Fluid chemistry and temperatures prior to exploitation at the Las Tres

Geothermics 35 (2006) 156–180
Fluid chemistry and temperatures prior to exploitation
at the Las Tres Vı́rgenes geothermal field, Mexico
Surendra P. Verma a,b,∗ , Kailasa Pandarinath a , Edgar Santoyo a ,
Eduardo González-Partida c , Ignacio S. Torres-Alvarado a ,
Enrique Tello-Hinojosa d
a
b
Centro de Investigación en Energı́a, UNAM, Privada Xochicalco S/N, Col. Centro,
Apartado Postal 34, Temixco 62580, Morelos, Mexico
Centro de Investigación en Ingenierı́a y Ciencias Aplicadas (CIICAp), Universidad Autónoma del Estado de
Morelos, Av. Universidad No. 1001, Col. Chamilpa, Cuernavaca 62210, Morelos, Mexico
c Centro de Geociencias, UNAM, Apartado Postal 1-742, Querétaro 76001, Querétaro, Mexico
d Gerencia de Estudios Geotermoeléctricos, Comisión Federal de Electricidad, Alejandro Volta 655,
Morelia 58290, Michoacán, Mexico
Received 4 April 2005; accepted 9 February 2006
Abstract
Generation of electricity at the Las Tres Vı́rgenes (LTV) geothermal field, Mexico, began in 2001. There
are currently nine geothermal wells in the field, which has an installed electricity generating capacity of
10 MWe . The chemical and temperature conditions prevailing in the field prior to its exploitation have been
estimated, including their central tendency and dispersion parameters. These conditions were computed on
the basis of: (i) geochemical data on waters from springs and domestic wells, and on geothermal well fluids
(waters and gases); most of the sampling took place between 1995 and 1999; (ii) fluid inclusion studies;
(iii) geothermometric data; and (iv) static formation temperatures computed using a modified quadratic
regression Horner method.
Fluid inclusion homogenization temperatures (in the 100–290 ◦ C range) suggest that there is a hightemperature fluid upflow zone near wells LV3 and LV4 in the southern part of the field. Computed average
chemical equilibrium temperatures for the geothermal fluids are ∼260 ◦ C, based on the Na/K and SiO2
geothermometers, and ∼265 ◦ C, based on the H2 /Ar, and CO2 /Ar geothermometers. In general, the fluid
inclusion homogenization temperatures are consistent with geothermometric data, as well as with static
formation temperatures. Some of the observed differences could be related to well interference effects and
different fluid production/sampling depths. The deeper geothermal waters show higher concentrations of
Cl, Na, K, B, Ba, but lower concentrations of SO4 , Ca, and Mg than the shallower waters. Fluid inclusion
∗
Corresponding author. Tel.: +52 55 5622 9745; fax: +52 55 5622 9791.
E-mail address: spv@cie.unam.mx (S.P. Verma).
0375-6505/$30.00 © 2006 CNR. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.geothermics.2006.02.002
S.P. Verma et al. / Geothermics 35 (2006) 156–180
157
salinities are also higher in the deeper rocks. The measured Na/Cl ratios of the geothermal well waters are
more or less uniform throughout the field and are very similar to that of seawater, strongly suggesting a
seawater component in the fluid of the LTV system.
The heat stored in the LTV geothermal system was estimated to be at least 9 × 1012 MJ, of which some
4 × 1011 MJ (equivalent to about 148 MWe for 30 years of operation, assuming a conversion efficiency of
∼35%) might be extracted using wells. These results indicate that the installed capacity at LTV could be
safely increased from the current 10 MWe .
© 2006 CNR. Published by Elsevier Ltd. All rights reserved.
Keywords: Hydrogeochemistry; Fluid inclusions; Static formation temperatures; Geothermometers; Seawater component;
Heat balance; Baja California; Mexico
1. Introduction
The Las Tres Vı́rgenes geothermal field (LTV) is located about 33 km northwest of the town of
Santa Rosalı́a in the state of Baja California Sur, Mexico (Fig. 1). Geothermal exploration in the
area began in 1982. To date, nine geothermal wells have been drilled in the field for production and
Fig. 1. Simplified tectonic setting of north-western Mexico showing the location of the Las Tres Vı́rgenes geothermal
field (LTV). BCS: Baja California Sur; BC: Baja California. The inset corresponds to the LTV and surrounding areas.
The numbered circles indicate the location of thermal springs and domestic wells. 1: Agua Caliente; 2: San Alberto; 3: La
Palma; 4: Santana; 5: La Reforma; 6: La Palmitas II; 7: El Mezquite; 8: El Álamo; 9: Yaqui; 10: El Palmito; 11: Rancho
Vı́rgenes; 12: Rancho Mezquital; 13: Santa Lucı́a; 14: La Cueva; 15: Bonfil; 16: San Regis; 17: San Ignacio. The areas
with a similar type of water are bounded by the dotted lines. The square area identified in the insert is shown in further
detail in Fig. 2.
158
S.P. Verma et al. / Geothermics 35 (2006) 156–180
re-injection of geothermal waters (Fig. 2). Two 5-MWe wellhead condensing units came on-line
in July 2001 (Gutiérrez-Negrı́n and Quijano-León, 2004; Bertani, 2005), making it the fourth and
latest geothermal field to come into operation in Mexico, after Cerro Prieto, Los Azufres, and Los
Humeros.
The geology of the Las Tres Vı́rgenes volcanic area has been discussed by Garduño-Monroy
et al. (1993), López-Hernández (1998), and Capra et al. (1998). Chemical data on the water and
gas of the system were presented by Portugal et al. (2000) and González Partida et al. (2001).
The LTV geothermal system is located within a Quaternary volcanic complex characterized by
three NE-SW aligned volcanoes and a caldera (Fig. 1). The heat source for the system seems to be
the magma chambers associated with this volcanic complex. The geothermal fluids are found in
fractures and faults of a Cretaceous granodiorite pluton below the Quaternary volcanics (Fig. 2).
We present the results of: (i) systematic fluid sampling and analyses in the LTV area, carried
out mainly during 1995–1999. The study covered waters from domestic water wells and thermal
springs, in addition to waters and gases from geothermal boreholes; (ii) petrographic and fluid
inclusion investigations of well cuttings and cores; (iii) application of Na/K, SiO2 , H2 /Ar, and
CO2 /Ar geothermometers; and (iv) computation of static formation temperatures based on well
log data. From the results obtained, we characterized the pre-exploitation (or natural-state) fluid
chemistry and temperature conditions in the LTV geothermal system. The estimated heat stored
in the system is also presented.
2. Geological setting
The LTV geothermal field is located some 200 km northeast of an extinct ocean trench (see the
paleo-subduction trench in Fig. 1), along the western edge of a deformation zone that is probably
linked to the dominantly extensional Quaternary fault systems associated with the formation and
opening of the Gulf of California (Garduño-Monroy et al., 1993; Capra et al., 1998).
The major volcanic features in the LTV area include: (1) the La Reforma caldera, formed about
1.2 Ma; (2) the El Aguajito caldera (0.8 Ma); and (3) the Tres Vı́rgenes complex, consisting of the
volcanoes El Viejo (∼0.44 Ma), El Azufre (more recent) and La Virgen (still active; Figs. 1 and 2).
The NE-SW alignment and ages indicate migration of volcanic activity to the southwest during
the last 0.8 Ma. According to Capra et al. (1998), the last eruptive phase of the La Virgen volcano
occurred about 6515 years B.P. and was characterized by both Plinian and Vulcanian eruptive
phases.
The basement in the LTV area is dominated by a biotite granodiorite emplaced at 81–84 Ma.
