'I

Journal of Volcanology and Geothermal Research, 45 (1991) 101 - 123 Elsevier Science Publishers B. V., Amsterdam 101

Exploration drilling and reservoir model of the Platanares geothermal system, Honduras, Central America

Fraser Goff', Sue J. Goff', Sharad Kelkar", Lisa Shevenell··1, Alfred H. Truesdellb , John Musgrave", Heinz RiifenachtC and Wilmer Floresd

UEarth and Space Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. bBranch of Igneous and Geothermal Processes, U.S. Geological Survey, Menlo Park, CA 94025, U.S.A.

cSwissboring Ltd., Guatamata City, Guatamata dUnidad Proyecto Geotermico, Empresa Nacional de Energia Electrica, Tegucigalpa, Honduras

lpresent address: Water Resources Center, Desert Research Institute, P.O. Box 60220, Reno, NV 89506, U.S.A.

(Accepted June 26, 1990)

ABSTRACT

Goff, F., Goff, S.l., Kelkar, S., Shevenell, L.. Truesdell, A.H., Musgrave. J. t Rufenacht, H. and Flores, W., 1991. Exploration drilling and reservoir model of the Platanares geothermal system, Honduras, Central America. In: F. Goff (Editor), Honduras - A Geothermal Investigation. J. Volcanol. Geotherm. Res., 45: 101-123.

Results of drilling,logging, and testing of three exploration core holes, combined with results of geologic and hydrogeochemical investigations, have been used to present a reservoir model of the Platanares geothermal system. Honduras. Geothermal fluids circulate at depths 2: 1.5 km in a region of active tectonism devoid of Quaternary volcanism. Large, artesian water entries of 160 to 165°C geothermal fluid in two core holes at 625 to 644 m and 460 to 635 m depth have maximum flow rates of roughly 355 and 560 llmin, respectively, which are equivalent to power outputs of about 3.1 and 5.1 MW(thermal). Dilute, alkali-chloride reservoir fluids (TDS ::s: 1200 mg/kg) are produced from fractured Miocene andesite and Cretaceous to Eocene redbeds that are hydrothermally altered. Fracture permeabillity in producing horizons is locally greater than 1500 md and bulk porosity is ::s: 6070. A simple, fracture-dominated, volume-impedance model assuming turbulent flow indicates that the calculated reservoir storage capacity of each flowing hole is approximately 9.7 x lcf I/(kg cm·2). Tritium data indicate a mean residence time of 450 YT for water in the reservoir. Multiplying the natural fluid discharge rate by the mean residence time gives an estimated water volume of the Platanares system of 2: 0.78 km3• Downward continuation of a 139°C/km "conductive" gradient at a depth of 400 m in a third core hole implies that the depth to a 225°C source reservoir (predicted from chemical geothermometers) is at least 1.5 km. Uranium-thorium disequilibrium ages on calcite veins at the surface and in the core holes indicate that the present Platanares hydrothermal system has been active for the last 0.25 m.y.

Introduction

Geothermal investigations in Honduras are part of a coordinated effort among the Em­presa Nacional de Energia Electrica (ENEE). the United States Geological Survey (USGS), and the Los Alamos National Laboratory and their consultants (LANL) to locate, evaluate and develop a geothermal resource for elec-

0377-0273/91/$03.50 © 1991 Elsevier Science Publishers B.V.

trical power generation. Initial studies (1985 to 1986) focused on ten areas: Platanares. San Ig­nacio, Azacualpa, Choluteca region (pavana), La Ceibaregion (Sambo Creek), El Olivar, Isla de Aguas Calientes, San Francisco de Ojuera, EI Cajon Dam, and Agua Caliente (west of San Ignacio) (Fig. 1). Further evaluation during 1986 concentrated on the three systems having the best apparent potential: Platanares, San Ig-

102

Caribbean Saa

Guatemala

0, 100 km L. ____ ...J'

Nicaragua

• Cities • Geothermal Sites (major) • Other Sites

- major roads

F. GOFF IT AL.

Fig. 1. Map of Honduras showing locations of major geothermal sites (large triangles) and other hot spring areas (small triangles) sampled during OUf investigations from 1985 to 1987.

