Geothermal reconnaissance of Northeastern Venezuela

0375-6505/89 $3.00 + 0.00 Pergamon Press plc © 1989 CNR.

Geothermics, Vol. 18, No. 3, pp. 403-427, 1989. Printed in Great Britain.

GEOTHERMAL

RECONNAISSANCE VENEZUELA

OF NORTHEASTERN

FRANCO URBANI Universidad Central de Venezuela, Departamento Geologla, Centro de Documentacion de lnformacion Geot~rmica Nacional, Apartado 47028, Caracas 1041A, Venezuela (Received October 1987; acceptedfor publication September 1988) Abstract--About 60% of Venezuela has been covered by a reconnaissance geothermal survey that includes geologic and water geochemical studies. The information is stored in a computerized data bank that holds data from 361 geothermal localities. The subsurface reservoir temperatures of the geothermal systems have been estimated using chemical geothermometry and mixing models and in many cases conceptual geothermal models have been postulated. Preliminary assessments of the northeastern Venezuelan geothermal systems indicate that the most promising system is Las Minas near El Pilar in the state of Sucre, with an estimated deep reservoir temperature of 200-220°C. Further studies are intended to evaluate its potential for electricity generation. Based on present data, other medium and low temperature systems in Venezuela appear useful for direct applications.

INTRODUCTION In 1800 the German naturalist Alexander von Humboldt was the first to make a scientific description of a Venezuelan hot spring: Las Trincheras, Carabobo (Fig. lb), having a boiling temperature of 97°C (Humboldt, 1814). In the last third of the 19th century, V. Marcano started to make quantitative water analyses of several hot springs (Urbani, 1986). In the first half of this century hot springs were described by A. Jahn, G. Febres-Cordero, O. Ostos, G. DelgadoPalacios, A. Otero and T. Briceno-Maas (Urbani, 1984). In 1969 a systematic study of the hot springs of central Venezuela was started (Urbani, 1969) and in 1975 the work continued in northeastern Venezuela, where geological and geochemical studies have shown that some systems in this area have potential for electricity generation (Urbani, 1977). In 1981 the Universidad Central de Venezuela (UCV) started a detailed "National Geothermal Inventory" project covering the central, eastern and southern regions (Zannin and Marifio, 1983; Rodriguez, 1983; Fermin, 1983; Hevia and Di Gianni, 1984). Geothermal systems of T~chira and M6rida were investigated by Burguera et al. (1981). The states of Zulia, Trujillo, Lara, Barinas and Portuguesa (Fig. lb) have not been covered to date. Northeastern Venezuelan geothermal manifestations and related features, such as sulfur deposits, acid-sulfate alteration zones and mud volcanoes, have been investigated in detail. At each locality all fluid emissions have been located on detailed maps and geologic maps of surrounding areas have been prepared. Samples of waters, rocks and mineral deposits have been taken from all thermal areas and cold background waters and a few gas samples have also been taken. The data from the inventory and from previous published and unpublished reports have been stored in a computerized data bank that contains geographical, geological and geochemical information from a total of 361 geothermal localities. The water analyses were processed with the computer program G E O T R V (Urbani, 1985b, 1986), which calculates several chemical geothermometers and mixing models. Those results and the geologic information were used to produce preliminary conceptual models of many Venezuelan geothermal systems and to 403

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estimate the temperatures of the aquifers that feed the hot springs. Further details of the methods used in the inventory, including field, laboratory, data processing and interpretations, are discussed elsewhere (Urbani, 1985a). The Venezuelan geothermal information has been condensed and a general assessment of the geothermal systems was carried out by Urbani (1984), showing that the most promising system for possible electrical generation is Las Minas, near El Pilaf, state of Sucre. Since I985 the Venezuelan Ministry of Energy and Mines, in cooperation with the International Institute for Geothermal Research of Italy, has been working on a new geochemical study at El Pilaf, but no results have yet been published.

Geothermal Reconnaissance of Northeastern Venezuela

405

G E O L O G I C A L SETTING The area covered in this report comprises the states of Sucre, northeastern Anzofitegui and northern Monagas (Fig. 1). The geology of the region has been reviewed in Gonzfilez de Juana et al. (1980), and a recent interpretation of the main tectonic features appears in Speed (1985). Geographically speaking, northeastern Venezuela consists of a northern cordillera of the Araya-Paria peninsulas, a central region with the Cariaco and Paria gulfs and the lower mountains and valleys between them. At the south there is a large mountainous massif. As summarized by Speed (1985), the Araya-Paria peninsulas are formed mainly of Mesozoic metamorphic rocks consisting of phyllites, schists and gneisses of the greenschist facies of the Araya-Tobago terrane (Fig. 2). In the vicinity of Cart~pano there are outcrops of the southernmost parts of the Lesser Antilles magmatic arc and forearc basin terrane which includes some ultramafic rocks. The Eastern Mountains Massif belongs to the foreland thrust belt terrane (Fig. 2). In the El Pilar region, on both sides of the El Pilar fault (Fig. 2B), the metamorphic rocks of Araya-Paria are thrust over sedimentary rocks typical of the Eastern Mountains Massif by a thrust of probable Eocene age. The central depression is controlled mainly by the El Pilaf fault system, whose nature and extent has been the subject of much controversy. According to Speed

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F. Urbani

(1985) it is a 300 km long right-lateral strike-slip fault to a point of suturing located near the town of El Pilar (Fig. 2A). This suture is attributed to an oblique collision of the Caribbean and South American plates at the southern side of the Lesser Antilles arc. The El Pilar fault system is seismically active with loci of up to 15 km depth (A. Singer, personal communication, 1984). The geology of the El Pilar region is critical to any understanding of the transition from the convergent plate boundary of the Antillean arc to the transform boundary of the E1Pilar system. The Eastern Mountains Massif is mainly built of Cretaceous and Tertiary marine sedimentary rocks. The most widely distributed units are Early Cretaceous platform sediments, mostly sandstones and limestones, and Late Cretaceous pelagic sediments, such as cherts, black shales and limestones. Besides the El Pilar fault, other major faults in northeastern Venezuela are the San Francisco and Urica right-lateral strike-slip faults (Fig. 2B). The Urica fault marks the western end of the Eastern Mountains Massif, and the San Francisco fault makes a major incision in the center of the massif. Most of the hot springs of northeastern Venezuela are located near these three major fault zones. Half way between the cities of Cartipano and El Pilar there is an area with about 50 small, isolated bodies of rhyodacite that are probably remnants of feeder conduits. However, no effusive volcanics occur in the area. These plugs are the youngest igneous rocks on the mainland of eastern Venezuela; they probably represent the southernmost extension of the Antillean arc magmatism (Schubert and Sifontes, 1984). There is a single K-Ar age of 5 Ma (Schubert and Sifontes, 1984). A geophysical survey by Vierbuchen (1984) shows a gravity low centered in the area of the igneous rocks that can be modelled as a large pluton of granitic composition. The modelled depth of the top of the pluton is 5 kin, and its southern extension lies at 2 km in horizontal distance from the Mundo Nuevo geothermal system. In the following sections the geothermal systems of northeastern Venezuela are discussed: (1) systems of northeastern Anzofitegui, located at the western end of the Eastern Mountains Massif (Fig. 1C); (2) systems of northern Monagas, located at the south and southeastern ends of the mountains (Fig. 1C); (3) systems of Sucre, roughly located in the area along the E1 Pilar fault (Fig. 2B). G E O T H E R M O M E T R I C AND W A T E R CLASSIFICATION METHODS USED In this preliminary assessment, much emphasis has been placed on chemical geothermometry in which the subsurface aquifer temperatures of the hot-water geothermal systems are estimated using the temperature-dependent solubility of chemical constituents. These methods are based on several assumptions, mainly water-rock equilibrium, negligible re-equilibration as the water rises towards the surface and no mixing of the hot reservoir fluids with cold surface waters. These restrictions can make it difficult to estimate subsurface temperatures of spring systems when few springs are available to sample (Fournier et al., 1974; Fournier, 1977, 1981). In Venezuelan geothermal systems only hot and cold spring water data are available, since no drilling has taken place. Therefore, where several spring water analyses are available, we shall use the geothermometers with caution to suggest subsurface temperature ranges. Several geothermometers that depend on the quartz and chalcedony saturation curves with conductive and adiabatic cooling (Fournier, 1981; Fournier and Potter, 1982) and the Na-K-Ca geothermometer of Fournier and Truesdell (1973) have been used to estimate subsurface temperatures. Waters with high Mg content have been corrected according to the method of Fournier and Potter (1979). The N a - K - C a geothermometer has been refined experimentally by Benjamin et al. (1983), and their formulation is supposed to give more reliable results at temperatures below IO0°C.