This pluton does not outcrop in the geothermal area, but has been intersected by all the geothermal
wells at a depth of approximately 1000 m (elevation of about −250 m above sea level). The plutonic
basement rocks are overlain by a sequence of sediments and pyroclastic rocks. From the bottom
up (Figs. 2 and 3), these are: (1) the Comundú Group, consisting of Miocene sandstones, andesitic
tuffs, breccias, and basalts; (2) the Santa Lucı́a andesites, which do not outcrop in the area; and
(3) the La Gloria Formation, consisting of even younger sandstones.
From the structural point of view, the LTV area is part of a NW-SE trending depression known
as the Santa Rosalı́a basin, which has persisted since the Late Miocene. The basin is bounded by
NW-SE extensional faults (Garduño-Monroy et al., 1993; López-Hernández, 1998) and its tectonic
blocks are sheared in a NE direction. A second NNE-SSW and N-S fault system, consisting mainly
of strike-slip faults, is related to the northwestward migration of the Baja California peninsula.
These faults cut the older extensional system as well as the Aguajito caldera and displace the
volcanoes of the Las Tres Vı́rgenes complex.
S.P. Verma et al. / Geothermics 35 (2006) 156–180
159
Fig. 2. Simplified geological map of the Las Tres Vı́rgenes geothermal field (modified after López-Hernández, 1998 and
Capra et al., 1998) showing the locations of geothermal wells and major faults. The schematic NW-SE cross-section
through wells LV5, LV1, LV8, LV4, LV3, and LV7 is shown in Fig. 3.
160
S.P. Verma et al. / Geothermics 35 (2006) 156–180
Fig. 3. Geologic cross-section drawn through six LTV wells (see Fig. 2 for location). The numbers beside the heavy black
dots correspond to average fluid homogenization temperatures (Th ; Table 4). The isotherms were drawn based on these
well temperature data. The black arrow indicates the area of hot upwelling fluid near the bottom of wells LV3 and LV4.
3. Methodology
Thermal waters from springs and domestic water wells were classified according to the modified Hill–Piper diagram using the computer program MHPT.BAS by Rao (1998). This approach
takes into greater account fluid electrical conductivity and the relative proportion of different types
of ions than the conventional Piper diagrams. Six LTV geothermal wells were sampled periodically between 1995 and 1999, although a few samples from 1992 and 1994 were also included
in our analysis. Diverse fluid chemical parameters, including major anions and cations, along
with electrical conductivity, were determined by means of conventional methods. Gases were
collected from five geothermal wells and analyzed using gas chromatography and wet-chemistry
procedures, as reported by Santoyo et al. (1991).
Two liquid (Na/K and SiO2 ; Verma and Santoyo, 1997) and two gas geothermometers (H2 /Ar
and CO2 /Ar; Giggenbach, 1991; Verma, 2002) were used to infer subsurface temperatures from
fluid samples taken from thermal springs and from domestic and geothermal wells.
S.P. Verma et al. / Geothermics 35 (2006) 156–180
161
Downhole temperature logs obtained during drilling were also used to compute static formation
temperatures (SFT), following the Horner method (Dowdle and Cobb, 1975), but the quadratic
regression model recently proposed by Andaverde et al. (2005) was used to correct for the nonlinear behavior of the measured temperature data. Error estimates were also obtained for these
quadratic regressions.
Petrographic analyses were performed on well cuttings collected in six geothermal wells
at 15 m depth intervals. This allowed us to identify the most common hydrothermal minerals
in the geothermal system, and to select the most suitable depth intervals for fluid inclusion
studies. Microthermometric measurements were made on doubly polished thin sections of well
cuttings and core samples that contained veins of hydrothermal minerals. The measurements
were performed on a calibrated Chaix-Meca heating–cooling stage. For each fluid inclusion,
three parameters were determined: (1) ice-melting temperatures (Tmi ), to estimate fluid salinities
(reported as weight percent, wt.%, NaCl equivalent); (2) homogenization temperatures (Th ), to
estimate minimum formation temperatures for the minerals being studied; and (3) dissociation
temperatures of CO2 clathrate (positive Tmi ) found in some samples.
Because of the large number of samples studied, it was decided to report only a synthesis of
the final statistical parameters (individual results are available on request from the first author).
Two common central-tendency parameters (median and mean) were estimated, along with the
conventional dispersion or scale parameter (standard deviation and, when the number of samples
in a given group was relatively large (n ≥ 5), the 95% confidence limits for the mean). For a smaller
number of samples, this statistical parameter becomes too large to be of much use because of
the very high values of Student’s t involved in the calculations (see any standard textbook on
Statistics, or Verma, 2005).
When the median and mean values are similar, it is said that a fair estimate of the central
tendency can be defined by any of the two parameters. For all sets of measurements, we detected
outliers with the basic model of normal data, using the SIPVADE computer program (Verma et al.,
1998). The remaining data (after outlier elimination) should then become “normal” according to
a large number of tests for normality, bringing the median and mean values into close agreement
(Barnett and Lewis, 1994; Verma, 2005). We therefore report only the mean values and the
associated basic statistical information (i.e., number of samples and standard deviation). The
95% confidence limits have not been included in the final tables, although they can be calculated
from the data given in the tables, but have been plotted in some diagrams and are discussed in
the text. The final data were rounded off before tabulation using the procedures described by
Bevington (1969) and modified by Verma (2005).
4. Results
The main results of our study are presented in this section, with their implications in
Section 5.
4.1. Composition of waters from thermal springs and domestic wells
The chemical compositions of the waters from thermal springs and domestic wells (part of the
data from Portugal et al. (2000) and González Partida et al. (2001)) are summarized in Table 1
and shown in Fig. 4. Based on the modified Hill–Piper diagram, the waters (all with a pH close
to neutral) are of 11 different types. None of the LTV waters have high salinities with respect
to typical geothermal waters; note that this terminology is relative only to the waters described
162
Table 1
Chemical composition of water samples from thermal springs and domestic wells of the Las Tres Vı́rgenes area, Mexico
Thermal springs and domestic wells
Stat Major anions
Cl−
All locations: carbonate, medium-to-high
salinity, low-to-medium sodium water
(B2C3S2) (n = 42)
x̄
s
x̄
s
x̄
s
x̄
55
283
25
21
96
370
43
80
43.6 280
1.8 10
14
–
Other physical–chemical parameters
Geothermometer
temperatures
K+
Li+
(mg/l)
Ca2+
Mg2+ SiO2
(mg/l)
B
(mg/l)
pH
T
(◦ C)
ϕ
(␮S/mm)
TA
(meq/l)
TNa/K
(◦ C)
TSiO2
(◦ C)
49
12
90
40
70
10
12
3.8
0.5
5.9
2.0
4.90
0.00
14
0.01
0.01
0.02
0.02
0.01
0.01
–
50
4
61
19
36.50
0.14
30
28
6
44
19
16.1
3.8
3.2
82.5
3.4
85
18
59.0
(0)
280
0.5
0.6
0.34
0.34
0.24
0.05
0.1
7.72
0.10
7.67
0.32
8.2
0.5
2.9
26.2
2.9
26.6
4.4
28
6
97
63
5
108
37
60.50
0.28
198
4.93
0.23
6.4
1.2
4.96
0.22
–
203
25
189
28
188
10
–
126.8
2.2
128
12
110
(0)
205
SO4 2−
Na+
11
8
24
23
10
10
850
x̄
s
x̄
136
25
120
323
23
220
800
500
28
235
29
120
18.2
2.8
1.6
0.15
0.03
–
185
39
38
93
43
21
130
20
60
1.18
0.27
0.2
7.66 28
0.10 4
7.9 23
250
50
83
5.7
0.5
4.0
200
19
96
154
9
111
x̄
140
260
280
240
11
0.07
69
22
94
0.33
7.95 35
170
4.6
154
133
s
x̄
100
284
70
275.2
230
600
50
310
6
20.3
0.03
0.16
16
140
7
67
21
102
0.21
1.20
0.40 11
7.68 24.0
34
265
1.3
4.78
32
188
12
138
s
x̄
25
1.2
192.0 250
200
1477
34
513
0.8
10
0.02
0.22
9
230
5
32
5
90
0.35
1.4
0.31 1.4
8
31
35
342
0.01
4.6
5
111
3
134
x̄
s
x̄
460
450
250
240
135.1 330
1000
1200
45
540
270
204
23
6
5.5
0.12
0.06
0.02
70
70
16.2
17
21
1.0
150
90
97
1.8
1.9
0.40
7.0 39
2.8 27
8.39 28
400
120
104.4
9.4
2.0
5.93
145
9
132.0
155
30
135
19
17
11
0.5
0.02
4.2
0.5
8
0.20
0.36
7
1.4
0.16
2.9
5
s
3.8
x̄
170
320
400
220
11
0.07
80
30
100
0.71
7.6
32
180
5.9
179
137
s
170
130
600
190
8
0.07
60
30
50
0.90
1.3
16
120
1.8
70
21
Stat: statistical parameter; n: number of measurements; x̄: mean; s: standard deviation; T: water temperature as measured at the surface; ϕ: electrical conductivity; TA: total
alkalinity; (–) not measured or calculated. Codes for water types (with anion and cation concentrations expressed in meq/l and ϕ in ␮S/cm; note that the unit ␮S/mm is used in
this table to be consistent with the international convention) are according to Rao (1998); for more details see caption to Fig. 4. Locations: 1 Cueva and La Cuevita; 2 Bonfil, San
Ignacio, El Tule, Regis, and Santa Lucı́a; 3 Rancho Mezquital; 4 Agua Agria; 5 La Palma and San Alberto; 6 Palmito; 7 Agua Caliente, Yaqui, Rancho Vı́rgenes, and Palmitas II;
