 m for depths .300 m. Cations were determined using induc-
tively coupled plasma atomic emission spectroscopy, alkalinity
by titration, ammonium inductively coupled plasma atomic
emission Kjeldahl digestion and anions by ion chromatography.
Water samples up to and including 86.5 m were acidified before
filtration; this meant that dissolved suspended solids were
reported in the chemical analysis, giving misleading results that
overestimated dissolved cations (high positive charge balances).
From 135 to 995 m samples were filtered prior to acidification,
with charge balances predominantly less than �5%. Analyses are
given in full in Table 4.
With depth, conductivity and temperature rise until c. 400 m
(Fig. 8), at which point there is a sharp increase in both. This
corresponds to the point at which the hole intersected the large
open fracture at 410 m. Individual chemical species show con-
trasting behaviour with depth. The major cations (Na, K, Mg, Ca),
lithium, strontium and chloride increase in the same way as
conductivity, with a sudden increase once the 410 m feeder had
been intercepted (Fig. 9). Observed pH decreases, from 7.6–8.1 to
5.8–6.0, probably reflecting the absence of bicarbonate buffering
at depth: alkalinity (expressed as mg l�1 CaCO3) decreases with
depth to very low values, as does dissolved sulphate.
The constancy of the chemical composition of the water below
400 m reflects a combination of two possible causes: the water
column within the hole is well mixed as a consequence of the
drilling operation and/or the contribution of water from the
fissure at 410 m is sufficiently strong to dominate water chem-
istry during drilling. There is no evidence from the water
chemistry of any substantial flow of water that is more saline
(i.e. deeper sourced) or less saline (i.e. derived from nearer the
surface) into the hole at depths greater than 410 m.
The deep brine encountered by the borehole has as dominant
solutes 27 700 mg l�1 chloride, 10 030 mg l�1 sodium and
5320 mg l�1 calcium. Comparing its composition with water
recovered from Cambokeels mine (Manning & Strutt 1990; Table
5), there is little doubt that the Eastgate Borehole has intersected
the same water system (over 15 years after the brine was sampled
in the mine) where it is deeper, more saline and warmer.
Importantly, the Eastgate Borehole water is about 25% more
saline than seawater, but less than half (36%) the salinity of the
water currently produced at the Southampton geothermal plant. It
is more than twice the salinity of thermal waters reported from
South Crofty Mine (Edmunds et al. 1984). As in South Crofty,
molar Cl/Br ratios for the Eastgate water are below that typical
of seawater (650), which could be attributed to loss of Cl via
precipitation of halite through evaporation; in contrast, the high
value for Southampton suggests halite dissolution. Molar Na/Li
ratios are very low for the Eastgate and South Crofty waters,
indicating substantial interaction with basement rocks, and their
molar Na/Br ratios are again consistent with water–rock inter-
action (whereas the Southampton water’s high Na/Li and Na/Br
ratios are consistent with halite dissolution). The Eastgate water
is close in composition to that identified by Cann & Banks
(2001) as being associated with fluorite zone mineralization.
Although unlikely to be a mineralizing fluid in its own right
(although silica minerals are likely to have precipitated) the
Eastgate water seems to have shared a common fluid–rock
interaction history with the waters responsible for fluorite miner-
alization. Psyrillos et al. (2003) discussed the role of water from
adjacent basins as part of the kaolinization process in the SW
England granites. As the South Crofty and Eastgate waters are so
similar in their major solute proportions, it is likely that
analogous processes led to their formation, and the Eastgate
waters may be derived ultimately from adjacent deep aquifers.
Using the chemical data given in Table 5, temperatures at
Fig. 7. Variation with depth of calculated heat production for the
Weardale Granite from the Eastgate and Rookhope boreholes.