Geothermal Reconnaissance of Northeastern Venezuela

407

During their ascent, hot waters of deep origin may mix with cold waters from shallow aquifers. This mixing makes it difficult to apply single-component geothermometers, but has less effect on geothermometers based on cation ratios. In this study and lacking other lines of evidence, mixing was suspected when samples plotted in a rather straight line fashion on Piper diagrams or on plots such as t vs C1, t vs SiO2, CI vs Na, etc. When this happened in non-boiling waters, the so-called "warm spring mixing model" (Fournier and Truesdell, 1974; Truesdell and Fournier, 1977) was applied. Rather than using data from individual warm and cold springs, we plotted all analyses of the system (warm and cold) on the t vs SiOe graph, computed the straight-line regression equation and found the temperature and SiO2 content of the intercept with the quartz saturation curve. The intercept temperature (tWSMM) is the estimate for the deep hot-water end member. The proportions of mixing can also be evaluated. In the case of boiling spring systems, some conservative components like CI may increase by evaporative concentration. Therefore, a system with several boiling springs having different C! contents can be formed by the mixing of an unknown single deep hot-water component with cold shallow water. This mixing can be evaluated by the use of the so-called "boiling spring mixing model" (Truesdell and Fournier, 1976; Fournier, 1979) that makes use of the enthalpy H vs CI plot to estimate the temperature (tBSMM) and CI content of the deep hot-water end member. Another step in this preliminary study was to classify the water analyses of each geothermal system by means of Piper diagrams, several X Y plots and the A-Fplot of D'Amore et al. (1983). The A - F plots proved very useful because they yield striking visual differences for different water types and yet are sensitive to small changes in composition. By consideration of water classification, estimated reservoir temperatures and geological setting, conceptual models of many Venezuelan geothermal systems have been developed. G E O T H E R M A L SYSTEMS OF N O R T H E R N A N Z O A T E G U I Figure 3 shows the location of hot and cold springs (with H2S odor) in northeastern

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408

F. Urbani Table 1. Field data and chemical analyses of some selected thermal and non-thermal spring waters from Anzo~itegui and Monagas (analysis reported in mg/1-l) Data from a selected spring water from each system General data of system

System name

t (°C) min-max

TDS av.

IDt

Elev. (m a.s.l.)$

Longitude

Latitude

24-26 36 23-24 23-52 26--45 33-34 26-52

305 270 447 432 933 454 313

An.2 An.7 An.10B An.26B An.19 An. 15 An.35

650 285 325 90 90 40 360

64°03'02'' 64° 3'32" 64°21'47'' 64°31 ' 3" 64°36'46" 64°37'54'' 65°20'13''

9°50'25'' 9°49' 4" 10° 2' 7" 10° 9'40" 10° 4'47" 10°11'45" 10° 1'53"

6 2

21-28 23-27

461 333

Mo.4 Mo.6

10 40

63° 5 ' 4 6 " 63°10'29"

t0°10'49" 10° 7'30"

7 3

23-27 25-36

631 500

Mo.ll Mo.18

80 120

63°21'48'' 63015'59''

9°59'16" 9°59'37"

12 4

23-26 25-28

337 473

Mo.36 Mo.43

460 240

63°39'2Y' 63°31'14"

No.*

(A) State of Anzo~itesui (1) Mundo Nuevo 6 (2) Urica 2 (3) Bergantin 12 (4) SanDiego 10 (5) Naricual 3 (6) Pozuelos 3 (7) Clarines 5 (B) (1) (a) (b) (2) (a) (b) (3) (a) (b)

State of Monasas Caripito LosMorros Azagua El Pinto Punceres Los Bafios San Francisco EI Guamo RioAragua

Geographical coordinates

10° 5' Y' 10° 0'19"

*Number of springs in the system. tSample identification. The first two letters are the initials of the state followed by a unique number asigned to each spring. Sm a.s.l. = elevation in meters above sea level. Chemical analyses were carried out by the American Public Health Association (1976) techniques: Ca by EDTA titration, SiO.~by colorimetry, Na and K by AA.

Anzo~itegui, d e s c r i b e d in detail by R o d r i g u e z (1983). S e v e n g e o t h e r m a l systems have b e e n d e f i n e d , a n d the data are s u m m a r i z e d in T a b l e 1, w h e r e a s the results of g e o t h e r m o m e t r y a p p e a r in T a b l e 2.

Mundo Nuevo Six cold springs with H2S o d o r discharge n e a r the t o w n of M u n d o N u e v o from the a l l u v i u m of Rio A m a n a . T h e a l l u v i u m covers U p p e r C r e t a c e o u s cherts a n d black shales. B a s e d o n b o t h t Q C a n d t N K C ( 1 ) e s t i m a t e s we suggest that the aquifers that feed these springs h a v e t e m p e r a t u r e s not higher t h a n 50°C. Mixing of cold a n d hot waters s e e m s p l a u s i b l e , b u t the r a n g e s in spring t e m p e r a t u r e , SiO~, a n d C! are too n a r r o w to define the h o t e n d m e m b e r t e m p e r a t u r e .

Urica T w o w a r m springs discharge n e a r the t o w n of U r i c a in a s w a m p y area. T h e springs issue from u n c o n s o l i d a t e d s e d i m e n t s of P l i o - P l e i s t o c e n e age very n e a r a n u n c o n f o r m a b l e c o n t a c t a b o v e E o c e n e - M i o c e n e rocks. N o faults have b e e n m a p p e d in the spring a r e a , b u t the large a n d n e o t e c t o n i c a l l y active U r i c a fault is located 2.5 k m to the southwest. B o t h t Q C a n d t N K C ( 1 ) suggest m a x i m u m a q u i f e r t e m p e r a t u r e s of 60-70°C.

Geothermal Reconnaissance of Northeastern Venezuela

409

Date

Flow (I rain -l)

pH

t (°C)

TDS

SiO:

Ca

Mg

Na

K

HCO 3

SO4

3 July 81 7 July 81 14 July 81 18 July 81 16 July 81 14 July 81 20 July 81

30 15 90 30 120 90 120

6.9 7.1 6.8 6.6 7.0 6.6 6.7

26 36 23 52 45 33 44

411 270 610 554 1080 565 429

14 21 9 26 18 9 17

85 42 140 24 52 140 11

14 8 5 10 17 15 17

50 42 87 170 87 41 130

1.6 8.6 2.0 11 20 1.2 1.0

390 260 595 420 465 360 355

32 4 20 28 5 140 24

15 15 42 75 65 37 50

0.4 0.4 0.2 0.6 0.8 0.6 0.4

20 May 81 25 May 81

900 200

6.8 7.2

28 27

475 347

13 10

140 110

0 5

41 14

1.2

0.8

445 375

28 11

30 7

0.2 0.1

28 May 81 5 June 81

60 180

7.2 7.2

27 36

949 2180

14 30

96 14

24 15

250 800

4.7 15

840 240

48 70

100 1100

0.3 0.0

30 June 81 10 May 81

15 30

7.0 7.0

26 28

377 420

19 17

96 26

12 14

10 120

0.9 1.6

190 335

140 40

5 39

0.2 0.3

C1

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Bergant[n This a r e a c o n t a i n s 12 low d i s c h a r g e , cold springs with H2S o d o r which s e e p f r o m U p p e r C r e t a c e o u s b l a c k shales a n d c o n c r e t i o n a r y p y r i t e - b e a r i n g p e l a g i c l i m e s t o n e s . G e o t h e r m o m e t e r e s t i m a t e s a r e less t h a n 50°C.

San Diego A l o n g t h e valleys o f the R i o N e v e r i n e a r San D i e g o t h e r e are 10 w a r m to h o t springs which issue m a i n l y f r o m s t r o n g l y f a u l t e d and f o l d e d shales a n d s a n d s t o n e s of E a r l y a n d L a t e C r e t a c e o u s age. This a r e a has the highest t e m p e r a t u r e springs of the state o f Anzo~itegui (53°C). G e o t h e r m o m e t e r e s t i m a t e s [ t Q C a n d t N K C ( 1 ) ] are r a t h e r c o n s i s t e n t in t h e r a n g e 50-70°C, b u t m i x i n g m a y b e n e c e s s a r y to e x p l a i n the t e m p e r a t u r e a n d c h e m i c a l v a r i a b i l i t y . A t vs SiO2 p l o t i n d i c a t e s a t e m p e r a t u r e o f 108°C ( t W S M M ) for the p o s s i b l e d e e p h o t aquifer.

Naricual T h e t o w n o f N a r i c u a l , k n o w n for its o l d coal m i n e , is n e a r t h r e e h o t springs which d i s c h a r g e f r o m c o a l - b e a r i n g s h a l y r o c k s o f T e r t i a r y age. T h e t Q C a n d t N K C ( 1 ) e s t i m a t e s a r e quite c o n s i s t e n t in t h e r a n g e 50-70°C.

Pozuelos N e a r the t o w n of P o z u e l o s o c c u r two h i g h - d i s c h a r g e w a r m springs, o n e o f which is u s e d as b o t t l e d m i n e r a l w a t e r . G e o t h e r m o m e t e r e s t i m a t e s of less t h a n 35°C a r e i n t e r p r e t e d to i n d i c a t e

410

F. Urbani Table 2. Chemical geothermometry of thermal and non-thermal spring waters from Anzo~itegui Geothermometers

System name

ID

Measured t (°C)

tQC

tNKC(4/3)

tNKCM

tNKC( 1 )