8 Santa Ana; 9 El Carrizo; 10 Reforma, El Azufre, and Mezquite; 11 Alamo.
S.P. Verma et al. / Geothermics 35 (2006) 156–180
Non-carbonate, low-to-medium salinity,
low sodium water (A2C2S1)1 (n = 4)
Non-carbonate, medium-to-high salinity,
low sodium water (A2C3S1)2 (n = 9)
Non-carbonate, low-to-medium salinity,
low sodium water (A3C2S1)3 (n = 2)
Carbonate, medium-to-high salinity, low
sodium water (B1C3S1)4 (n = 1)
Carbonate, high-to-very high salinity, low
sodium water (B1C4S1)5 (n = 4)
Carbonate, medium-to-high salinity, low
sodium water (B2C3S1)6 (n = 1)
Carbonate, medium-to-high salinity,
low-to-medium sodium water
(B2C3S2)7 (n = 9)
Carbonate, high-to-very high salinity,
low-to-medium sodium water
(B2C4S2)8 (n = 2)
Carbonate, high-to-very high salinity,
medium-to-high sodium water
(B2C4S3)9 (n = 1)
Carbonate, high-to-very high salinity, very
high sodium water (B2C4S4)10 (n = 6)
Carbonate, medium-to-high salinity,
medium-to-high sodium water
(B3C3S3)11 (n = 3)
HCO3 −
(mg/l)
Major cations
S.P. Verma et al. / Geothermics 35 (2006) 156–180
163
Fig. 4. Hill–Piper classification diagram of waters from thermal spring and domestic wells in the LTV area, using
the computer program by Rao (1998). Water types (e.g., A2C2S1) are given in Table 1. A star indicates the average
composition of all samples. The fields are: A1 (Ca & Mg < HCO3 , Ca & Mg > Na & K, Cl & SO4 < HCO3 , residual
NaHCO3 negligible, non-carbonate waters); A2 (Ca & Mg > HCO3 , Ca & Mg > Na & K, Cl & SO4 > HCO3 , residual
NaHCO3 negligible, non-carbonate waters); A3 (Ca & Mg < HCO3 , Ca & Mg < Na & K, Cl & SO4 > HCO3 , residual
NaHCO3 negligible, non-carbonate waters); B1 (Ca & Mg < HCO3 , Ca & Mg > Na & K, Cl & SO4 < HCO3 , residual NaHCO3 present, carbonate waters); B2 (Ca & Mg < HCO3 , Ca & Mg < Na & K, Cl & SO4 < HCO3 , residual
NaHCO3 present, carbonate waters); B3 (Ca & Mg < HCO3 , Ca & Mg < Na & K, Cl & SO4 > HCO3 , residual NaHCO3
present, carbonate waters). The C followed by a number represents the specific electrical conductivity values in ␮S/mm;
C1 (<2.5 ␮S/mm) is low-salinity water, C2 (2.5–<75 ␮S/mm) is low-to-medium salinity water, C3 (75–<225 ␮S/mm)
medium-to-high salinity water, and C4 (225–625 ␮S/mm)
is high-to-very high salinity water. The S numbers are the
sodium adsorption ratios (SAR; defined as SAR = Na/ (Ca + Mg)/2) such that S1 (<10) is low-sodium water, S2
(10–<18) is low-to-medium sodium water, S3 (18–<26) is medium-to-high sodium water, and S4 (≥26) is very high
sodium water.
here. The most common types are (Table 1): (i) non-carbonate, medium-to-high salinity, low
sodium waters (nine samples from five locations); (ii) carbonate, medium-to-high salinity, lowto-medium sodium waters (nine samples from four locations); and (iii) carbonate, high-to-very
high salinity, very high sodium waters (six samples from three locations). In most cases, the
164
S.P. Verma et al. / Geothermics 35 (2006) 156–180
median (not given in Table 1) and mean values of the physical–chemical parameters showed a
close agreement, indicating that no outlying observations existed in the initial data set (i.e., the
chemical parameters represented a “normal” statistical sample). The overall mean (or median)
composition for all the thermal waters (42 samples; Table 1; Fig. 4) corresponds to the carbonate,
medium-to-high salinity, low-to-medium sodium water type.
4.2. Composition of geothermal well waters
Table 2 presents the final statistical parameters of the chemical data on water samples from six
geothermal wells (LV1, LV2, LV3, LV4, LV5, and LV8). Before outlier detection and elimination,
there were, in some cases, differences between the median and mean values (data not tabulated),
suggesting the presence of outlying observations. These two parameters became indistinguishable
after applying the SIPVADE statistical tests. The average compositions of the geothermal waters
were also used in their classification, applying the modified Hill–Piper diagram. Fig. 5 shows that
Fig. 5. Hill–Piper classification diagram for waters from individual LTV geothermal wells. The stars indicate the average
composition of all samples (see Fig. 4 for more details). Note that, because of the very high salinity, all samples fall
outside the salinity field in the diagram proposed by Rao (1998).