Table 3. Summary of thermal properties for granite from the EastgateBorehole
Average heat production 4.07 �W m�3
Thermal conductivity 2.99 W m�1 K�1
Surface heat flow at Eastgate 115 mW m�2
D. A. C. MANNING ET AL.378
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Table 4. Water compositional data (in mg l�1) for samples taken from the Eastgate Borehole (135 m and deeper)
Sample: E9 E10 E11 E12 E13 E14 E15 E16 E17 E18 E19 E20Date (2004): 11/9 11/9 11/9 12/9 15/9 15/9 16/9 16/9 16/9 6/10 6/10 7/10Depth (m): 135 144 152 166 181 192 214.5 234 236.5 266.5 275 300Temperature (8C): 14 14.1 14.1 14.3 14.6 15 14.9 16.4 17 17.2 17.9 18.9Cond. (field) (�S cm�1): 3750 3870 3900 4390 5150 5780 5680 11820 14940 27390 27130 26150pH: 7.6 7.8 7.8 7.8 7.9 8.1 8.1 7.7 7.8 6.4 6.8 6.9Cond. (lab) (�S cm�1): 3600 3794 3867 4215 5204 5502 5802 13290 14560 18740 16100 16030Alkalinity: 256 246 240 246 250 256 246 202 206 110 142 126
Nitrate ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5Chloride 559 648 826 761 1037 1123 1231 3895 4262 7449 5810 5906Sulphate 466 444 399 412 389 392 378 290 289 269 291 290BromideAmmoniumCalcium 222 228 231 244 293 316 339 807 969 1170 976 1066Magnesium 9.5 8.5 8.4 8.6 8.1 9.6 8.2 16.8 16.2 25 22.4 20.4Sodium 484 510 521 552 676 705 730 1984 2248 2697 2205 2243Potassium 40.1 42.9 44.1 42.3 51.2 52.4 52.5 103 111 159 138 140Iron ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 0.4Manganese 0.5 0.7 0.6 0.8 0.8 0.8 1.1 1.7 2 2.2 1.5 1.6Zinc ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 0.2 0.2 ,0.1 ,0.1 ,0.1Copper ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1Lead ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2Lithium 3.4 3.7 3.8 4.3 5.5 5.9 6.2 16.1 17.4 25.1 20.1 20.5Silicon 6 6 6 6 6 6 6 6 6 5 5 4Strontium 8.1 8.5 8.7 10.0 12.5 13.7 14.3 39.6 45.65 76.8 60.5 61.8BariumCharge balance (%) 6.2 5.4 0.5 5.3 5.4 5.1 4.5 5.5 8.3 �7.5 �5.6 �4.3
Sample: E21 E22 E23 E24 E25 E26 E27 E28 E29 E29A E30 E31Date (2004): 8/10 19/10 27/10 28/10 29/10 2/11 6/11 9/11 25/11 30/11 1/12 2/12Depth (m): 335 411.5 485 561 590 674 725 770 847 910 951 995Temperature (8C): 19.2 26 24.5 26 25.5 26.2 26.6 26.5 26.6 26 27 –Cond. (field) (�S cm�1): 26010 181000 181200 188900 190100 279600 210903 221014 212000 215000 239000 –pH: 6.9 6.2 6.4 6.4 6.3 6.4 6.4 6.4 5.8 5.9 6 6Cond. (lab) (�S cm�1): 23210 65400 65700 66200 66500 66800 63200 64000 65800 66500 66200 65200Alkalinity: 130 60 56 56 58 54 56 50 54 58 47 56
Nitrate ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5Chloride 9486 28750 25840 26970 25660 28560 25730 25700 28400 30000 31200 27800Sulphate 255 48.5 52.2 44.7 46.9 47.8 68 69 41.2 44.3 43.4 45.3Bromide 140 140 150 160 140 140 130 160 150 150 140Ammonium 11 – – – – – – 11 12 11 11Calcium 1595 5285 5256 5424 5345 5250 5410 5620 5312 5375 5264 5009Magnesium 28.6 72.4 71.9 72.8 72.1 73.1 68.9 69.1 79.2 79.2 78.2 75.3Sodium 3333 9630 9580 9940 10000 9790 9930 9940 10100 10300 11000 10100Potassium 201 631 642 646 667 656 782 551 689 708 646 638Iron ,0.1 0.4 ,0.1 0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1Manganese 4 20.3 19.1 18.3 19.8 17.6 21.5 22 19 18.7 19.4 19Zinc ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1Copper ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1Lead ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2Lithium 30.7 90.6 91.7 93.2 93 93.5 92.8 94.8 94.3 91.3 91.3 89.5Silicon 5 6 4 4 3 3 6 4 5 4 5 6Strontium 103 343 344 352 350 353 304 313 311 315 313 305Barium 12.9 13.6 12 13 13.3 12.5 14.7 13.8 12.3 12.2 11.5Charge balance (%) �7.0 �5.6 �0.5 �1.0 1.5 �4.8 1.4 1.8 �3.4 �5.3 �5.8 �3.5
Alkalinity is measured as mg l�1 as CaCO3.