(1) Mundo Nuevo

An.1 An.2 An.3 An.4 An.5 An.6

26 26 24 24 24 24

44 50 24 24 27 29

12 14 -7 -3 -3 0

4~ 40 21 24 31 32

(2) Urica

An.7

36

65

70

70

(3) Bergantin

An.9 An.10A An.10B An. 10(2 An. 11 An.12 An.12M An.13 An.14 An.16 An.17 An.20

23 23 23 23 23 24 23 24 23 24 24 23

24 55 33 29 29 24 13 37 47 13 33 33

-5 -5 14 24 16 36 - 13 4 1 0 -23 -6

32 39 44 48 48 57 32 43 43 28 20 29

(4) San Diego

An.23 An.24 An.25 An.26A An.26B An.26C An.26D An.28 An. 29

44 43 29 43 52 51 50 53 35

65 47 33 57 74 74 55 57 37

174 98 149 39 103 57 47 117 22

An.18 An.19 An.22

26 45 32

47 59 74

49 99 54

(6) Pozuelos

An.15 An.31A An.31B

33 33 34

33 57 33

0 14 14

35 26 27

(7) Clarines

An.32 An.33 An .34 An.35 An.36

26 26 36 44 52

13 37 65 57 65

78 81 46 44 23

58 60 73 69 61

(5) Naricual

84 48 28 47 -39

58

70 60 58 64 62 68 65 64 17 56 46 48

Abbreviations and symbols for use in Tables 2, 3 and 5: ID = spring identification; tCH chalcedony geothermometer with conductive cooling; tQC quartz geothermometer with conductive cooling; tNKC(I/3), tNKC(4/3) and tNKCM, Na-K--Ca geothermometers b = 1/3, b = 4/3 or with Mg correction; tNKC(1) Na-K---C geothermometer of Benjamin et a/. (1983) t < 100*C; values with * show preferred values following the rules of the Na-K-Ca-Mg geothermometers; cold appears under tNKCM when R > 50.

that these waters owe their origin to shallow circulation and never attain temperatures hotter than the spring temperature.

much

Clarines

T h i s l o c a l i t y ( F i g . 1 C ) h a s six h o t a n d c o l d s p r i n g s d i s c h a r g i n g f r o m a c o m p l e x t h r u s t z o n e involving Late Cretaceous to Late Tertiary formations. Geothermometers g i v e e s t i m a t e s o f less

411

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Fig. 4. A - F plots of D ' A m o r e et al. (1983) for the geothermal systems of the states of Anzofitegui and Monagas. This plot is useful for water classification and the A - F parameters are calculated in mg I -t units as follows, where S A and S C are the sums of anions and cations, respectively:

A = [(HCO3 SO4)/SA]IO0:

B = [(SOJSA) - (Na/SC)]IO0; C = [(Na/SC- CI/SA]IO0; D = [(Na- Mg)/SC]IO0; E = [((Ca + Mg)/SC) - (HCO3/SA)]IO0; F = [(Ca - Na - K)/SC]IO0. -

than 80°C, and the plots of t vs C1 and t vs SiO 2indicate mixing with a deep hot water component of 90°C (tWSMM). An overall view of the geothermometer results for the systems described above reveals that tNKC(4/3) and tNKCM give quite erratic values, while tNKC(1) and tQC are much more consistent with each other. The chalcedony geothermometer (tCH) is not given in Table 2 because it produces values lower than spring temperature. From this region two water samples (An. 11 and An.26) from San Diego and Bergantin were analysed for stable isotopes, and the values plot on Craig's world meteoric line. The A - F plots for the geothermal systems of Anzo~itegui show two trends, one for the Ca-HCO 3 waters of Pozuelos, Mundo Nuevo, Bergantin and Naricual and the other for the Na-HCO3 type of San Diego and Clarines (Fig. 4a). Looking at this region as a whole, the recharge area of the thermal springs occurs in the high mountains to the east. The mountains consist mainly of sandstones and limestones of Early Cretaceous and black shales of Late Cretaceous age and reach elevations of up to 2700 m. Due to the relatively low temperature estimates in this region, only low-temperature direct applications are possible. G E O T H E R M A L SYSTEMS OF NORTHERN MONAGAS The warm springs in this region are located near the towns of El Guamo, San Francisco, Rio Aragua, El Pinto and Caripito (Fig. 5) and form three clusters that will be referred to as Caripito, El Pinto and San Francisco. Each site has two groups of fluids of different chemical composition. These springs have been described in detail by Zannin and Marifio (1983). Tables 1 and 3 present a summary of the field, analytical and geothermometric data. Caripito region

The Los Morros de Caripito group has six cold to warm springs, some of high discharge, aligned along the western" side of the northwest-trending San Juan Graben. The highest discharge spring of this group is used as a bathing spa. The Azagua group has two springs which

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" - - : .............

k

"*'--'C

I

d)

\\'\

PUNCERES 63°30 ' WATER

SPRINGS.

c

GRABEN ~

',, LOS\ K\,..~ ,.~ MO~OS\

\.--.~j. k

~

DE

\

"-, ~ u . C A R I P I T O

~.~ c A ~ L P ~ 0

...... EL PINTO

I

10 km

Fig. 5. Geologic map of northern Monagas and location of springs. The cross section represents a regional structural model from Castro and Zamora (1982).

discharge from the northwest-trending Azagua fault zone. In both areas the water discharge from black limestones and shales of Upper Cretaceous age. The results of chemical classification and geothermometers show that both groups are rather similar, with temperature estimates [tQC and tNKC(1)] in the range 30-50°C. The Ca-HCO3 type water and the large discharge of the main springs of the area indicate that the recharge area lies in the high mountains to the west, where extensive areas of karst topography occur. El Pinto

This cluster has 10 cold to warm springs with H2S odor. The springs issue close to a regional unconformity between Cretaceous limestones and early Tertiary well consolidated shales and sandstones, covered by younger sediments of late Tertiary age. The unconformity also controls the distribution of more than 50 oil seeps. As seen in Table 1, two types of water are found here. The Punceres group has indicated reservoir temperatures [tQC and tNKC(1)] of 50-70°C, but there is evidence of mixing that may permit a temperature of more than 180°C for the deep hot water component. The Los Bafios group has the higher discharge of the cluster and has the highest temperature and salinity in the state of Monagas (maximum values of 36°C and 4300 mg i-l). The geothermometer temperatures range between 60-80°C (tQC) and 95-125°C [tNKC(1/ 3) and tNKC(h)]. Several plots of t, SiOe and CI suggest mixing, and their evaluation allows an estimate (tWSMM) of about 150°C. It seems plausible that these springs are a mixture of deep Na-CI hot water (plus hydrocarbons) coming from the marine Tertiary oil-bearing basin to the south, with colder C a - H C O 3 water. Recharge probably originates in the high mountains to the north (see cross section in Fig. 5), which also have karst topography.

Geothermal Reconnaissance of Northeastern Venezuela

413

San Francisco The springs of this area are located along valleys formed by the San Francisco fault, in which the geology is rather complex due to much faulting and folding. The E1 Guamo spring group comprises 12 cold Ca-HCO3 water springs with H2S; some springs are now covered by lake water at El Guamo reservoir. These low discharge springs have rather low geothermometer estimates of 50-70°C for tQC, whereas the tNKC(1) estimates are usually colder than the spring temperature. These waters do not seem to have been much hotter at depth. One spring discharges from a travertine which includes small amounts of stilbite. The Rio Aragua group comprises four low discharge cold to warm Na-HCO3 springs with H2S odor. The geothermometers estimates are very coincident around 40-70°C [tQC and tNKC(1)]. The plots of t vs CI and CI vs Na suggest mixing, but the SiO2 values are too low and variable (10-26 mg l -l) to permit estimation of tWSMM temperature. As can be seen in Table 3, the tNKC(4/3) geothermometer applied to the systems described

Table 3. Chemical geothermometry of thermal and non-thermal spring waters from Monagas Geothermometers

System name (1) Caripito (a) Los Morros

(b) Azagua (2) El Pinto (a) Punceres

(b) Los Bafios

(3) San Francisco (a) El Guamo

(b) Rio Aragua

ID

Measured t (°C)

tQC

Mo.1 Mo.2 Mo.3 Mo.4 Mo.5 Mo.8 Mo.6 Mo.22

26 26 27 28 28 26 27 23

Mo.ll Mo. 12 Mo. 14 Mo. 15 Mo.26 Mo.27 Mo.29 Mo.18 Mo. 19 Mo.25 Mo.30 Mo.31 Mo.32 Mo.33 Mo.34 Mo.35 Mo.36 Mo.37 Mo.38 Mo.39 Mo.40 Mo.45 Mo.41 Mo.42 Mo.43 Mo.44

Abbreviations as in Table 2.

tNKC (1/3)

tNKC (4/3)

tNKC (1)

47 47 47 47 47 41 37 50

-2 2 1 0 - 1 -25 -11 37

29 30 32 35 39 20 23 42

27 25 29 27 29 30 24 36 27 24

50 37 52 52 57 61 44 79 74 61

51 48 104 67 57 57 65 155 202 37

61 57 75 69 62 60 63 93 115 54

25 25 25 24 25 25 26 25 23 23 25 23 25 25 28 28

57 57 57 69 74 44 61 52 52 47 50 41 37 63 57 37

14 14 2 -7 5 2 -8 -6 7 -7 -23 -33 46 29 39 97

33 37 18 20 23 21 19 23 26 19 12 9 38 45 60 79

127 116 97

414

F. Urbani

above gives very erratic results. This shows that tNKC(4/3) is not well calibrated in low temperature systems. On the other hand, the formulation proposed by Benjamin et al. (1983) produces more consistent values. Some large discharge springs of northern Monagas are appropriate for bathing spas, as already exist at Los Morros de Caripito and in a rudimentary way at Los Bafios. The latter spa could be much improved, since it has a better temperature than the waters of Caripito (38°C vs 28°C). The relatively high temperatures estimated at Los Bafios (near 150°C) should provide enough incentive to continue more detailed exploratory work. G E O T H E R M A L SYSTEMS OF SUCRE Most of the springs in this state are located in the geographic depression between the Araya-Paria cordillera to the north and the Eastern Mountain Massif to the south (Fig. 2A). This region is broadly controlled by the E1 Pilar Fault system. A geothermal inventory of the state was previously carried out by Hevia and Di Gianni (1984). In this work the springs will be grouped into three broad geographic regions: (1) Paria gulf; (2) Cuman~i-Cariaco--Putucual; (3) Las Minas-Rio Casanay (Fig. 6). Paria Gulf region The springs of this region are located around the western termination of the gulf, usually near the boundary between Quaternary alluvium and the mountainous surroundings. In this context there are springs at the northern margin of the basin (Maraval, Qda. Seca and lqo Carlos) and four groups of springs at the western end of the basin in the Cumacatal-Catana area. These springs seem broadly aligned along a NW-SE trend that parallels the major fault trend in the region. The distribution of hydrocarbon seeps also follows the same structural trend as the springs. Field, analytical and geothermometric data from these springs are presented in Tables 4 and 5a. The Maraval spring discharges from Mesozoic calcareous schist whereas the Qda. Seca springs seep from a brackish water swamp in Quaternary alluvium near metamorphic rocks. Calcareous schists and marbles abound in the area and may be the source of the high calcium concentration in the thermal water, whereas the high Na and C1 concentrations in Qda. Seca are due to seawater contamination. Geothermometer estimates give a range of 55-65°C for Maraval and