Table 2
Summary of the chemical composition of water samples from geothermal wells in the Las Tres Vı́rgenes geothermal field, Mexico
Well (wellhead elevation,
sampling depth, sampling
elevation)
Stat
ϕ (␮S/mm)
Concentration (mg/l)
Anions
Cations
Other chemical parameters
Cl−
HCO3 −
SO4 2−
Na+
K+
n
x̄
s
n
x̄
s
n
x̄
s
n
x̄
s
n
x̄
s
n
x̄
s
87
7880
340
2
4400
4700
100
7690
700
63
9650
1030
72
8570
330
23
5620
280
64
77
14
2
130
110
98
54
42
61
33
41
68
47
40
23
13
10
89
70
18
2
230
210
100
83
19
41
39
12
70
96
27
23
70
29
86
4435
120
2
2600
2400
100
4400
350
59
5500
360
72
4900
200
23
3260
110
86
700
60
2
400
450
100
740
70
59
1000
230
72
770
50
23
387
9
All wells (1270–2420 m vertical
depth; −622 to −1700 m
elevation)a
n
x̄
s
347
8100
1200
316
50
40
325
77
31
342
4620
640
342
760
180
Ca2+
Mg2+
SiO2
B
87
19
8
2
13
14
100
19
7
60
20
7
72
21
7
23
5.7
0.4
89
270
44
2
120
160
100
139
38
62
285
60
72
335
47
23
293
19
88
0.34
0.14
2
0.30
0.14
92
0.17
0.10
57
0.10
0.08
71
0.32
0.20
22
0.08
0.04
89
460
110
1
490
–
81
770
240
44
310
180
63
230
90
23
420
140
89
151
30
2
96
101
100
187
26
60
189
22
72
176
27
23
57
10
88
0.19
0.12
2
0.8
1.0
49
0.25
0.21
18
0.14
0.04
39
0.40
0.28
23
0.40
0.30
344
19
8
348
250
90
332
0.23
0.17
301
470
260
346
167
43
219
0.27
0.23
Fe
Geothermometer
Mn
56
0.278
0.035
2
0.14
0.11
47
0.17
0.18
18
0.14
0.03
39
0.20
0.14
–
–
–
162
0.21
0.13
As
Ba
Rb
47
10.5
1.8
2
4.4
4.4
24
9.6
1.6
10
9.2
4.6
5
9.8
3.2
–
–
–
58
0.36
0.06
2
0.20
0.26
50
0.87
0.26
17
1.48
0.44
44
0.16
0.10
–
–
–
21
5.15
0.30
–
–
–
18
4.50
0.30
34
6.0
0.9
18
5.4
0.8
–
–
–
91
5.4
0.9
88 171
9.9
0.6
2.5
0.5
TNa/K
TSiO2
84
2300
220
1
2200
–
100
2290
210
63
2780
200
72
2650
160
23
1680
80
86
260
6
2
238
36
100
267
7
59
273
17
72
260
6
23
234.2
3.3
89
250
26
1
256
–
81
320
60
44
213
42
63
194
20
23
241
34
351
2390
380
342
262
13
301
250
60
S.P. Verma et al. / Geothermics 35 (2006) 156–180
LV1 (741 m elevation) (1780 m
vertical depth; −1039 m
elevation)
LV2 (648 m elevation) (1270 m
vertical depth; −622 m
elevation)
LV3 (720 m elevation) (2134 m
vertical depth; −1414 m
elevation)
LV4 (720 m elevation) (2420 m
vertical depth; −1700 m
elevation)
LV5 (739 m elevation) (1815 m
vertical depth; −1076 m
elevation)
LV8 (725 m elevation) (1656 m
vertical depth; −931 m
elevation)
Li+
Temperature (◦ C)
The actual drilling length was somewhat greater than the vertical depth reported for each well; the vertical depth is the sampling depth corrected for well inclination; elevations
given in meters above sea level (m asl). n: number of measurements; x̄: mean; s: standard deviation. The geothermometers used were (Verma and Santoyo, 1997): TNa/K :
temperature estimated from Na/K geothermometer; TSiO2 : temperature estimated from SiO2 geothermometer.
a Waters from individual wells were classified using the computer program of Rao (1998). All geothermal well waters are of the type B2C4S4 (carbonate, high-to-very high
salinity, very high sodium water). For more details see legend to Table 1.
165
166
S.P. Verma et al. / Geothermics 35 (2006) 156–180
the waters from the geothermal wells are chemically similar. In fact, all average compositions
of their waters are of the carbonate, high-to-very high salinity, very high sodium water type.
Note that the well samples fall outside the electrical conductivity–sodium adsorption ratio (C–S)
part√of the diagram of Rao (1998); the sodium adsorption ratios (SAR) are defined by SAR =
Na/ (Ca + Mg)/2. This fact suggests that the diagram should be modified to include high-to-very
high salinity, high-to-very high sodium waters.
The results of periodic sampling of the geothermal wells between 1995 and 1999 are shown in
Fig. 6, highlighting the changes observed in geothermal water chemistry with time. Significantly
higher concentrations than “normal” were observed for Cl in LV4 during 1997 (Fig. 6a), for
HCO3 in LV1 and LV4 during 1996 (Fig. 6b), for Na in LV5 during 1998 (Fig. 6d), for K in LV4
during 1997 (Fig. 6e), and for SiO2 in well LV3 during 1998 (Fig. 6f). On the other hand, lower
concentrations of HCO3 were noted in LV3 during 1998 and 1999 (Fig. 6b), of SO4 in LV5 during
1998 and 1999 (Fig. 6c), and of K in LV1 during 1998 (Fig. 6e).
4.3. Composition of geothermal gases
Gas compositions (Table 3) were determined for samples from five wells (LV1–LV5). The gas
phase at the LTV geothermal field represents a relatively small fraction (overall mean value of
∼2 wt.%) of the separated steam phase. This proportion of non-condensable gases is comparable
to those at Cerro Prieto and Los Azufres, but much smaller than at Los Humeros (Tello Hinojosa,
2005). Carbon dioxide is dominant in this small gas phase, representing ∼97–99 wt.% of the total
gas content in the field (Table 3). No outlier detection and elimination was required for the gas
composition data because the median and mean values showed a closer agreement for gas than
for the liquid samples. This similarity suggests a relatively homogenous distribution of different
gases in the geothermal reservoir, especially for CO2 , the dominant non-condensable gas.
4.4. Hydrothermal alteration
Most rocks in the LTV geothermal field show hydrothermal alteration. The degree of alteration
is low (on average ∼10% of the total rock; 1–5% near the surface and up to 40% in the deeper
parts of the wells), especially if compared to other active geothermal systems in Mexico, such as
Cerro Prieto (Schiffman et al., 1984) or Los Azufres (Torres-Alvarado, 2002).
Hydrothermal alteration minerals were deposited in fractures and vesicles, and replaced primary minerals. Argillic alteration predominates at the surface where the main minerals are
zeolites (mordenite and stilbite), associated with quartz, calcite, chlorite, smectite, and mixed
layer illite-smectite. This low-temperature, low-degree alteration zone extends downward to the
first appearance of epidote at around 1000 m depth (elevation of ∼−250 m asl) in all wells. A
propylitic alteration zone begins at this depth, represented by minerals such as epidote, quartz,
calcite, wairakite, sericite, chlorite, mixed layer chlorite-smectite, as well as marcasite, chalcopyrite, and pyrite. Abundant secondary calcite and quartz were observed at the top of the propylitic
alteration zone, close to the contact of the Comundú Group with the granodioritic basement
(Fig. 3).
4.5. Geothermometry of thermal springs and geothermal wells
Subsurface temperatures estimated from two chemical (Na/K and SiO2 ) geothermometers for
springs and domestic water wells are also reported in Table 1. These temperatures are significantly
S.P. Verma et al. / Geothermics 35 (2006) 156–180
167
Fig. 6. Change with time of selected chemical parameters of waters from four LTV geothermal wells. There are not
sufficient data on wells LV1 and LV8 to determine any time-related variations.
higher than the actual measured temperatures (Table 1). The Na/K geothermometer temperatures
for all non-carbonate and some carbonate waters are higher than the corresponding SiO2 temperatures.