GE
OT
HE
RM
AL
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WE
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UK
37
9
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which the water may have equilibrated can be estimated
(Truesdell 1984; Table 6). The quartz geothermometer gives a
temperature of 38 8C (less than the observed bottom-hole tem-
perature of 46 8C), whereas the alkali geothermometers give
temperatures between 146 and 191 8C, similar to those calculated
by Manning & Strutt (1990). It is likely that the water has lost
silica by precipitation of quartz close to the surface and possibly
in the recent geological past (quartz precipitation is abundant
within fractures in the Slitt Vein). The alkali geothermometers
suggest that the water achieved equilibrium with respect to Na,
K and Ca at depths of 3–4 km (assuming a geothermal gradient
of 40 8C km�1). This suggests that the water in the borehole
forms part of a deep circulation system, which appears still to be
active given the similarity of the water to that encountered at
Cambokeels between 15 and 20 years ago.
Geothermal potential
Geophysical logging of the settled water column in the borehole,
3 days after the end of drilling, indicated the magnitude of the
geothermal resource potentially developable in this vicinity. The
bottom-hole temperature at 995 m was 46.2 8C, which yields a
mean geothermal gradient estimate for this borehole of
38 8C km�1. This compares very favourably with the UK average
(c. 21 8C km�1; Downing & Gray 1986), from which a bottom-
hole temperature of only 30–35 8C would be predicted at this
depth. As the geothermal gradient is likely to continue on the
same linear trend as logged from 411 m to 995 m, the implica-
tion of the measured bottom-hole temperature at 995 m is that a
borehole sunk to a typical ‘production’ depth of about 1800 m
would be expected to return a bottom-hole temperature in the
range 75–80 8C.
Given that the heat production capacity of the Weardale
Granite at Eastgate is similar to that previously calculated by
Downing & Gray (1986), the key issues relate to the availability
of natural groundwater to act as a transmission fluid for heat
produced in the granite at various distances from the borehole.
The extraordinary transmissivity of the major fracture at around
410 m depth provides unequivocal evidence of the association of
highly permeable fractures with the Slitt Vein structure. Although
the deeper fractures were not quite so permeable, geophysical
logging indicates that they are still significant water-bearing
structures. The occurrence of permeable fractures associated with
the Slitt Vein at depth means that there is no need to artificially
introduce water to the geological environment, in contrast to the
usual assumption that similar UK granites are (at best) HDR
prospects (Downing & Gray 1986; Barker et al. 2000). Instead of
an HDR prospect, therefore, the Eastgate resource may be
classified as a ‘low-enthalpy hydrogeothermal resource hosted in
vein-bearing granites’. This is the first time that this category of
geothermal resource has been described (see Downing & Gray
1986; R. A. Downing, pers. comm.). Similar veins occur within
the nearby (also radiothermal) granites of the Lake District and
Wensleydale. Elsewhere, the Lecht Mine has a similar structure
in the radiothermal granite of the Eastern Highlands, and the SW
England Batholith hosts many analogous lodes, some known to
Fig. 8. Variation of (a) water temperature and (b) electrical conductivity
with depth in samples taken for analysis.