64 o

I

O K M

QUATERNARY COYER

63°30 '

CARIBBEAN

- -

CRETACEOUS SEDIMENTARY ROCKS MESOZOIC METAMORPHIC ROCKS

~ ~

SEA

~

MAJOR HIGH ANGLE FAULTS CHUPARIPAL TRUST FAULT CONTACT OF QUATERNARY ROCKS ISTPLED SIDE) WITH OLDER ROCKS

63 °

r,

~) HOT SPRINGS HYDROCARBON SEEPS (~) AREA WITHMANY (>SO) SMALL /UIO~W~fEES OF RHYODACITIC ROCKS (SMs)

-]

IN CUMACATAL REGION 1 2. 3 4

SANIA ANA (~JARIMAN CATANA MARE-MARE

Fig. 6. Generalized geological and location map of the central part of Sucre.

o

,o

i

KM

,.o

Geothermal Reconnaissance of Northeastern Venezuela

415

44(tQC)-118°C (tNKCM) at Qda. Seca. This last value may be too high due to seawater contamination. The lqo Carlos springs issue from Quaternary alluvium only a few tens of meters from outcrops of calcareous schist. The springs deposit calcite, to produce small crater-like mounds. Chemical geothermometry gives a wide span of results from 100 to 200°C (tNKCM). Calculations with double the amount of Ca so as to overcome the loss of Ca by precipitation result in the same range. These high temperature estimates could be partially caused by seawater contamination, but the linear t vs CI and t vs SiO2 relationships suggest underground mixing. Analytical data from samples collected in 1976 allow an estimate of a hot-water end member of the order of 150-200°C (tWSMM). Further work is desirable in this area. In the Cumacatal-Catana region the hottest spring is at Mare-Mare in a swampy area surrounded by seawater channels ("carlos"). Travertine is currently being deposited by the springs, and the waters are of the Na-C1 type. The Catana springs flow from outcrops of carbonate rocks of the E1 Cantil Formation. These springs occur near an oil seep, and are Ca-HCO3 in character. The geothermometers for Mare-Mare and Catana give estimates in the range 60-76°C, not much above the maximum spring temperature of 58°C. Plots of t vs C1 and t vs SiO2 suggest mixing with a possible hot end member near 80°C (tWSMM). The Guarim~in and Santa Ana springs discharge from outcrops of Early Cretaceous carbonate rocks and are only slightly warm; chemical geothermometry estimates less than 50°C. At Santa Ana there are two vigorously bubbling Ca-HCO3 cold springs (Su.24 and Su.26) with high CO2 and CH4 contents (Table 6). Discharge temperatures in this region decrease toward the NW [Mare-Mare (55-58°C), Catana (38-48°C), Guarim~in (28-30°C) and Santa Ana (24--26°C)]. Apparently all the springs recharge in the mountains to the west, where carbonate rocks occur. The water is heated during its underground transit and moves eastward toward the mountain piedmonts near the alluvial flatlands of the Paria basin. There the fluids mix with hydrocarbons (oil and CH4) from the sedimentary basin.

Cuman(l-Cariaco-Putucual region This region is dominated by the Cariaco Gulf and the structurally related wide alluvium filled valley which continues eastward to Rio Casanay (Fig. 6). The basin is broadly controlled by the El Pilar fault system, and warm and hot springs are located along its southern side. This zone corresponds to the northern piedmont of the Eastern Mountain Massif, and it has good recharge areas in sandstones and limestones of Early Cretaceous age. On the other hand, the Putucual area is at the terminus of the San Juan Graben mentioned in the section about Monagas. Some of the springs are contaminated by seawater (Table 4). Geothermometer temperatures for this group are shown in Table 5b. The Pantofio group is characterized by dilute, high discharge, warm springs which are mainly used in bathing spas. The springs issue from alluvium. Geothermometry indicates temperatures of less than 80°C. At Putucual, the springs discharge from alluvium, are warm to hot (28-48°C) and have low discharge rates. One spring actively deposits calcite. Geothermometers estimate temperatures of less than 55°C. The Cariaco group of springs occurs very near the coast line of the Cariaco Gulf. One spring is artesian a few meters offshore. These springs may be partially contaminated by seawater. Two slightly different water types are found at Cotua and at Punta Gorda-Cachamaure (Table 5b). Geothermometers give values in the range 50-110°C for tQC and up to 95°C for tNKCM. The Punta Gorda group shows evidence of mixing, allowing an estimate of the order of 150°C (tWSMM) for the hot end member. However, the data have a narrow range of values

F. Urbani

416

Table 4. Field data and chemical analyses of some selected thermal and non-thermal spring waters from Sucre (analyses reported in mg l -t) Data from a selected spring water from each system General data of system System name

No.

t (°C) rain-max

Geographical coordinates TDS av.

ID

Elev. (m a.s.l.)

Su.1 Su.2 Su.5 SuUCV33 Su.6A Su.96 Su.24

Longitude

Latitude

80 5 8 0 70 40 50

62°33'49" 62*43'25" 63° 4' 1" 63° 4'30" 63° 7' 4" 63° 9 ' 1 9 " 63"11' I 1"

10°38'42" 10o36'48 " 10°35, 2" 10o26'40 '' 10°27'13 '' 10*28'21" 10°29'34"

(C) Paria Gulf Region (1) (2) (3) (4) (5) (6) (7)

Maraval Qda. Seca* lqoCarlos* Mare-Mare* Catana Guarim~in Santa Ana

I 1 5 2 2 2 7

32 30 35-51 55-58 28-40 28-30 24-26

(D) Cumana--Cariaco-Putucual (1) Pant.rio 17 (2) Putucual* 3 (3) Cariaco* 13 (4) Santa Marfa 1 (5) Los Ipures 5 (6) Tacal 2 (7) Araya* 1

region 25-37 28--47 33--60 27 31--47 25-28 36

(E) Las Minas-Casanay region (1) Las Minas Rio Janeiro 1 Aguas Calientes 10 Chirriaderos 1 Las Minas 5 Mina Alemana (2) Mundo Nuevo (a) acid-SO4 (b) Ca-HCO3 (3) Rio Casanay

1750 9490 4895 2110 911 447 423 526 1132 1492 270 2638 462 15280

Su.43 Su.22F Su.84 Su.90 Su.93 Su. 102 Su.I01

60 20 5 375 32 70 15

63°26' 4" 63°17'40"' 63041'59" 63°33'48" 64* 8 ' 5 7 " 64°15'55" 64012' 7"

10°28'57" 10"24'12" 10028' 8" 10019'10" 10"23'25" 10"21'59" 10"36'19"

99-100 59-98 94-100 34-96

310 850 420 607

1

29

164

Su.15w Su.7A Su.14w Su. 18w Su.17w Su.16

132 73 155 280 232 190

63°11'48" 63"12'36" 63"13' 8" 63012'28" 63012'00" 63"12'20"

10"31'46" 10"31'35" 10"31'16" 10"31'52" 10"32'12" 10031'30 "

13 8 2

30-97 25-45 32-37

552 248 371

Su.45A Su.52 Su.36

350 206 190

63°15' 16" 63"15'29" 63"17'45"

10°30'57 " 10"31' 6" 10031' 7"

Abbreviationsas in Table 1. *Seawater contaminationmay exist. Samples with ID's terminating in w are from West Japan Engineering Consultants Inc. (1984).