The subsurface temperatures for geothermal wells estimated from two liquid (Na/K and SiO2 )
and two gas (H2 /Ar and CO2 /Ar) geothermometers are summarized in Tables 2 and 3, respectively. These geothermometers reflect the temperatures at the location where the fluids were
168
S.P. Verma et al. / Geothermics 35 (2006) 156–180
Table 3
Summary of the composition of gases from geothermal wells in the Las Tres Vı́rgenes geothermal field, Mexico
Well
Year of
sampling
Temperature (◦ C)
Stat Concentration (% m/m)
Cg
CO2
H2 S
NH3
H2
Ar
N2
CH4
Geothermometer
TH2 /Ar TCO2 /Ar
LV1
1995
1996
All years
(1995–1996)
n
x̄
s
n
x̄
s
n
x̄
s
32
1.4
1.6
20
2.4
2.5
52
1.8
2.0
32
97.9
1.6
20
97.1
0.9
52
97.6
1.4
32
1.2
0.8
20
1.8
0.6
52
1.4
0.8
32
0.13
0.09
20
0.16
0.07
52
0.14
0.08
32
0.049
0.023
19
0.056
0.040
51
0.052
0.030
5
0.49
0.14
5
0.016
0.009
5
0.015
0.014
5
0.22
0.16
6
0.043
0.015
7
0.024
0.012
–
LV2
1998
n
x̄
s
5
2.2
0.5
5
98.1
0.5
5
1.09
0.20
LV3
1995
n
x̄
s
n
x̄
s
n
x̄
s
n
x̄
s
8
6.2
1.0
11
1.30
0.29
23
0.8
0.5
42
1.9
2.2
9
97.1
1.6
16
98.9
0.6
23
99.26
0.41
48
98.7
1.1
9
0.90
0.17
16
1.0
0.6
23
0.67
0.39
48
0.8
0.5
9
9
0.122 0.039
0.036 0.016
11
13
0.054 0.044
0.026 0.040
23
–
0.066
0.024
43
22
0.08
0.042
0.04
0.032
12
0.48
0.34
1996
1998
All years
(1995–1998)
LV4
1997
n
x̄
s
12
1.76
0.39
12
97.9
0.7
12
0.34
0.16
LV5
1997
n
x̄
s
9
1.48
0.41
9
97.4
1.5
9
0.5
0.6
126
98.1
1.l
126
1.0
0.7
All
Entire period n
wells (1995–1998) x̄
s
120
1.8
1.9
9
0.030
0.010
121
0.16
0.18
14
14
22
0.039 1.71 0.015
0.010 0.36 0.005
17
16
20
0.023 0.88 0.08
0.010 0.37 0.07
31
30
42
0.031 1.3
0.04
0.012 0.6
0.06
13
0.033
0.016
8
11
0.0140 0.04
0.0033 0.05
7
0.030
0.008
93
0.042
0.030
5
0.051
0.014
14
265
20
16
290
36
30
278
32
14
253
5
16
266
12
30
260
12
5
0.030
0.017
5
272
30
5
278
30
6
9
2.7
0.052
0.7
0.036
1
10
0.17 0.133
–
0.042
–
–
6
263
18
7
274
37
–
6
251
7
7
268
23
–
7
2.3
1.2
13
269
29
13
260
19
8
245
29
8
260
19
3
248
8
3
242.6
2.3
59
270
32
59
261
16
19
0.10
0.06
11
11
1.11 0.047
0.44 0.013
6
2.9
0.9
65
59
0.032 1.4
0.023 0.9
5
0.117
0.033
82
0.06
0.06
Stat: statistical parameter; n: number of measurements; x̄: mean; s: standard deviation. Cg: amount of the gas phase in
steam phase, expressed in wt.%. Helium (He) was below the detection limit of the gas chromatography procedures; it
could be measured only in well LV2 for 1998 sampling (n = 5, He concentration = 0.0009 ± 0.0004 wt.%; the error is one
standard deviation of five measurements).
last in chemical equilibrium, probably near the bottom of the wells. The estimated temperatures
for the Na/K and SiO2 geothermometers are, respectively: 273 ± 17 and 213 ± 42 ◦ C for LV4;
267 ± 7 and 320 ± 60 ◦ C for LV3; 260 ± 6 and 194 ± 20 ◦ C for LV5; 260 ± 6 and 250 ± 26 ◦ C
for LV1; 234 ± 3 and 241 ± 34 ◦ C for LV8; and 238 ± 36 and ∼256 ◦ C (only one measurement)
for LV2. Thus, although standard deviations for the SiO2 temperatures are higher than for the
Na/K temperatures, partly because of analytical problems (Verma, 2000; Verma et al., 2002), both
S.P. Verma et al. / Geothermics 35 (2006) 156–180
169
geothermometers give fairly consistent (i.e., statistically similar) results for wells LV1, LV8, and
LV2. The Na/K temperatures, on the other hand, are higher than the SiO2 temperatures for wells
LV4 and LV5.
The gas geothermometers (H2 /Ar and CO2 /Ar; Table 3) also provided generally consistent
temperatures: ∼275 ◦ C for LV2, ∼270 ◦ C for LV1, ∼265 ◦ C for LV3, ∼255 ◦ C for LV4, and
∼245 ◦ C for LV5.
4.6. Estimated static formation temperature and homogenization temperatures
The static formation temperatures and fluid inclusion results are summarized in Table 4. Fluid
inclusions were observed in quartz, calcite, and epidote between 631 and −1647 m asl (Table 4).
Only two-phase, liquid-rich inclusions were observed in the samples at room temperatures. All
inclusions homogenized to the liquid phase. The size of the inclusions varied between ∼5 and
10 ␮m. Mean salinity values (wt.% NaCl equivalent) estimated from fluid inclusions (all individual
data are presented in Table 4) are as follows: ∼9% for LV1, ∼12% for LV3, ∼5% for V4, ∼4%
for LV5, ∼3% for LV7, and ∼1% for LV8.
Measured homogenization temperatures (Th ) of the fluid inclusions varied from ∼100 to
∼290 ◦ C for elevations from ∼700 to ∼−1700 m asl, respectively (Fig. 3). Homogenization temperature isotherms were drawn for the cross-section through all the geothermal wells for which
Th data were available (Fig. 3).
Static formation temperatures and homogenization temperatures (Th ) are compared in Fig. 7. In
well LV1 (Fig. 7a), the SFT are slightly higher than the Th for shallow depths but lower for greater
depths. In well LV4 (Fig. 7c) and, to some extent, in well LV3 (Fig. 7b), the SFT are consistently
lower than the Th . For the other wells, LV5 (Fig. 7d) and LV8 (Fig. 7e), the differences are fairly
small.
Similarly, selected geothermometer temperatures are compared with SFT and Th in Fig. 8. The
Na/K temperatures are higher than the corresponding SFT for wells LV1, LV3, LV4, and LV8
(Fig. 8a). These temperatures for wells LV1 and LV8 are significantly higher than the corresponding Th at the bottom of these two wells (Fig. 8c), but lower for well LV4. Temperatures based on
the CO2 /Ar geothermometer are higher than the SFT for wells LV1 and LV3 (Fig. 8b) and also
higher than the Th for well LV1 (Fig. 8d). These CO2 /Ar temperatures are lower than the SFT for
well LV5 (Fig. 8b) and also lower than the Th for wells LV4 and LV5 (Fig. 8d).
5. Discussion
5.1. Chemistry of the thermal manifestations
Thermal springs and domestic water wells in the LTV area show compositions ranging from
non-carbonate, low-to-medium salinity, low-sodium waters to carbonate, high-to-very high salinity, very high sodium waters (Table 1). Similar types of waters seem to occur in different parts
of the study area (see locations enclosed by dotted lines in the inset in Fig. 1). These waters
originate at shallower depths than the geothermal fluids sampled in the deep wells, and yield
estimated aquifer temperatures ranging from ∼96 to 203 ◦ C (TNa/K ) and ∼110 to 205 ◦ C (TSiO2 ).
The geothermometric data indicate that the spring and domestic well waters, whose temperatures at the surface vary from ∼23 to 97 ◦ C (Table 1), are likely to have a significant deep
component.