Fig. 9. Variations with depth in (a) pH, (b) alkalinity (mg l�1 CaCO3)
and sulphate, (c) calcium, sodium and chloride and (d) lithium and
strontium.
D. A. C. MANNING ET AL.380
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have transmitted brines similar to that found at Eastgate
(Edmunds et al. 1984). It may be that further exploration in
geological settings similar to that at Eastgate would lead to the
identification of other geothermal resources of significant magni-
tude.
Table 7 compares aspects of the Eastgate Exploration Borehole
with the existing Southampton Borehole. Given that the amount
of water that may be produced from Eastgate is very similar to
that yielded by the Southampton Borehole, the latter is an
appropriate model for possible future development at Eastgate.
The Eastgate water has the advantage that it is much less saline
than the Southampton water (44 500 mg l�1 total dissolved solids
(TDS) compared with 124 440 mg l�1 TDS at Southampton).
Given the findings from the Eastgate exploration borehole, it
is reasonable to predict that a further borehole sunk to 1800 m
could provide a resource similar in magnitude to that at South-
ampton. However, because the Southampton borehole was drilled
to intersect a more or less horizontal aquifer unit, a production
borehole at Eastgate would need to intersect fractures associated
with the Slitt Vein at the target depth. From the experience of
drilling this exploration borehole, it is unlikely that a depth
of 1800 m could be reached without intersecting a number of
shallower fractures carrying cooler water. Further casing close to
the total depth would therefore be needed, so that shallower
feeders could be sealed off before drilling on in pursuit of
sufficient large fractures at depth to yield a viable low-enthalpy
resource. Alternatively, a new production borehole could be
deliberately drilled off-centre from the target vein structure, with
directional drilling techniques being used to deviate the lower-
most parts of the borehole until they contact the permeable
fractures along the vein azimuth. Implementation of either of
these options would be both the most significant engineering
Table 5. Water compositions (in mg l�1) for samples taken from the Eastgate Borehole (mean of 10 samples below 400 m), compared with analysesreported for water from Cambokeels Mine (Manning & Strutt 1990), from the Southampton geothermal project (Downing & Gray 1986), and for seawater
Eastgate deepbrine
Cambokeels minewater 1988
South Crofty 380level
Southamptongeothermal water
Seawater
Temperature (8C): 26.1 16 41.5 76Electrical conductivity (�S cm�1): 65591Discharge (m3 day�1): 1600 300 1700pH: 6.2 6.50 6Alkalinity (mg l�1 as CaCO3): 55 111 62
Sodium 10028 7345 4300 41300 10770Potassium 660 340 180 705 380Magnesium 73.8 121 73 752 1294Calcium 5323 5236 2470 4240 412Ammonium 11.2 36Iron ,0.1 4.75 4.1Manganese ,0.1 4.50 1.26Zinc ,0.1Copper ,0.1 0.023Lead ,0.2Lithium 92.4 70.5 125 31 0.18Strontium 328 134 7.9Barium 12.9 0.52Silica (SiO2) 9.6 34.2 32.1Chloride 27692 24600 11500 75900 19350Sulphate 50.1 145 1230 2712Nitrate ,5 ,0.2Bromide 146 43 91 67TDS 44482 37713 19002 124440 35000Molar Cl/Br 427 602 1877 650Molar Na/Li 32.7 10.4 402 18060Molar Na/Br 239 348 1577 558
Table 6. Temperatures derived from borehole water compositions usingchemical geothermometers (Truesdell 1984)
Geothermometer Eastgate Cambokeels
Quartz 38 43NaK (Fournier) 184 169NaK (Truesdell) 146 129NaKCa 191 159
Table 7. Comparison of aspects of the Eastgate Exploration Borehole with the existing production facility at Southampton
Eastgate (existing borehole only) Southampton
Temperature (8C) 27 (bulk, mixed water in late stages of drilling)46 (bottom-hole temperature)
76 (water)
Depth (km) 1 2Yield (m3 day�1) .1600 860 (originally 2330 in 1987)Salinity (mg l�1) 44500 124500Power (MW) c. 0.75 1.4 (originally 2 in 1987)
GEOTHERMAL EXPLORATION WELL, WEARDALE, UK 381
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challenge and the most significant risk element in proceeding to
full-scale geothermal energy production at Eastgate.