( t e m p e r a t u r e 34-46°C, SiO2 13-39 mg l - t ) , i n d i c a t i n g that m o r e i n f o r m a t i o n is n e e d e d to s u p p o r t this high e s t i m a t e . Los I p u r e s is a g r o u p of springs l o c a t e d in a valley c o n t r o l l e d by a N E - S W t r e n d i n g fault that cuts E a r l y C r e t a c e o u s s a n d s t o n e s . T h e springs d e p o s i t calcite to f o r m small t r a v e r t i n e m o u n d s . T h e g e o t h e r m o m e t e r s b r a c k e t the 5 0 - 9 5 ° C r a n g e . S o m e ion plots m a y suggest m i x i n g , b u t n o t e n o u g h i n f o r m a t i o n is available to e s t i m a t e the h o t e n d m e m b e r t e m p e r a t u r e . T h e T a c a l a n d A r a y a springs have low discharge rate a n d low t e m p e r a t u r e s . T h e A r a y a spring discharges n e a r the e v a p o r a t i o n p o n d s of a salt p r o c e s s i n g facility a n d has a very high T D S c o n t e n t . G e o t h e r m o m e t e r s e s t i m a t e t e m p e r a t u r e s in the 50-600C r a n g e . A l l springs in the r e g i o n are a p p a r e n t l y r e c h a r g e d f r o m E a r l y C r e t a c e o u s s a n d s t o n e s a n d l i m e s t o n e s a n d to a lesser e x t e n t f r o m L a t e C r e t a c e o u s cherts, b l a c k shales a n d l i m e s t o n e s . T h e springs h a v e surface t e m p e r a t u r e s of u p to 60°C, a n d g e o t h e r m o m e t e r e s t i m a t e s are u s u a l l y less t h a n 100°C. T h e springs with high discharge rates a n d m o d e r a t e t e m p e r a t u r e s are used for

Geothermal Reconnaissance of North eastern Venezuela

Date

18 May 81 19May 81 21 May 81 13 Jan. 76 22May 81 1May 82 11 June 81 24 July 81 9 June 81 18 Aug. 81 19 Aug. 81 20 Aug. 81 8 May 81 6 May 81

81 25 May 81 81 81 81 4 June 81 30 July 81 1 Aug. 81 14 July 81

Flow (lmin-1)

53 100

5

200 300 200

pH

t (°C)

6.3 7.3 7.6 6.6 6.6 6.6 6.7

31 30 45 58 40 30 24

1780 9480 5010 2140 1000 625 440

21 12 55 21 26 11 6

310 , 320 580 260 240 210 110

7,1 6.7 7.0 7.0 7.8 6.9 7.0

37 45 60 27 47 28 36

807 1950 2440 270 2800 550 15300

9 33 71 7 28 16 33

7.8 7.0 7.3 2.6 2.7 7.1

99 94 96 94 97 29

3310 3270 2420 810 1360 164

5.0 6.7 7.1

91 45 37

776 206 404

TDS

SiO 2

Ca

Mg

417

Na

K

HCO3

SO4

CI

F

19 48 24 48 96 10 17

360 3300 1100 450 12 9.0 28

10 280 200 37 6.8 2.0 1.3

1390 5420 2360 1060 1240 715 340

38 12 850 2 1 7 80

340 2900 1000 740 8 9 28

<0.05 <0.05 <0.05 1.3 0.1 0.2 <0.05

110 160 12 48 13 150 880

21 85 17 29 14 27 460

130 500 920 14 1100 3.0 3700

22 6.0 43 0.8 82 5.0 9.0

195 1500 1950 281 2100 280 29

22 33 120 16 100 110 3

280 400 300 14 470 96 9200

0.9 1.3 2.0 (I.2 0.6 0.2 0.3

148 159 90 44 55 16

120 180 170 24 300 44

1 5 26 3 17 2

840 850 620 3.2 3.9 5.1

300 240 160 1.9 3.1 0.6

50 695 160 0 0 90

87 200 66 720 980 44

1800 130(I 1300 5 6 6

1.7 1.4 2.2 4.1 4.6 0.2

15 26 13

14 40 70

1 16 25

-0.5 5.5 35

1.2 1.9 2.0

0 180 235

700 18 110

4 8 30

0.5 0.3 0.3

bathing at Pantofio and Cachamaure. More exploration may locate aquifers in the 60-80°C range, which could be used in fish-related processing facilities.

Las Minas-Rio Casanay region This is by far the most important geothermal region of Venezuela. The springs roughly follow the trend of the El Pilar fault system (Fig. 6), although at a local scale some springs seem to be associated with other NW-SE and SW-NE trending faults near their intersections with the El Pilar zone. Between Rio Casanay and Las Minas there is a mountainous zone that connects the mountains of the Araya-Paria metamorphic cordillera and the Eastern Mountain Massif. This zone interrupts the lowlands associated with the Cariaco and Paria Gulfs. Another major geologic feature of this region is the Chuparipal thrust that places the northern metamorphic rocks over younger sedimentary rocks. The geology of the area is very complex. It has been studied by several workers, and rather different stratigraphic and tectonic interpretations have been reached (Christansen, 1961; Metz, 1964; Vignali, 1977; Vierbuchen, 1978, 1984; Campos, 1981). From west to east the hot springs are clustered in three areas (Fig. 6): (1) near Rio

418

F. Urbani

Table 5(a). Chemical geothermometers of thermal and non-thermal spring waters from the Paria gulf region, Sucre Geothermometers Measured System name

ID

t(°C)

tQC

tCH

tNKC (l/3)

tNKC (4/3)

tNKCM

tNKC (1)

Na-HCO3 w a t e r t y p e (1) Maraval Su.1

31

65

33

(2) Qda. Seca

Su.2

30

44

12

201

230

118"

87

(3) (4o Carlos

Su.4 Su.4f Su.5 Su.5a Su.5b Su.5c Su.5w

51 50 45 38 35 42 50

47 105 106 110 107 113 95

15 75 76 81 77 84 64

215 139 216

205*

144 146 214

166 97* 165 78* 114 103 147

79* 103" 155'

65 66 65 77 70 70 59

SuUCV32 SuUCV33

55 58

63 65

31 33

190 165

125 104

67* 76*

59 59

48 38

74 72

4l 40

20 17

13 13

(4) Mare-Mare

C a - H C O 3 water type (5) Catana Su.6A Su.6B

54*

57

186"

(6) Guafim~n

Su.96 Su.97

30 28

41 47

9 15

47 7

9 15

(7) Santa Ana

Su.24 Su.25 Su.27A Su.27B Su.27D Su.98 Su.99

24 25 26 26 26 26 25

19 19 18 52 50 41 70

-9 -9 -10 20 18 9 38

2 -34 -8 - 18 5 8 -11

31 24 14 36 37 9 6

Abbreviations as in Table 2.

Casanay (Fig. 7); (2) Las Minas (Fig. 8a); (3) Mundo Nuevo (Fig. 8b). This region is the only one in Venezuela having acid-sulfate springs and argillic alteration zones characterized by vegetation-free surfaces with barren, bleached rocks. The local name "Azufrales" (sulfur mine) stems from the presence of sulfur crystals and the typical H2S odor. On a regional scale the Mundo Nuevo-Las Minas zone is located at the intersection of the El Pilar fault system and two large faults or lineaments with SW-NE and SE-NW trends (Fig. 6). This tectonic setting may facilitate the ascent of deep fluids. Approximately 5 km north of this area there are outcrops of rhyodacite intrusives having a single age date of 5 Ma; these are the youngest volcanics known in eastern Venezuela (Fig. 6).

Rio Casanay The geology of the area is shown in Fig. 7. The area is crossed by the east-west flowing Rio Casanay, along which two Mg-rich warm springs discharge (average elevation 155 m a.s.1.). At the southern margin of the wide valley at higher elevation (average 240 m a.s.l.) there are four small (tens of meters) inactive argillic acid-sulfate alteration zones with ancient deposits that include anhydrite, gypsum and silica sinter (chalcedony and opaline silica). Such zones discharge no fluid, although at Los Cerritos (Su.32), during the rainy season, the ground is saturated with warm water. The high Mg content of the springs is attributed to the marie and ultramafic rocks that are regionally distributed along and near the Chuparipal thrust fault (Fig. 7). The waters are

Geothermal Reconnaissance of Northeastern Venezuela

419

Table 5(b). Chemical geothermometers of thermal and non-thermal spring waters from the Cuman~i-Cariaco-Putucual region, Sucre Geothermometers Measured System name

tNKC (1/3)

tNKC (4/3)

t (°C)

tQC

tCH

Su.19 Su.19A Su.20 Su.21 Su.41 Su.42 Su.43 Su.43M Su.44 Su.66 Su.67 Su.68 Su.70 Su.71 Su.72 Su.95

32 29 32 32 33 29 37 30 25 32 34 33 31 30 33 31

77 78 65 69 77 67 33 86 88 67 72 69 65 55 61 65

45 46 33 36 45 35 3 54 57 35 40 36 33 22 29 33

(2) Putucual

Su.22 Su.23 Su.39

47 39 28

54 53 44

21 21 12

116

56* 32* 132

23*

40 40 40

(3) Cariaco (a) Cotua

Su.84 Su.85 Su.86A Su.86B Su.87 Su.88A Su.88B Su.89 Su.77 Su.76 Su.75 Su.74

60 42 41 35 54 55 38 34 34 33 41 46

119 118 118 63 73 50 79 63 59 47 75 91

90 89 89 30 41 18 48 30 27 15 43 59

174 171 172 178 167 178 181 178 292

224 211 210 241 208 150 161 246 185

22* 53* 53* 32* 60* 19' 20* 24* cold*

94 92 91 97 93 71 74 98 53

284 295

146 184

cold* cold*

44 52

(4) Ipures

Su.92 Su.93 Su.94A Su.94E Su.94G

40 47 31 35 35

85 77 65 51 59

54 45 32 19 26

209 204 206 213 219

274 269 259 266 294

51" 59* 55* 53* 41"

94 95 92 91 95

(5) El Tacal

Su.102 Su.103

27 25

55 52

22 20

(1) Pantofio

(b) Punta Gorda-

Cachamaure

191

197 226 214 231

9* 67* 23* 20* 114 56* 82* 105 56* 100 111 111 93 80* 94* 70*

10" 10"

tNKCM

tNKC (1)

ID

cold*

98* 91" cold* 81" 70*

30 45 38 26 55 39 40 50 38 41 48 44 45 44 44 47

25

Abbreviations as in Table 2.