170
Table 4
Static formation and homogenization temperatures and other fluid inclusion parameters in the Las Tres Vı́rgenes geothermal wells, Mexico
Tmi
(◦ C)
Tmi average
(◦ C) (n)
Salinity (wt.%
NaCl equivalent)
Well LV1 (longitude: 112.560388◦ W; latitude: 27.526996◦ N; wellhead elevation: 741 m asl)
110
110
631
131 ± 0.1
Qz
101
110
110
631
–
Cc
100
250
250
491
144.9 ± 4.3
Qz
116–126
500
500
241
159.2 ± 1.1
Cc
148–163
600
600
141
160.5 ± 0.1
Cc
162–171
700
700
41
160.1 ± 0.4
Cc
170–188
1100
1057
−316
196.0 ± 1.0
Qz
219–238
1200
1150
−409
203 ± 7
Qz
232–264
1400
1325
−584
213.0 ± 4.0
Qz
219–252
1600
1500
−759
212.1 ± 1.3
Qz
219–252
1750
1633
−892
211.8 ± 1.3
Qz
220–222
1820
1695
−954
212.1 ± 1.3
Ep
213–234
101 (3)
100 (1)
121 (6)
152 (7)
165 (15)
177 (14)
231 (40)
243 (6)
225 (27)
223 (22)
221 (24)
217 (25)
–
–
+4.5 to +6.6a
−0.4
−0.4 to −5.7
−1.6 to −15.8
−1.2 to −2.4
−13.0 to −17.7
−4.3 to −12.0
−1.4 to −16.0
−1.4 to −2.2
−5.0
–
–
+5.1 (5)
−0.4 (17)
−1.8 (10)
−11.7 (14)
−1.5 (40)
−15.1 (4)
−10.1 (27)
−6.2 (22)
−1.6 (24)
−5.0 (25)
–
–
n.c.
–
3.0
15.6
2.5
18.6
14.0
9.3
2.7
7.8
Well LV3 (longitude: 112.555591◦ W; latitude: 7.506226◦ N; wellhead elevation: 720 m asl)
570
570
150
102.5 ± 4.2
Cc
580
580
140
–
Cc
920
920
−200
160.4 ± 2.5
Qz
1202
1202
−482
189.8 ± 1.9
Qz
1202
1202
−482
–
Ep
1647
1647
−927
217.5 ± 0.1
Qz
1830
1830
−1110
218.3 ± 0.1
Qz
1940
1940
−1220
227.3 ± 0.1
Qz
1940
1940
−1220
–
Ep
2000
2000
−1280
238.3 ± 0.1
Qz-Cc
2150
2150
−1430
–
Qz-Cc
109 (4)
118 (6)
125.5 (4)
230 (23)
241 (25)
232 (27)
261 (18)
247 (9)
243 (10)
262 (21)
261 (12)
–
+0.4 to +2.2a
–
−6.0 to −12.3
–
−14.9
−13.9
−2.0
−2.0
−15.7
–
–
+1.0 (6)
–
−6.8 (23)
–
−14.9 (27)
−13.9 (18)
−2.0 (9)
−2.0 (10)
−15.7 (21)
–
–
n.c.
–
10.2
–
18.5
17.7
3.4
3.4
19.2
–
196 (4)
207 (23)
217 (24)
222 (37)
242 (40)
243 (3)
290 (24)
−1.2
−5.2
−5.2
−1.7
−1.0
−0.4
−6.5 to −8.7
−1.2 (4)
−5.2 (23)
−5.2 (24)
−1.7 (37)
−1.0 (40)
−0.4 (3)
−8.6 (24)
2.0
8.3
8.3
2.9
1.7
0.7
12.4
Vertical
depth (m)
Elevation
(m asl)
SFT (QR)
Host
mineral
Th range
(◦ C)
109
114–127
125–126
226–237
227–264
231–235
256–271
242–259
237–259
261–263
261–263
Well LV4 (longitude: 112.555948◦ W; latitude: 27.506421◦ N; wellhead elevation: 720 m asl)
800
800
−80
149.4 ± 2.8
Qz
194–197
900
900
−180
174.9 ± 1.1
Qz
201–210
1101
1085
−365
189.8 ± 1.1
Qz
214–220
1191
1175
−455
183.4 ± 4.0
Qz
217–225
1311
1280
−560
188.1 ± 2.4
Qz
237–244
1588
1540
−820
193 ± 6
Cc
236–249
2452
2367
−1647
250 ± 11
Qz
287–292
S.P. Verma et al. / Geothermics 35 (2006) 156–180
Th average
(◦ C) (n)
Depth along
the well (m)
Table 4 (Continued )
Tmi
(◦ C)
Tmi average
(◦ C) (n)
Salinity (wt.%
NaCl equivalent)
Well LV5 (longitude: 112.560800◦ W; latitude: 27.526747◦ N; wellhead elevation: 739 m asl)
405
405
334
–
Cc
146–158
498
498
241
110.0 ± 3.8
Qz
164–170
606
606
133
124.1 ± 2.6
Qz
180–187
706
706
33
137.4 ± 4.1
Cc
189–196
906
895
−156
164.5 ± 3.2
Qz
215–217
906
895
−156
–
Cc
189–196
1206
1182
−443
195.4 ± 1.2
Qz
231–250
1302
1272
−533
200.0 ± 1.6
Qz
230–250
1797
1745
−1006
219.3 ± 2.2
Qz
265–276
150 (6)
168 (23)
185 (27)
194 (27)
216 (6)
194 (27)
244 (30)
248 (27)
270 (28)
−0.6
−0.3
−1.2
−2.8
−0.4
−2.8
−4.6
−3.6
−4.2
−0.6 (6)
−0.3 (23)
−1.2 (27)
−2.8 (27)
−0.4 (6)
−2.8 (27)
−4.6 (30)
−3.6 (27)
−4.2 (28)
1.0
0.5
1.9
4.6
0.7
4.6
7.3
5.8
6.6
Well LV7 (longitude: 112.533518◦ W; latitude: 27.497852◦ N; wellhead elevation: 523 m asl)
1098
1098
−575
–
Qz
213–217
1203
1203
−680
–
Qz
223–230
1250
1250
−727
–
Qz
207–213
215 (14)
227 (23)
211 (15)
−2.5
−1.0
−1.1
−2.5 (14)
−1.0 (23)
−1.1 (15)
4.2
1.7
1.9
Well LV8 (longitude: 112.543706◦ W; latitude: 27.519266◦ N; wellhead elevation: 725 m asl)
399
399
326
–
Cc
155–159
500
500
225
–
Cc
154–165
598
598
127
109.6 ± 0.9
Cc
172–185
700
700
25
124.6 ± 2.8
Cc
168–169
900
890
−165
152.4 ± 1.6
Qz
187–195
1000
985
−260
162.3 ± 2.2
Qz
200–212
157 (11)
160 (9)
180 (30)
169 (25)
191 (20)
207 (29)
−0.4
+4.5 to +5.0a
−0.6
−0.5
−0.5
−1.4
−0.4 (10)
+4.6 (9)
−0.6 (30)
−0.5 (25)
−0.5 (20)
−1.4 (29)
0.7
n.c.
1.0
0.9
0.9
2.4
Vertical
depth (m)
Elevation
(m asl)
SFT (QR)
Host
mineral
Th range
(◦ C)
Samples from well LV2 (longitude: 112.5634975◦ W; latitude: 27.5299214◦ N; wellhead elevation: 648 m asl) were not included in this study. The depth along the well in meters
refers to the actual length (not vertical depth) of well drilled to recover the samples; the vertical depths are slightly smaller depending on the angle of drilling, which in most
cases was not in the vertical direction. m asl: meters above sea level; SFT (QR): static formation temperature (quadratic regression); Th : homogenization temperature (◦ C); Tmi :
ice-melting temperature (◦ C); n: number of inclusions measured; Qz: quartz; Ep: epidote; Cc: calcite. n.c.: not calculated.
a Dissociation temperature of CO clathrate (◦ C).
2
S.P. Verma et al. / Geothermics 35 (2006) 156–180
Th average
(◦ C) (n)
Depth along
the well (m)
171
172
S.P. Verma et al. / Geothermics 35 (2006) 156–180
Fig. 7. Average homogenization temperatures (Th ) vs. static formation temperatures (SFT) for five LTV geothermal wells.
Error bars are shown for both variables, representing the range (minimum to maximum value) for Th and mean ± standard
deviation values for SFT (Table 4). The diagonal lines indicate the exact correspondence between values of SFT and Th .