nacio, and Azacualpa. Of these, Platanares was chosen as the best site for geothermal development because of relatively high estimated subsurface reservoir temperatures (- 225°C), large natural discharge of the hot spring system (- 3300 11m), and well-defined structural setting. In late 1986 to middle 1987, three slim, core holes were drilled at Platanares to obtain information on subsurface tempera­tures and temperature gradients, stratigraphy, hydrothermal alteration, fracturing, chemi­stry, and possible inflows of hydrothermal fluids. This paper presents a prefeasibility stage model of the Platanares geothermal system by integrating data from the core holes with sur­face studies. Previous reports on the area have been presented by GeothermEx (1980), Flores (1980), Eppler et al. (1986), Heiken et al. (1986, 1987). F. Goffetal. (1986, 1987a, b,1988b), S. Goff et .al. (1987, 1988), Bargar (1987), Truesdell et aI. (1987a, b), and Hoover and Pierce (1988).

Geologic background

The Platanares geothermal site is located along the Quebrada del Agua Caliente just east of the villages of Platanares and San Andres and about 16 km west of Santa Rosa de Copan (Santa Rosa de Copan topographic quadran­gle, UTM coordinates 928/327). The Plata­nares region is mountainous and deeply dissected. Elevations of nearby ridges exceed 1400 m, whereas the hot springs along the quebrada are at 700 to 800 m. An active gold mine occurs in the town of San Andres and an inactive antimony mine (El Quetzal Mine) lies about 2 km north of San Andres (Fig. 2).

Rocks in the Platanares area (Heiken et aI., 1986, 1987, 1991-this volume) consist of Paleozoic schists and gneiss in the north, which are juxtaposed along La Bufa Fault (a normal fault) against Oligocene to Miocene andesites, basin-fill sedimentary rocks, tuffs and tuf­faceous sedimentary rocks to the south. Late

Tpm

.~<:'O,c=:::::::: -..... ..... <.,~ -

Tpm

o km I

TpmD Padre Miguel Group, 14.5 Ma J T'~ Subinal Formation TERTIARY

Tml·"";"",, Matagalpa Formation, 37.4 Ma

IKTvaL,"';J Valle de Angeles Group CRETACEOUS-EOCENE

P,me::;::::] Metamorphic Basement; (Includes some igneous

rocks)

,</ Fault, ball on downthrown side

/,/ Contact

0- Hoi Spring Area

.' Exploration Core Hole

WF Waterfall

L/ Approximate Surface Limit of '" Geothermal Manifestations

A·

/"' A

a-f

Cross-Section (Fig. 10)

Location of Samples for Age Determination (Tables 9 and 10)

PALEOZOIC?

Fig. 2. Simplified geologic map of the Platanares g~thermal site showing locations of major hot spring areas and three exploration core holes in relation to rock units and structure (adapted from Heiken et al., 1991-this volume).

~ 5 ~ ~ o ~

r r

~ ~ o ~

rn ~ ~ ~

I ~ ~

~ ~ :;: !"

I ~

8

104

Cretaceous to Eocene red beds and con­glomerates of the Valle de Angeles group lie in fault contact with both Paleozoic and Tertiary rocks west, east, and northeast of the geother­mal area and unconformably underlie tuf­faceous rocks in the Quebrada de las Juniapas I km east of the area. Quaternary terrace gravels cap the Tertiary rocks near Platanares village and patches of silica-cemented river gravels locally fill the canyon bottom near the hot springs.

Potassium-argon dates indicate that the tuffs (Padre Miguel Group) are Miocene in age (14.5 ± 0.3 Ma) (Heiken et aI., 1991-this volume) and suggest that the andesitic lavas (Matagalpa Formation) are Oligocene in age (;;,: 25 to 37.5 Ma) (Bargar, 1991-this volume). Paleozoic rocks are locally intruded by a diorite stock north ofEI Quetzal Mine and the geother­mal site. One date on an altered diorite dike from EI Quetzal mine suggests an age ;;,: 100 Ma.

The geothermal area is highly faulted and fractured and occupies a structural graben trending in a northwest direction (see also, Aldrich et aI., 1991-this volume and Aiken et aI., 1991-this volume). Of primary importance is La Bufa Fault which marks the northern limit of hot spring activity, and the Quebrada del Agua Caliente fault zone, which controls the surface discharge of 95070 of the hot water.