very dilute (TDS = 371 mg 1-'), and geothermometers give estimated temperatures in the range 47-55°C (tQC). These waters may be mixed with groundwater from the Rio Casanay alluvium. The inactive "azufrales" are interpreted to be either (I) the remnants of an ancient geothermal system now exhausted or with a much diminished activity, or (2) an active system below a lowered water table. According to local sources, degradation of the forest has lowered the water table in this region, so we prefer the second alternative. Mundo Nuevo In the vicinity of the village of Mundo Nuevo there is an elongated zone of about 2000 x 300 m characterized by "azufrales" and cold to boiling acid-sulfate springs (Fig. 8b). The springs

420

F. Urbani

Table 5(c). Chemical geothermometers of thermal and non-thermal spring waters from Las Minas-Rio Casanay region, Sucre Geothermometers

Locality

ID

Measured t(°C)

(1) Las Minas-A~uas Calientes Na-CI water type (with silica sinter deposition) Rfo Janeiro Su. 15w 99 Na-Cl water type (with calcite deposition) Aguas Su.7A 93 Su.7B 79 Calientes Su.7w 95 Su.8 84 Su.9 85 Su.10A 92 Su.10B 89 Su.ll 90 Su.12 84 Su.28 59 Chirriaderos Su.14w 96 Ca-HC03 water type Su.16 Mina Aiemana

tQC

tQA

tNKC (1/3)

tNKC (4/3)

tNKCM

160

154

280

251

279*

165 163 158 92 85

158 156 152 93 87

83 85 98 29 131

86 87 99 40 128

257 259 267 259 261 250 256 263 258 304 241

217 212 204 213 215 213 220 219 205 171 187

244* 213" 185" 215" 217" 208* 245* 240* 246* 206* 132"

tNKC (1)

28

55

-12

1

Su.57 Su.58 Su.59 Su.65 Su.52 Su.53 Su.78 Su.83

29 25 33 37 45 42 42 35

29 29 52 44 74 96 63 102

-17 17 0 -t0 13 1 -1 25

-15 22 24 13 15 12 13 13

Su. 36 Su. 31

37 32

47 55

18 0

35 12

(2) Mundo Nuevo Ca-HCO 3 waters

(3) Rio Casanay Mg-HC03 waters

Abbreviations as in Table 2.

usually have low water discharge, and some ponds vigorously bubble CO2-rich gas. Along a ENE trend there are two large alteration zones displaying small aligned cone shaped hills up to 10 m in elevation that resemble phreatic explosion cones made up of sandstone fragments. The "azufrales" show ancient mineral deposits of gypsum and silica minerals (chalcedony and opal). Areas surrounding the active acid-sulfate zone have several cold to warm Ca-HCO3 springs with low flow. Geothermometric data (Table 5c) for the Ca-HCO3 waters show a temperature range from 25 to 100*C for tQC, whereas tNKC systematically gives estimates below discharge temperatures. This appears to be steam-heated groundwater composed of condensed steam, CO2 and shallow surface water overlying or surrounding a hotter vapor-rich zone (Mahon et al., 1980; Goff et al., 1985). On the other hand the acid-sulfate waters are not suitable for the application of SiO2 and Na-K-Ca-Mg geothermometers. Acid-sulfate waters seem to consist of condensed steam and oxidized H2S mixed with various proportions of shallow groundwater (White et al., 1971). Therefore, the acid-sulfate area of Mundo Nuevo is apparently related to a deep neutral-chloride water with temperatures equal to or greater than 200°C.

B.2.b B.2.b B,2,a

A.3 A.4

C.7 D.2 E.2.a E.2.a E.2.a E.2.a E.2.a

Mo.14 Mo.18 Mo.26

An.ll An.26

Su.26 Su.39 Su.50C Su.58 Su.60A Su.60B Su.85

66.74 91.45 71.66 86.35 82.70 86.50 78.33

2.19 15.92

7.38 12.92 7,48

CO2

18.36 <0.01 2.27 2.56 5.24 4.27 5.28

13.65 17.84

52.32 70.18 35.48

CH4

0.02 <0.01 <0.01 <0.01 <0.01 <0.01 0.01

<0.01 0.02

0.02 0.03 0.02

C2H4

10.33 6.04 20.89 9.06 9.99 8.52 13.25

66.65 57.55

40.35 15.88 57.10

Nz

3.95 1.74 4.28 1.86 1.58 0.18 2.66

16.31 7.64

0.20 1.52 0.11

02

0.21 0.10 0.25 0.14 0.15 0.09 0.15

0.82 0.69

0.35 0.15 0.55

Ar

<0.005 <0.005 <0.005 <0.005 0.005 0.005 0.005

<0.005 0.030

0.050 0.020 0.080

He

<0.005 <0.005 <0.005 <0.005 0.050 0.040 <0.005

<0.005 <0.005

<0.005 <0.005 <0.005

H:

Analyses gas by W. C. Evans, isotopes by L. D. White, U.S.G.S., Menlo Park, CA, courtesy of !. Barnes. *Geothermal system code as in Tables 1 and 4. t l n CO2.

System*

ID

Gases (vol. %)

99.61 99.33 99.35 99.97 99.72 99.60 99.68

99.62 99.69

100.67 100.70 100.82

Total

-23.1 -19.1 - 12.3 - 19.6 -18.5 -11.9 -21.9

-21.7 -22.7

-26.9 -23.4 -19.5

6D

-4.22 -3.58 -3.04 -4.113 -3.99 -2.00 -4.05

-4.1)4 -4.17 -7.50 -3.16 -3.09 -7.05 -3.47 -2.98

613Ct

Water

-3.81 -3.40 -3.69

6'xO

Table 6. Chemical composition of gases and stable isotope composition of some Venezuelan hot spring waters

35.01 31.25 30.47 28,34 35.48 28.02

~IxOt

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422

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464

465

466

467

1163

1162

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CONCEALED TRACE EL PILAR FAULT CHUPARIPAL THRUST

MESOZOIC METAMORPHIC ROCKS B

SERPENTINITES

UTM COORDINATES

Fig. 7. Simplifiedgeologicmap of the Rio Casanayarea. Adapted from Hevia and Di Gianni (1984).

Las Minas In this general area (Figs 6 and 8a) the springs are located on the northern slope of the west--east trending valley of the Rio Chaguaramas. Acid-sulfate and Na-CI water are present. At an elevation of 230-280 m a.s.l, there are acid-sulfate springs and argillic alteration zones (El Salvaje and Buena Esperanza), whereas at lower elevations, Na-CI waters appear in the Aguas Calientes group (70-90 m), Rio de Janeiro (132 m) and Los Chirriaderos (155 m). Las Minas group. The locality of Las Minas was exploited when sulfur mining started in the middle of the 19th century. The mined areas occur in argillic altered Lower Cretaceous sandstones. Sulfur-impregnated bleached rocks were carried from El Salvaje to Mina Alemana (Fig. 8a), where they were steam-treated to extract the sulfur. At Las Minas there are three large alteration zones known as El Salvaje, Buena Esperanza and Mina Alemana. The last area is relatively inactive with only a cold, low discharge Ca-HCO3 spring. The El Salvaje and Buena Esperanza areas are strongly active, with vigorously boiling springs, abundant gas emissions and fumaroles having conduits coated with sublimated orthorombic sulfur crystals. Cortese (1904) reports red cinnabar together with sulfur. There are also low discharge, cold to warm springs with sulfur deposited by biogenic activity (filamentous bacteria). The waters of these localities are SO4 rich, and the pH is mainly in the 2-3 range. However, the main boiling springs of El Salvaje and Buena Esperanza in some years become near-neutral in pH during the dry season. All this suggests a plumbing system that has much shallow mixing. Old silica sinter is observed at El Salvaje, whereas at Buena Esperanza gypsum is currently forming around the alteration zone. Aguas Calientes group. This group has Na-CI water, and the main springs are Chirriaderos,

423

Geothermal Reconnaissance of Northeastern Venezuela LAS MINAS - AGUAS C A L I E N T E S

a

L477 \

1476

~

I,

'~

1479

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1480

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BUENA \ " ~ . E SPEFt~NZA ]EL ~: \ "\ RIO DE ^ "\ '~ .JANEIRO / " . SALVAJE ~. / x ~\ /~Z, ./~/ I . ~ S u 18 ..~ ~ x'x / "-~.~-~/

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,g~J~'-~...---" "? //AGuAS / I - - ' - - ~ . , , ' / ~ ( " C AL'E"TES _' Su12~Su7 _/ .... I / ..... ~---. ':'.to / , "";~Su 16 -~,~111./ ~- ~ I { MINA....... , Su13~. ~ ~... - ---.- - -,~., ,~g M,,~AS I , ALEMANA ~Su28 I'"t~. ~ " R~O C H A ~ u ~ " / AGUAS CALIENTF-JS % ~ • ..q. ,,~

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~) N a - C t SPRINGS, WARM TO BOILING, pH = 7.2

CREEKS DIRT ROAD

(~ C a - HCO 3 SPRINGS, COLD TO WARM, pH = 6.6 ~) SMALL ACID-SO 4 ALTERATION ZONE (ALL INACTIVE) '..i )

LARGE ACID-SO 4 ALTERATION ZONES

b

MUNDO NUEVO

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Fig. 8. Location map of geothermal features in (A) Las Minas-Aguas Calientes and (B) Mundo Nuevo areas. After Urbani (1984). Both areas are underlain mainly by Early Cretaceous sandstones.