5.2. Chemistry of the geothermal well fluids
The chemistry of the fluids of individual geothermal wells showed changes during the
1995–1999 period (Fig. 6). The averages for different wells are characterized by relatively large
standard deviations (Table 2). The variations in a given well may be a consequence of the fact
that water is being drawn into the borehole from different depths, which will depend on the fluid
flow regime in the reservoir and the length of the borehole production liner or open interval(s).
S.P. Verma et al. / Geothermics 35 (2006) 156–180
173
Fig. 8. Comparison of different estimates of subsurface temperatures for five LTV geothermal wells. (a) SFT–Na/K
geothermometer temperatures; (b) SFT–CO2 /Ar geothermometer temperatures; (c) Th –Na/K geothermometer temperatures; (d) Th –CO2 /Ar geothermometer temperatures.
Equilibrium (Na/K and SiO2 ) geothermometer temperatures for deep geothermal fluids were
calculated to be ∼260 ◦ C (±13 ◦ C for TNa/K and ±60 ◦ C for TSiO2 ) on average (Table 2), which
are generally consistent with fluid inclusion data (i.e., Th ) for rock samples taken at the bottom of
the wells (Table 4). Below ∼−1000 m asl (Fig. 3), isotherms based on Th measurements indicate
the presence of a high-temperature fluid upflow zone near the deeper parts of wells LV3 and LV4,
which also corresponds to the zone of most intense hydrothermal alteration. Another upflow zone
may also exist at the western side of the deeper part of well LV5, but more work is needed to
confirm and determine its importance and areal extent.
The shallowing of the 150 and 200 ◦ C isotherms to the area immediately NE of well LV4
(Figs. 2 and 3) seems to be a consequence of this upwelling phenomenon, enhanced by the
presence of fractures and faults. The lower temperatures observed in the shallower region of well
LV3, as compared to the nearby well LV4, may be caused by the presence of an inclined fault
(not shown in Fig. 2), affecting well LV3 (Fig. 3).
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Wells drilled close to the faults (LV1, LV2, and LV5) show widespread argillic alteration
near the drilling pads, suggesting upwelling of hot fluids. The zones of thermal upwelling
must be thoroughly investigated and taken into consideration when siting new wells in the
field.
The LTV geothermal system is liquid-dominated (Portugal et al., 2000; González Partida et
al., 2001). The average composition of its geothermal waters is: ∼8100 Cl, ∼4600 Na, ∼60 K,
∼50 HCO3 , ∼80 SO4 , and ∼170 B (all values in mg/l; Table 2). These waters are supersaturated
with respect to calcite, explaining its abundance in well cuttings, fractures, and as an alteration
product of primary minerals.
In the six LTV wells that were studied, most of the fluid inclusion homogenization temperatures
(Th ) do not reach the boiling curves (Fig. 9a–f). Since no vapor-rich inclusions were observed,
we conclude that boiling has not been an important process in the LTV geothermal system. The
low content of non-condensable gases in the fluids produced by most geothermal wells (Table 3)
also supports this hypothesis.
At shallow depth, well LV3 has relatively lower temperatures than in nearby LV4, whereas,
at depth, both wells show similar temperatures (Figs. 3, 9b and c). Well LV1 shows a thermal
inversion below −600 m asl (Fig. 9a), thus presenting lower temperatures than in neighboring
well LV5 (Fig. 9d). These data suggest that unidentified fractures and faults may exist near wells
LV3 and LV1 (Fig. 3).
The changes in geothermal well fluid composition (Fig. 6) may reflect the effect of the varying
amounts of fluids from different depths and of different composition that were drawn into the
wells at the time of sampling.
The main chemical components in the geothermal well waters are given in Fig. 10. The deeper
waters appear to be richer in Na (Fig. 10e), Cl (Fig. 10a), and K (Fig. 10f) than the shallower
waters. The concentration–elevation (or depth) relationships for these elements (ions) obtained
from linear regression of all data (see values of n in Fig. 10) are statistically significant at the 99%
confidence level (the linear correlation coefficient, r, is greater than the respective critical value of r
(see Bevington (1969), Ebdon (1988), or Verma (2005), for the critical values and the significance
of this test). Likewise, B (Fig. 10g), Ba (Fig. 10h), Na + K (Fig. 10i), salinity of the fluid inclusions
(Fig. 10j), and TNa/K (Fig. 10k), also show a similar statistically significant (at the 99% confidence
level) relationship, in this case decreasing concentration with increasing depth, whereas several
other chemical parameters (SO4 in Fig. 10b, Ca in Fig. 10c, and Mg in Fig. 10d) show an inverse
relationship (i.e., they increase with elevation). The remaining chemical parameters (Table 2)
do not show such a systematic behavior. In other words, the deeper geothermal waters show
higher concentrations of Cl, Na, K, B, Ba, and Na/K geothermometer temperatures, but lower
concentrations of SO4 , Ca, and Mg than the shallower waters. Deep geothermal fluids are generally
enriched in the first group of elements (e.g., Cl, Na, K, and B; Nicholson, 1993) and not in the
second group (e.g., SO4 , Ca, and Mg); these chemical differences are therefore consistent with
upwelling around wells LV3 and LV4. The Na/Cl ratio, however, remains practically unchanged,
or statistically “equal” (see overlapping 95% confidence limits and also very small value of r in
Fig. 10m) for all the geothermal waters.
5.3. Different temperature estimates
Static formation temperatures reflect the present thermal regime in a geothermal field,
whereas Th give the temperatures prevailing at the time of secondary mineralization. Any
significant differences between SFT and Th could help reveal the temperature history of a
S.P. Verma et al. / Geothermics 35 (2006) 156–180
175
Fig. 9. Plots of average fluid inclusion homogenization temperatures (Th ) vs. elevation for six LTV geothermal wells.
The horizontal error bars represent the range (minimum–maximum) for Th values. For comparison, the boiling point vs.
depth curves (Haas, 1971) for pure water and saline water (10 wt.% NaCl equivalent) are drawn.
given system. In the LTV, significantly higher Th values recorded in some wells (LV1, LV3,
and LV4; Fig. 7) indicate that the geothermal field probably had higher temperatures in the
past.
Geothermometer temperatures correspond to those prevailing at the locations where the respective chemical components of the geothermal fluids were in equilibrium. In the LTV geothermal
field, higher geothermometer temperatures than SFT or Th (Fig. 8) may indicate that the fluids
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Fig. 10. Average chemical characteristics of waters from LTV geothermal wells as a function of elevation (see Table 2 for
more details). Dotted lines in each diagram are regression lines for the entire data set (n: total number of data regressed; r:
linear correlation coefficient). All concentrations are in mg/l; salinities of fluid inclusions in wt.% NaCl equivalent; Na/K
geothermometric temperature in ◦ C. In (m), the horizontal bars correspond to the 95% confidence limits of the respective
mean values. Included for comparison are the Na/Cl values for seawater from Bearman (1989; letter a); Goldberg (1963;
b), Demicco et al. (2005; c); Home (1969; d); Bruton et al. (1997; e), and Prol-Ledesma et al. (2004; f). Also shown on the
right is a schematic profile extending from the LTV area to the nearby Gulf of California (note the exaggerated elevation
scale).
originated from greater depths than the bottom of the wells since there is no evidence that the
geothermal system is heating up.
5.4. Evidence for the presence of a seawater component
Based on isotopic and hydrochemical data, Portugal et al. (2000) excluded the presence of
seawater from the Gulf of California in the deep parts of the reservoir. Instead, they proposed that
the geothermal brines in the Las Tres Vı́rgenes system represented a fossil, hence concentrated,
“paleofluid” formed during glacial periods at the end of the Late Pleistocene (∼18–11 ka) and Early
Holocene (∼11–8.9 ka). One should remember that the average annual rainfall in the region is only
around 62 mm. The geothermal reservoir was thus assumed to be essentially a stagnant system
under hydrostatic equilibrium conditions. However, we consider that the evidence presented by
Portugal et al. (2000) is rather weak. The stable isotope data shown by these authors (their
Fig. 11) are consistent with the involvement of seawater because a three-component mixing of
meteoric, magmatic, and seawater cannot be ruled out. Similarly, the only chemical data they
considered were B and SiO2 concentrations (their Fig. 7), for which a three-component mixing
with seawater cannot be ruled out as a viable mechanism. Temperature-dependent water–rock
interaction processes and a consequent increase of several elements such as B and SiO2 will
further complicate this simple scenario. The general increase in the B concentrations of the deeper
geothermal waters (Fig. 10g) may indeed be due to this water–rock interaction under geothermal
conditions.