Hydrothermal alteration varies from intense to non-existent (Bargar, 1991-this volume; Heiken et aI., 1991-this volume). Intense alteration occurs in the San Andres mine, in EI Quetzal mine (Roberts and Irving, 1957), along the Quebrada del Agua Caliente fault zone, and locally along the Rio Lara and other faults within the northwest-trending graben. At least two hydrothermal events have effected the area:

(I) The first event or series of events is associated with Au, Hg, Sb, As and Cu mineralization, and quartz veins in the mine areas. This event is evident in stibnite-bearing veins exposed in Quebrada del Agua Caliente

F. GOFF ET AL.

upstream of the waterfall and in core from hole PLTG-I, which penetrated over 500 m of frac­tured, altered andesite laced with gold-bearing quartz veins.

(2) The second hydrothermal event is associated with the present hot springs and hydrothermal fluids. It is apparently responsi­ble for calcite veins that cut quartz veins observed along the Quebrada del Agua Caliente and some calcite veins observed in core from holes PLTG-I and PLTG-3. Present hot springs deposit silica sinter at the surface; thus hydrothermal fluids at depth are probably con­tributing to silicification and alteration in­itiated at an earlier time. The relative ages of the hydrothermal events are discussed later in more detail.

Thermal features

Over 100 thermal springs and seeps varying from 35°C to slightly above boiling issue in the Platanares area. Although most thermal springs discharge near river level along both the Quebrada del Agua Caliente and Rio Lara, many hot springs occur along the quebrada walls as much as 35 m above river level. The greatest number of near-boiling to boiling springs discharge from the quebrada along a 1-km stretch downstream from the waterfall and are associated with several low-pressure steam vents that smell of H2S and NH3, clustered near core hole PL TG-3. Boiling springs also oc­cur upstream from the waterfall as far away as La Bufa Fault. All boiling springs are deposi­ting sinter. A large variety of hot springs of mixed chemical composition are interspersed with the boiling springs suggesting dilution of deep fluid with shallow groundwaters. No single hot spring has a flow exceeding 150 I/min but the combined discharge along the quebrada averages ;;,: 3300 l/min, which can raise the temperature of the river to about 40°C during dry seasons. Several hot springs and seeps (:5 60°C), which appear to be part of a small, southward-flowing hydrothermal plume, issue

EXPLORATION DRILLING AND RESERVOIR MODEL OF PLATANARES, HONDURAS 105

from fractured tuffs along a I-kmstretch of the Rio Lara and one short tributary stream (F. Goff et aI., 1988a). No gas emissions or sinter deposits are associated with these springs. Details of the location, deposits, and

geochemistry of the Platanares hot springs are described by F. Goff et al. (1986, 1987a, b, 1988b), Truesdell et al. (l987a, b), and Janik et al. (1991-this volume).

Fig. 3, Photo looking northwest of PLTG-l while erupting from large 160°C water entry at -630 m before flow tests of February- March, 1987.

106

Exploration core holes

Three diamond core holes were drilled in the Platanares geothermal site during the last stages of prefeasibility study. Two of the holes struck copious artesian hydrothermal fluids. Project objectives, drilling operations, casing schedules, mud program, well head diagrams,

TABLE I

F. GOFF ET At.

etc. are described in detail by S. Goff et al. (1987,1988). Hole PLTG-I (Fig. 3) is located within the Quebrada del Agua Caliente fault zone adjacent to the river along a stretch con­taining few hot springs. The hole penetrated 563 m of altered andesites (Matagalpa Forma­tion) containing various mineralized veins, vugs and fractures, and bottomed at 650.4 m in

Stratigraphy, depth of water entries, and casing points for exploration core holes, Plafanares, Honduras

PLTG-I

Elevation (a.m.s.!.) -74Om

Stratigraphy

Alluvium (QuaL) 0 to 7.2 m Terrace deposits (QuaL) Padre Miguel Fm.

(Miocene) Subinal Fm. (Mioc. - Olig.) -Matagalpa Fm. (Olig.) 7.2 to 563 m Valle de Angeles Gp.