We acknowledge funding by the Weardale Task Force, made up of One
NorthEast, Durham County Council, Wear Valley District Council and
Lafarge Cement. We are grateful to Rob Westaway for his help in
reviewing this paper. This project received funding from One NorthEast
through the County Durham Sub Regional Partnership. The project was
part-financed by the European Union, European Regional Development
Fund.
References
Bachler, D. & Kohl, T. 2005. Coupled thermal–hydraulic–chemical modelling
of enhanced geothermal systems. Geophysical Journal International, 161,
533–548.
Banks, D., Skarphagen, H., Wiltshire, R. & Jessop, C. 2004. Heat pumps as a
tool for energy recovery from mining wastes. In: Giere, R. & Stille, P.
(eds) Energy, Waste, and the Environment: a Geochemical Perspective.
Geological Society, London, Special Publications, 236, 499–513.
Barker, J.A., Downing, R.A., Gray, D.A., Findlay, J., Kellaway, G.A.,
Parker, R.H. & Rollin, K.E. 2000. Hydrogeothermal studies in the United
Kingdom. Quarterly Journal of Engineering Geology and Hydrogeology, 33,
41–58.
Bott, M.H.P. & Masson-Smith, D. 1953. Gravity measurements over the northern
Pennines. Geological Magazine, 90, 127–130.
Bott, M.H.P. & Masson-Smith, D. 1957. The geological interpretation of a
gravity survey of the Alston Block and the Durham coalfield. Quarterly
Journal of the Geological Society of London, 113, 93–117.
Brown, G.C., Ixer, R.A., Plant, J.A. & Webb, P.C. 1987. Geochemistry of
granites beneath the north Pennines and their role in orefield mineralization.
Transactions of the Institution of Mining and Metallurgy, 96, B65–B76.
Caneta Research 1999. Global warming impacts of ground source heat pumps
compared to other heating/cooling systems. Report to the Renewable and
Electrical Energy Division, Natural Resources Canada (Ottawa). Caneta
Research, Mississauga.
Cann, J.R. & Banks, D.A. 2001. Constraints on the genesis of the mineralization
of the Alston Block, Northern Pennine Orefield, northern England. Proceed-
ings of the Yorkshire Geological Society, 53, 187–196.
Creaney, S.D. 1980. Petrographic texture and vitrinite reflectance variation on the
Alston Block, north-east England. Proceedings of the Yorkshire Geological
Society, 42, 553–580.
Dickson, M.H. & Fanelli, M. 2005. Geothermal Energy: Utilization and
Technology. Earthscan, London.
DiPippo, R. 2005. Geothermal Power Plants. Principles, Applications and Case
Studies. Elsevier, Oxford.
Downing, R.A. & Gray, D.A. (eds) 1986. Geothermal Energy—the Potential in
the United Kingdom. HMSO, London.
Dunham, K.C. 1987. Discussion of G. C. Brown et al. Geochemistry of granites
beneath the north Pennines and their role in orefield mineralization.
Transactions of the Institution of Mining and Metallurgy, 96, B65–B76.
Transactions of the Institution of Mining and Metallurgy, 96, B229–B230.
Dunham, K.C. 1990. Geology of the Northern Pennine Orefield. Volume 1—Tyne to
Stainmore. Economic Memoir of the British Geological Survey, HMSO,
London.
Dunham, K.C., Dunham, A.C., Hodge, B.L. & Johnson, G.A.L. 1965. Granite
beneath Visean sediments with mineralization at Rookhope, northern Pennines.