S.P. Verma et al. / Geothermics 35 (2006) 156–180
177
We propose a different interpretation for the LTV hydrochemical data. Since the Na/Cl ratio
in the geothermal well waters (0.57 ± 0.03; n = 341) is practically indistinguishable from that of
seawater (∼0.55 from the concentration data reported by Bearman, 1989; ∼0.56 compiled by
Bruton et al., 1997; and ∼0.60 given by Prol-Ledesma et al., 2004, for a surface seawater sample
close to the study area), a seawater component could quite possibly be present in the deep waters in
the LTV area (see Fig. 10a–m for individual well data and seawater Na/Cl compiled from different
sources in Fig. 10m; see also the schematic profile in the lower right part of Fig. 10). The Na and
Cl concentrations in the geothermal brines are also very high (Table 2), much higher (by at least
an order of magnitude) than, for example, at the Los Humeros geothermal field. López Mendiola
and Munguı́a Bracamontes (1989) reported 12 geothermal water analyses for wells in that field.
The median values for Los Humeros geothermal waters were: Na, ∼150 mg/l; Cl, ∼80 mg/l; and
Na/Cl, ∼1.7. On the other hand, the values for the LTV geothermal waters are similar to those
for the Tiwi geothermal field, Philippines, located close to the seashore (Bruton et al., 1997).
These authors inferred a significant component of seawater in the Tiwi reservoir fluids because
the Na/Cl and Cl/Br ratios were similar to those of seawater. For the Cerro Prieto geothermal
field, the Na/Cl ratio of the geothermal well waters is ∼0.55 (Angulo et al., 1987), again close to
the seawater ratio. At the Los Azufres field, located far from the ocean, although the Na and Cl
concentrations in the geothermal well waters are high (González-Partida et al., 2005), the average
Na/Cl ratio (∼0.65) is significantly different from that of seawater (∼0.55), suggesting a different
fluid source.
We should also note that the actual concentrations of any elements such as Li, B, Ba, As, Si, and
K that are present at minor-to-ultra-trace levels in seawater (Bearman, 1989) will likely change
in a geothermal system because of water–rock interactions. The latter processes, however, will
not significantly change the ratio between Na and Cl, two major components in seawater. This
ratio can therefore be used as a tracer for seawater in a geothermal system, but not other ratios
involving elements that are modified considerably by geothermal processes.
Concentrated “seawater-type” fluids may not be the only ones present in the LTV geothermal
system, which could also contain meteoric waters (despite the sparse rainfall in the area), paleofluids (as proposed by Portugal et al., 2000), and magmatic fluids (because of the active magmatic
system beneath the volcanic complex (Fig. 1). We should stress that the chemistry of the reservoir
fluids will reflect the effects of water–rock processes, i.e., the characteristics of the recharging
fluids will change as they interact with the rocks in the subsurface. In addition, the geothermal
fluids may mix and be diluted by shallow, colder meteoric waters as they rise to the surface, thus
forming the low-salinity fluids observed in the fluid inclusions collected at shallower depths, in
thermal springs, and in domestic water wells.
Identification and quantification of the different components of the reservoir fluid could prove
useful when determining the commercial lifespan of the LTV geothermal system. If the “fossil
fluid” model (with no marine input) of Portugal et al. (2000) is valid, then a carefully conceived
re-injection program will be required to return all geothermal waters to the reservoir, in order
to reduce the risk of shortening the lifetime of the field. On the other hand, if we are correct
in our hypothesis that a significant amount of seawater from the Gulf of California recharges
the geothermal system, then the re-injection program need not be so intensive (environmental
considerations permitting), and the lifespan of the field could be longer. However, since no active
recharge path of seawater to the geothermal system has yet been identified and the environmental
impact of geothermal fluids must be minimized, we strongly recommend re-injection of the
geothermal waste water in the LTV field. Further detailed work is needed to better understand
this complex geothermal system.
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5.5. Heat balance of the LTV geothermal field
The heat balance of the LTV geothermal system can be estimated using the volumetric reservoir
model proposed by Sánchez-Upton (in preparation). Following a conservative approach, since a
larger area could have been chosen, we estimated the amount of heat stored in the system to
be roughly ∼9 × 1012 MJ. This value was obtained assuming a rock volume of ∼15 km3 (based
on the area of the wellfield, ∼15 km2 , and a reservoir thickness of 1 km; see Figs. 2 and 3,
respectively), an average temperature of ∼232 ◦ C (Fig. 3), a rock density of 2670 kg m−3 , and a
specific heat capacity of 1.0 kJ kg−1 K−1 (Yoshinobu et al., 1998). Of this, roughly 4 × 1011 MJ
can be extracted at the surface by assuming certain reservoir parameters (i.e., effective rock
porosity of ∼4%, initial pressure of ∼9 MPa, and fluid enthalpy of ∼1000 kJ kg−1 ). The heat that
can be extracted (4 × 1011 MJ) for 30 years of operation would correspond to either ∼148 MWe
(assuming a conversion efficiency of ∼35%: Koroneos et al., 2003) or ∼55 MWe (for a typical
condensing unit with an efficiency of ∼13%: Sánchez-Upton, in preparation).
If a larger area is chosen for the geothermal field (for example, geophysical studies indicate that
the geothermal system may extend over ∼70 km2 ; Flores-Armenta and Jaimes-Maldonado, 2001),
the amount of stored heat in the system would increase proportionally. More reliable estimates
could be obtained in the future through detailed modeling of the primary (magmatic) heat source
and reservoir studies. These preliminary heat balance estimates and the geochemical evidence do
seem to indicate, however, that geothermal electricity generation in this field could be increased
from its current installed capacity of 10 MWe in future.
6. Conclusions
The liquid-dominated Las Tres Vı́rgenes geothermal system is related to young volcanism
(<1 Ma). Hydrothermal alteration has affected most rocks in the field to varying extent. Argillic
alteration, calcite deposition, and propylitization are the most prevalent. Fluid inclusion homogenization temperature (Th ) measurements generally agree with equilibrium temperatures estimated
on the basis of the chemistry of the produced geothermal fluids, as well as with static formation
temperatures computed using a modified quadratic regression Horner method.
Homogenization temperatures suggest the presence of a high-temperature upflow zone at depth
in the vicinity of wells LV3 and LV4. High salinity inclusions in deeper samples may support
the hypothesis of the existence of a fossil “paleofluid” in the geothermal system as proposed by
Portugal et al. (2000), but they are also consistent with a possible recharge of the deep geothermal
system with seawater from the Gulf of California. The latter hypothesis is corroborated by high
concentrations of Cl and Na and the similarities between the Na/Cl ratio of the geothermal fluids
and seawater. The highly concentrated deep geothermal fluids may mix with, and be diluted by,
shallow groundwater to form the dilute fluid observed in the inclusions found in the shallow
portions of the studied boreholes, as well as in the samples collected from thermal springs and
domestic water wells.
Acknowledgements
Three of the authors (K. Pandarinath, E. Santoyo, I.S. Torres-Alvarado) are grateful to DGAPAUNAM (projects IN-104703 and IN-105502-3) for their partial support. Thanks are also extended
to J.N. Moore, R. Gunderson, an anonymous reviewer, M.J. Lippmann, and M.H. Dickson for
their numerous useful suggestions that helped to improve the manuscript.
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179
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