Aguas Calientes and Rio de Janeiro (Fig. 8a). Los Chirriaderos (Su.14) is a boiling spring actively depositing calcite with a large (60 m long and about 5-10 m high) travertine deposit having a central fissure very similar to Soda Dam Spring in New Mexico (Goff and Shevenell, 1987). In the central fissure there are found grape-shaped masses of pisolitic calcite, formed in strongly agitated pools having CO2-rich emissions. The main spring at the Aguas Calientes locality (Su.7) also precipitates calcite and is surrounded by a large partially eroded travertine deposit made up of calcite and aragonite locally cross-cut by coarsely crystalized calcitic "dikes" and "sills". A smaller nearby spring (Su.13) contains old gypsum and anhydrite deposits. The Rio de Janeiro spring (Su. 15) only deposits silica, but some halite is present as efflorescences. The near-neutral Na-Cl waters produce rather dissimilar geothermometer estimates (Table 5c). Values in the range 150-160°C are obtained for tQC and tQA, whereas tNKCM gives estimates of up to as much as 279°C for the most concentrated spring of Rio de Janeiro (TDS = 3310 mg l -], Cl = 1820 mg 1-1, with silica sinter but no calcite deposition); more diluted water with active calcite deposition indicates temperatures of 200--240°C. The temperature and chemical variation of the springs of the Aguas Calientes group (which includes Rio de Janeiro

424

F. Urbani

and Chirriaderos) suggest rather shallow mixing, which together with the silica precipitation could be the cause of the low temperatures estimated from the silica geothermometers. Possible mixing in this group of boiling springs can be assessed using the "boiling spring mixing model" of Truesdell and Fournier (1976). This model estimates a deep hot component of the order of 200-210°C. Preliminary results of a current geochemical study using new water and gas chemistry and isotopic analysis by the Ministry of Energy and Mines of Caracas and the International Institute for Geothermal Research of Italy suggest reservoir temperatures of more than 220°C (P. Varela, personal communication). Three samples of this group analysed for stable isotopes (Table 6) plot to the right of Craig's world meteoric line. The sample most enriched in deuterium and oxygen-18 is Su. 15 from Rio de Janeiro, which is the most concentrated spring and gives the highest tNKCM temperature estimate. The amount of oxygen-18 enrichment may indicate considerable water-rock interaction. Possibly, spring water Su. 15 is representative of the composition of the deep hot reservoir fluid. In an attempt to estimate the depth of the 200-220°C reservoir we used the method proposed by Hurter (1984) based on conductive cooling models ~vith planar (fault plane) conduit geometry. With curves generated by this method the estimated depth would be of the order of 1 km. This is probably a lower limit because her method assumes conductive gradients. On the other hand, using the boiling-point curve for liquid water under hydrostatic pressure given by Haas (1971) the minimum depth of the 200-220°C reservoir may lie in the 300-500 m range. Model o f Las M i n a s - M u n d o Nuevo system

The Mundo Nuevo and Las Minas areas should be considered as part of the same geothermal system. With the above information, a conceptual model is proposed in which a deep Na-C1 type reservoir is present at depths of 0.5-1 km and temperatures of 200-220°C. Water rises and cools adiabatically and partially mixes with colder groundwater, forming an intermediate reservoir at a few hundred meters depth with enough residence time to reset the Na-K-Ca geothermometer at a temperature range of 150--160°C. By further vertical ascent, fluids boil at shallow depths, vapor-rich zones develop that produce fumaroles and acid-sulfate alteration zones (Mundo Nuevo, El Salvaje and Buena Esperanza). At the same time the boiled reservoir fluids move laterally to the south, reaching the surface at lower elevation and producing boiling springs of Na-C! type. Along this last path mixing with cooler groundwater may occur. This model resembles the drilled geothermal systems of Bac6n-Manito, Philippines (de LeOn, 1983), El Tatio, Chile (Cusicanqui et al., 1976), Valles Caldera, New Mexico (Vuataz and Goff, 1986) and Mokai, New Zealand (Hulston et al., 1981). Subsurface temperature cannot be estimated at Mundo Nuevo because of the absence of Na-CI waters. A suggestive but highly speculative idea is that Mundo Nuevo is above the hottest part of the deep reservoir. Due to its higher elevation (350--410 m a.s.I.) only fumaroles and gas emissions reach the surface. At the same time, deep fluids flow laterally to the lower topographic regions to the east (Las Minas-Aguas Calientes) and to a smaller extent to the west towards Rio Casanay. The heat source for this geothermal system has been controversial and will probably continue to be so, even if more data are gathered. The two classic explanations are tectonic (e.g. Soulas, 1982) and igneous (e.g. Urbani, 1984). The first is based primarily on the deep seismically active El Pilar fault system that may allow deep circulation of meteoric water recnarged from the mountains mainly north of Las Minas. The second explanation is based on the close proximity of the system to the 5 Ma old rhyodacite intrusive bodies that, according to Vierbuchen (1984), are the sparse surface expression of a large granitic pluton. On the basis of his gravity model the pluton has a volume of 200--400 km 2. If all the igneous bodies really are of 5 Ma (a single K-Ar date is available) this could hardly drive today's active geothermal system according to the age

Geothermal Reconnaissance o f Northeastern Venezuela

425

vs volume cooling curves of Smith and Shaw (1975). Nevertheless, it is provocative to suggest that a younger diapir may exist. If this diapir exists the ideas given to support the first hypothesis could be merged with the second one. This is the only area in Venezuela with high enough estimated temperatures to warrant feasibility studies for electricity generation. The rural areas and small to medium size cities of the region (El Pilar and Cartipano) could greatly benefit from a geothermal plant, since this area is far from the main hydroelectric transmission lines of central Venezuela and the electricity is locally generated by burning oil. CONCLUSIONS This work presents a geothermal survey of northeastern Venezuelan geothermal systems and recognizes three types of geothermal resources. High temperature resources The Las Minas-Mundo Nuevo system located to the southwest of El Pilaf is the most promising for electricity generation. It may have a deep reservoir of 200-220°C at depths as shallow as 1 km. Unfortunately, there are no drilling data available and the geologic information is rather sparse. Detailed geophysical and hydrogeologicai studies have not yet been carried out. Medium temperature resources Geothermal systems with estimated temperatures in the range of 100-150°C have not been sufficiently documented; the only systems that may have temperatures in this range are El Pinto in Monagas and lqo Carlos and Cariaco in Sucre. More detailed studies need to be carried out on these systems. L o w temperature resources Systems with estimated temperatures between 60 and 100°C occur in several localities around the Eastern Mountain Massif at San Diego, Naricual and Clarines in Anzo~itegui, and Qda. Seca, Pantofio, Cariaco and Los Ipures in Sucre. Besides obvious use for bathings spas, these resources could be useful in direct applications for food-related industries, but must be able to compete with the low cost of Venezuelan electricity and oil.

The structural and tectonic setting and rock types associated with hot spring systems in the state of Sucre show many similarities to hot spring systems associated with the San Andreas fault zone and San Franciscan-Great Valley rock assemblage in California (McLaughlin, 1981). In particular, the E1 Pilar geothermal area has some geologic and tectonic similarities to The Geysers-Clear Lake geothermal system (Golf et al., 1978). At The Geysers steam field, the system issues from complexly folded, faulted and thrusted Cretaceous to Eocene sedimentary rocks and ophiolites cut by Late Tertiary strike-slip faults. Adjacent to The Geysers is the Clear Lake volcanic field (Hearn et al., 1981), which ranges in age from 2.0 to 0.01 Ma. A 30 mgal gravity low (Isherwood, 1975) and other geophysical data indicate the presence of magma at depths as shallow as 7 km. The similar offset of a possible igneous heat source relative to the hot spring system along a zone of strike-slip movement is striking. On the other hand there is a major difference between The Geysers and El Pilar: the former is a vapor-dominated system, whereas the geochemistry of the latter indicates a hot-water system. The geologic and geochemical data indicate that the Las Minas-Mundo Nuevo system is a promising geothermal prospect. Acknowledgements--The author wishes to thank the Venezuelan institutions: Consejo de Desarrollo Cientifico y Humanistico (CDCH) of the Universidad Central de Venezuela. Fondo de Investigaciones (FONINVES) and the Consejo Nacional de Investigaciones Cientificas y Tecnol6gicas (CONICIT) that financed the geothermal inventory

426

F. U r b a n i

project during 1981-1984. Many thanks are due to the students who worked on the project. A. Fermin, N. Di Gianm, A. Hevia, N. Marifio, J. A. Rodrl'guez and G. Zann/n, and to the very many Venezuelan institutions and individual~ that helped in several stages of this project. This report was prepared while the author was on sabbatical leave at the Earth and Space Sciences Division, Los Alamos National Laboratory, New Mexico. The Organization of American States financed this visit. Fraser Goff at Los Alamos provided critical reviews at several stages during preparation of the manuscript, which greatly improved it