Quarterly Journal of the Geological Society of London, 121, 383–417.
Edmunds, W.M., Andrews, J.N., Burgess, W.G., Kay, R.L.F. & Lee, D.J. 1984.
The evolution of saline and thermal groundwaters in the Carnmenellis granite.
Mineralogical Magazine, 48, 407–424.
Fitch, F.J. & Miller, J.A. 1967. The age of the Whin Sill. Geological Journal, 5,
233–250.
Fraser, A.J. & Gawthorpe, R.L. 2003. An Atlas of Carboniferous Basin Evolution
in Northern England. Geological Society, London, Memoirs, 28.
Goulty, N.R. 2005. Emplacement mechanism of the Great Whin and Midland
Valley dolerite sills. Journal of the Geological Society, London, 162, 1047–
1056.
Karweil, J. 1955. Die Metamorphose der Kohlen vom Standpunkt der physika-
lischen Chemie. Zeitschrift der Deutschen Geologischen Gesellschaft, 107,
132–139.
Kolditz, O. & Clauser, C. 1998. Numerical simulation of flow and heat transfer
in fractured crystalline rocks: application to the Hot Dry Rock site in
Rosemanowes (UK). Geothermics, 27, 1–23.
Lund, J.W., Lienau, P.J. & Lunis, B.C. (eds) 1998. Geothermal direct-use
engineering and design handbook. Geo-Heat Center, Oregon Institute of
Technology, Klamath Falls, OR.
Manning, D.A.C. & Strutt, D.W. 1990. Metallogenetic significance of a North
Pennine springwater. Mineralogical Magazine, 54, 629–636.
Psyrillos, A., Burley, S.D., Manning, D.A.C. & Fallick, A.E. 2003. Coupled
mineral–fluid evolution of a basin and high: kaolinization in the SW England
granites in relation to the development of the Plymouth Basin. In: Petford,
N. & McCaffrey, K.J.W. (eds) Hydrocarbons in Crystalline Rocks.
Geological Society, London, Special Publications, 214, 175–195.
Richards, H.G., Parker, R.H. & Green, A.S.P. et al. 1994. The performance
and characteristics of the experimental hot dry rock geothermal reservoir at
Rosemanowes, Cornwall (1985–1988). Geothermics, 23, 73–109.
Sanner, B., Karytsas, C., Mendrinos, D. & Rybach, L. 2003. Current status of
ground source heat pumps and underground thermal energy storage in
Europe. Geothermics, 32, 579–588.
Truesdell, A.H. 1984. Chemical geothermometers for geothermal exploration. In:
Henley, R.W., Truesdell, A.H. & Barton, P.B. (eds) Fluid–Mineral
Equilibria in Hydrothermal Systems. Society of Economic Geologists, Re-
views in Economic Geology, 1, 31–44.
Webb, P.C., Lee, M.K. & Brown, G.C. 1987. Heat flow–heat production
relationships in the UK and the vertical distribution of heat production in
granite batholiths. Geophysical Research Letters, 14, 279–282.
Webb, P.C., Tindle, A.G., Barritt, S.D., Brown, G.C. & Miller, J.F. 1985.
Radiothermal granites of the United Kingdom: comparison of fractionation
patterns and variation of heat produced for selected granites. In: High Heat
Production (HHP) Granites, Hydrothermal Circulation and Ore Genesis.
Institution of Mining and Metallurgy, 203–213.
Younger, P.L. 2000. Nature and practical implications of heterogeneities in the
geochemistry of zinc-rich, alkaline mine waters in an underground F–Pb
mine in the UK. Applied Geochemistry, 15, 1383–1397.
Younger, P.L. & Adams, R. 1999. Predicting mine water rebound. Environment
Agency R&D Technical Report, W179.
Received 13 February 2006; revised typescript accepted 13 July 2006.
Scientific editing by Mike Fowler
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