(Cretaceous - Eocene) 563 to 650.4 m

Water entries

Casing points

Surface conductor PW casinga

HW casing NW casing (HQ rods) Liner (NQ rods) Open hole

1. - 30 m (22 I/min) 2. 49.7 m (35 IImin) 3. 70.0 m ("small") 4. 183 m (25 IImin) 5. 252 m (> 300 IImin) 6. 441 m ("small") 7. Several small entries

before 563 m 8. 574 m (several eruptions) 9. 625 to 644 m

(> 300 IImin)

10.2 m 70.0m 80.1 m

588.4 m

588.4 to 650.4 m

PLTG·2

-120m

o to 13.25 m

13.25 to 311 m 311 to 428 m

1. 80 m (lev small") 2. 170 m (5 - 10 IImin) 3. 395 m ("small")

11.65 m 77.35 m

394.0 m

394.0 to 401 m (bottom of hole to

428 m lost)

PLTG·3

-710 m

o to 6.85 m

6.85 to 288.7 m 288.7 to 362.4 m 362.4 to 622 m

622 to 679 m

I. 26 m (> 200 IImin) 2.32 m (> 150 IImin) 3. 172 m ("small") 4. 200 m (> 200 IImin) 5. 459 m (> 150 IImin) 6. Several small entries from

480 to 550 m 7.622 to 635 m

(>300 I/min)

7.0m 26.5 m 130.0 m 387.7 m 675.0 m 675 to 679 m

a Drill rods and casing used in the diamond coring industry are refered to by letters (PQ, HQ. NW. etc.) that designate diameter and wall thickness. See S. Goff et al. (1988) for exact dimensions used in this project.

EXPLORATION DRILLING AND RESERVOIR MODEL OF PLATANARES, HONDURAS 107

hydrothermally altered shales and conglome­rates of the Valle de Angeles Group (Table I). The hole is cased and cemented with HQ drill rods from 588 m to surface and is open from 588 m to total depth (TD). Internal diameter is approximately 7.60 cm (3.0 in.) from top to bottom. A large entry of artesian, hot water from fractured andesite at 252 m nearly caused loss of the hole. Hydrothermal fluids produced during flow tests of PLTG-I (described below) originate from a fractured zone between 622 and 640 m.

Hole PL TG-2 is located about I km south of the Quebrada del Agua Caliente, a few hundred meters north of the Rio Lara. The hole pene­trated fractured tuffs and tuffaceous sediments of the Padre Miguel Group to a depth of311 m and bottomed at 428.4 m in lightly altered silt-

stones, sandstones, and conglomerates. Petro­logy and paleomagnetic work suggest that this sedimentary sequence is part of the Subinal Formation (Heiken et ai., 1991-this volume; J. Geismann, Univ. New Mexico, unpubi. data, 1987). A small, artesian water entry at about 170 m chemically resembles hot springs that issue along the Rio Lara (F. Goff et ai., 1987a; Janik et ai., 1991-this volume) and two other small water entries were encountered during coring operations. Because of drilling pro­blems, the well was completed at 401 m. HQ drill rods are cemented in the hole to 394 m.

Hole PLTG-3 (Fig. 4) is located just west of the Qubrada del Agua Caliente, about 30 m above river level, within the area of most vigorous hot spring and low-pressure steam discharge. The hole penetrated 289 m of Padre

Fig. 4. Photo looking northwest of flowline, muffler, and weirbox during flow tests of PLTG-3, June 1987 (see Fig. 6). Equipment in left foreground is mud tanks and pump used for coring operations. Men at weir box are measuring flow rates.

108

Miguel tuffs, sedimentary rocks of the Subinal Formation from 289 m to 362.4 m, hydrother­mally altered andesitic lavas (Matagalpa For­mation) from 362.4 to 622 m, and red beds (siistones, sandstones, and conglomerates) of the Valle de Angeles Group to total depth of 679.0 m. The hole is cased and cemented with HQ drill rods from 387.7 m to surface bu~ due to instability the hole is lined with NQ drill rods to total depth (inside diameter 6.03 cm). A very large water entry at only 26 m in fractured, silicified tuff suddenly propelled a loaded PQ core barrel weighing about 50 kg to a height of 15 m above the rig floor. Other large water en­tries were encountered at depths of 459 m and 622 to 635 m.

Physical properties of core

A suite of physical property measurements was obtained on eight samples of core from PLTG-I and PLTG-2 (Table 2). Bulk densities of red bed and hydrothermally altered andesitic lava samples range from 2.5 to 2.6 g/cm3

TABLE 2

F. GOFF ET AL.

whereas tuff samples have an average of 1.9 g/cm3. There is a general decrease in porosity with depth in PL TG-2 but variable porosity in the altered rocks of PL TG-l. Permeability varies tremendously and is generally less than 0.10 md for the rock matrix, but may exceed 1400 md in fractured intervals. Three samples of fractured core from the main producing zone of PLTG-I were analyzed for horizontal permeability at 160°C and a range of confining pressures bracketing the lithostatic pressure (Table 3). Although permeability decreases with increase in confining pressure, small frac­tures only 5 to 8.5 cm in length can have permeabilities of 300 md. Clearly, secondary permeability dominates in the production zones in andesitic lavas and in the underlying red bed sequence.