REFERENCES American Public Health Association (1976) Standard Methods for Examination of Water and Wastewater, 14th Edn. Benjamin, T., Charles, R. and Vidale, R. (1983) Thermodynamic parameters and experimental data for the Na-K-Ca geothermometer. J. Volcan. Geotherm. Res. 15, 167-186. Burguera, J. L., Burguera, M. and Sampol, M. S. (1981) Descripcion geologica y relacion mineral6gica de las fuentes termales del estado M(~rida, Geotermia, Caracas 3, 26--45. Campos, V. (1981) Une transversale de la chaine Caraibe et de la marge V6n~zu~lienne, clans le secteur de Cartipano (V~n~zu~la Oriental). Structure g~ologique et evolution gdodynamique. Universit~ de Bretagne Occidentale, Dr 3(~me cycle, unpublished thesis. Castro, M. and Zamora, L, (1982) Geologia petrolera del flanco norte de la subcuenca de Maturfn, Venezuela Oriental. Una sintesis. Lagov~n S.A., Caracas, Geology archives, unpublished report. Christiansen, R, M. (1961) Geology of the Casanay-El Pilar region of central Sucre. Universit) of Nebraska, Department of Geology, unpublished Ph.D. thesis. Cortese, E. (1904) A quicksilver deposit. Engng Min. J. 78, 741. Cusicanqui, H., Mahon, W. A. and Ellis, A. J. (1976)The geochemistry of the El Tatio geothermal field, northern Chile. Proceedings of the Second U.N. Symposium on the Development and use of Geothermal Resources, San Francisco, CA, 1975, Vol. 2, pp, 1065-1073. D'Amore, F., Scandiffio, G. and Panichi, C. (1983) Some observations on the chemical classification of ground water. Geothermics 12, 141-148. Fermin, A. (1983) lnventario geot~rmico de la region central, Geotermia, Caracas, Colecci6n Libros no. 3. Fournier, R. O. (1977) Chemical geothermometers and mixing models for geothermal systems. Geothermics 5, 41-50. Fournier, R. O. (1979) Geochemistry and hydrologic considerations and the use of enthalpy---chloride diagrams in the prediction of underground conditions in hot-spring systems. J. Volcan. Geotherm. Res. 5, 1-16. Fournier, R. O. (1981) Applications of water geochemistry to geothermal exploration and reservoir engineering. In Geothermal Systems: Principles and Case Histories (Edited by L. Rybach and L. J. P. Muffler), pp. 1t0--143. John Wiley and Sons, New York. Fournier, R. O. and Potter, R. (1979) Magnesium correction to the Na-K-Ca chemical geothermometer. Geochirn. cosmochim. Acta 43, 1543-1550. Fournier, R. O. and Potter, R. (1982) A revised and expanded silica (quartz) geothermometer. Geotherm. Res. Coun. Bull. 11,3-12. Fournier, R. O. and Truesdell. A. H. (1973)An empirical Na-K-Ca geothermometer for natural waters. Geochim. cosmochim. Acta 37, 1255-1275. Fournier, R. O. and Truesdell, A. H. (1974) Geochemical indicators of subsurface temperatures. Part I I. Estimation of temperature and fraction of hot water mixed with cold water. J. Res. U.S. geol. Surv. 2,263-270. Fournier, R. O., White, D. E. and Truesdell, A. H. (1974) Geochemical indicators of subsurface temperatures. Part I. Basic assumptions. J. Res. U.S. geol. Surv. 2,259-262. Goff, F., Donnelly, J. M.. Thompson, J. M. and Hearn, B. C. (1978) Geothermal prospecting in The Geysers-Clear Lake area. northern California. Geology 5,509-515. Goff. F., Gardner, J., Vidale, R. and Charles, R. (1985) Geochemistry and isotopes of fluids from Sulphur Springs, Valles Caldera, New Mexico. J. Volcan. Geotherm. Res. 23,273-297. Goff. F. and Shevenell, L. (t987) Travertine deposits of Soda Dam, New Mexico and their implications for the age and evolution of the Valles Caldera hydrothermal system. Bull. geol. Soc. Am. 99,292-302. Gonz~ilez de Juana, C., Arrozena, J. M. and Picard, X. (1981) Geologia de Venezuela y de Sus Cuencas Petroliferas. Edic. Foninv~s, Caracas. Haas, J. L. (1971) The effect of salinity on the maximum thermal gradient of a hydrothermal system at hydrostatic pressure. Econ. Geol. 66,940-946. Hearn, B. C.. Donnelly, J. M. and Goff, F. (1981 ) The Clear Lake Volcanics: tectonic setting and magma sources. Prof. Pap. U.S. geol, Surv. i141, pp. 25--46. Hcvia, A. and Di Gianni, N. (1984) Inventario geot~rmico del estado Sucre, Geotermia, Caracas, Colecci6n Libros n o . 4. H u m boldt, A. ( 1814) Relation Historique du Voyage aux Regions Equinoxiales du Nouveau Continent. F. Schoell, Paris. Hulston. J. R.. Henley, R. W., Glover, R. B. and Cox, M. A. (1981) Stable isotope and geochemical reconnaissance of the Mokai geothermal system, Taupo volcanic zone. In Proceedings of the New Zealand Geothermal Workshop, University of Auckland, New Zealand, pp. 81-86.

G e o t h e r m a l Reconnaissance o f Northeastern Venezuela

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Hurter, S. J. (1984) Termometria quimica aplicada a exploraqao de dep6sitos geotermais. In Simposio Brasileiro sobre t(cnicas exploratorias aplicadas a geologia, pp. 78--97. Sociedade Brasileira de Geologia, Nucleo Bahia. Anais. Isherwood, W. F. (1975) Gravity and magnetic studies of The Geysers-Clear Lake geothermal region. California. Proceedings of the Second U.N. Symposium on the Development and use of Geothermal Resources, San Francisco, CA, 1975, Vol. 2, pp. 1065-1073. Mahon, W. A., Klyen, L. E. and Rhode, M. (1980) Neutral sodium/bicarbonate/sulfate hot waters in geothermal systems. J. Japan Geotherm. Energ. Ass. 17, 11-24. McLaugblin, R. J. (1981) Tectonic setting of pre-Tertiary rocks and its relation to geothermal resources in The Geysers-Clear Lake area. Prof. Pap. U.S. geol. Surv. 1141, pp. 3-24. Metz, H. L. (1964) Geology of the E1 Pilaf fault, state of Sucre, Venezuela. Princeton University, Department of Geology, unpublished Ph.D. thesis. Rodriguez, J. A. (1983) Inventario geotermico del estado Anzo~itegui, Geotermia, Caracas, Coleccion Libros no. 3. Schubert, C. and Sifontes, R. S. (1984) La riolita Plioceno tardia de Cartipano, estado Sucre, Venezuela: i,Extremo sur del arco volc~inico de las Antillas Menores? Acta cient, venez. 34,262-266. Smith, R. L. and Shaw, H. R. (1975) Igneous-related geothermal systems. In Assessment of Geothermal Resources of the United States--1975 (Edited by D. E. White and D. L. Williams), pp. 58-83. U.S. Geological Survey Circular 726. Soulas, J. P. (1982) Investigaciones neotectonicas en la zona nororiental de Venezuela (Abs.). Acta cient, venez. 33 (Supl. 1), 348. Speed, R. C. (1985) Cenozoic collision of the Lesser Antilles Arc and continental South America and the origin of El Pilar fault. Tectonics 4, 41-70. Truesdell, A. H. and Fournier, R. O. (1976) Calculation of deep temperatures in geothermal systems from the chemistry of boiling springs water of mixed origin. Proceedings of the Second U.N. Symposium on the Development and Use of Geothermal Resources, San Francisco, CA, 1975, Vol. 1, pp. 837-844. Truesdell, A. H. and Fournier, R. O. (1977) Procedure for estimating the temperature of a hot-water component in a mixed water using the plot of dissolved silica vs. enthalpy. J. Res. U.S. geol. Surv. 5, 49-52. Urbani, F. (1969) Notas preliminares sobre algunas fuentes termales de la cordillera de la costa. Bol. Soc, venez. Geol. 4, 21--44. Urbani, F. (1977) Geoquimica de las aguas termales del area de El Pilaf-San Antonio del Golfo, edo. Sucre. In Memorias V Congreso Geol6gico Venezolano, Vol. 3, pp. 1061-1065. Urbani, F. (1984) Evaluaci6n de los recursos geot6rmicos de Venezuela, Geotermia, Caracas, Coleccion Libros no. 5, Vols 1-3. Urbani, F. (1985a) Metodologia para la evaluaci6n de los recursos geotermicos en la fase de reconocimiento. In Memorias VI Congreso Geol6gico Venezolano. Vol. 7, pp. 4360--4399. Urbani, F. (1985b) GEOTRV, un programa en Fortran para la evaluacion de los recursos geot6rmicos en la etapa de reconocimiento. In Memorias VI Congreso GeolOgieo Venezolano, Vol. 9, pp. 5932-5972 (includes full listing). Urbani, F. (1986) GEOTRV computer program for geothermal exploration. Energ. Explor. Exploit. 3, 317-318. Urbani, F. (1986) Las fuentes termales en la obra de Vicente Marcano (1848-1891). Geotermia. Caracas 17, 1-31. Vierbuchen, R. (1978) The tectonics of NE Venezuela and the SE Caribbean Sea. University of Princeton, Department of Geology, unpublished Ph.D. thesis. Vierbuchen, R. (1984) The geology of the El Pilaf fault zone and adjacent areas of northeastern Venezuela. Mem. geol. Soc. Am. 162, pp. 189-212. Vignali, M. (1977) Geology between Casanay and El Pilar (abstract with map). In VIH Caribbean Geological Conference, Curaqao (abstracts), p. 215. Vuataz, F. and Goff, F. (1986) Isotope geochemistry of thermal and nonthermal waters in the Valles Caldera, Jemez Mountains, northern New Mexico. J. geophys. Res. 91(B2), 1835-1853. West Japan Engineering Consultants Inc. (1984) Report on fact-finding survey of north east geothermal region in Venezuela. Geotermia, Caracas 13, 1-28. White, D. E., Muffler, L. J. P. and Truesdell, A. H. (1971) Vapor-dominated hydrothermal systems compared with hot-water systems. Econ. Geol. 66, 75-97. Zannin, G. and Marifio, N. (1983) Inventario geot6rmico del estado Monagas. Geotermia, Caracas, Coleccion Libros no. 1.