Temperature logs

Although many temperature measurments were obtained by Kuster Tool during coring operations, those closest to eqUilibrium condi-

Selected physical properties of samples from PLTG·I and PL TG·2core holes, Platanares, Honduras (Terra Tek, Salt Lake City, Utah)

Sample Depth Rock Permeability Porosity Grain Bulk No. (m) type ('70) density density

Horz Horz-90 Vert (g/cm3) (g/cm3) (md) (md) (md)

Core hole PLTG-I

49-9C-I 158.25 Andesite <0.01 <0.01 <0.01 5.8 2.68 2.56 73-16-2 259.39 Andesite 1481 <0.01 110" 1.9 2.67 2.64

146-4-2 565.00 Shale 0.01 <0.01 <0.01 6.4 2.71 2.58 163-9-2 648.85 Conglomerate 0.46 <0.01 0.02 3.3 2.68 2.62

Core hole PLTG-2

46-1 154.10 Tuff 393 0.06 <0.01' 31.6 2.56 1.82 82-2B 310.0 Tuff 0.10 0.10 0.08 23.6 2.60 2.01 85-2 322.7 Shale 0.02 0.01 0.02 12.6 2.76 2.48

109-7-1 425.3 Shale <0.01 <0.01 <0.01 6.1 2.72 2.58

a Vertical fracture present.

EXPLORATION DRILLING AND RESERVOIR MODEL OF PLATANARES, HONDURAS 109

TABLE 3

Horizontal permeability measured on three samples of metaconglomerate, PLTG-l core hole, Platanares. Honduras (Terra Tek, Salt Lake City, Utah)

Sample Depth Effective stress (psi)a Description No. (m)

656 1156 1656 Permeability (md)

158-20 635 318.43 300.17 289.87 Core with large open fractures partially filled with quartz and calcite dipping 60 to 90°; width 0.5 to 2.0 mm, length 5 to 8.5 cm.

159-28 637 41.21 31.91 28.42 Core with fractures mostly filled with quartz dip-ping 60 to 85°; width 0.5 to 1.0 mm, length 6 to 11 cm.

160-24 640 0.67 0.51 0.45 Core with essentially quartz-filled fractures dipping 60 to 70°; width 0.2 to 0.5 mm, length 4.5 to 6cm.

a Test parameters: Temp. = 160°C; deionized water; permeability measured horizontal along fractures.

0

-50

-100

-150

-200

-250

E -300

.c; a -350

'" -400 0

-450

-500

-550

-600

-550

-700 0

LIQUID

PLTG-3 @26m

PLTG-3

PLTG-1

Boiling Curve (0 wt% NaCI)

VAPOR

20 40 SO 80 100 120 140 1SO 180 200 220 240 260 280

Temperature (OC)

Fig. 5. Plot of static temperature versus depth for the temperature logs of Table 4 compared to the boiling curve for pure water.

110

tions are shown in Table 4 and in Figure 5. The PL TG-I log was taken about 4 weeks after completion of the coring operations and im­mediately after allowing the hole to flow at wide open conditions for 2 weeks (to clean the bore of drilling fluids). Because PL TG-I is cased and cemented to 588 m, all flow is from the 625 - 644 m production zone in green, fractured metaconglomerate. A temperature of

TABLE 4

Temperature data of exploration core holes, Platanares, Hondurasa

PLTG-I PLTG-2 PLTG-3 February 27, 1987 March 9, 1987 June 12, 1987 (Kuster toolb ) (Temp. probe) (Kuster toolb )

Depth Temp. Depth Temp. Depth Temp. (m) ('C) (m) ('C) (m) ('C)

150 134 0.0 36.4 450 154.9 200 147 26.7 45.7 460 154.9 250 155 49.0 50.8 470 156.2 300 159.5 75.7 59.1 480 157.0 350 160 102.5 65.4 490 156.2 400 160 124.7 68.3 500 156.2 450 160 147.0 73.3 520 156.2 500 160 165.7 72.0 530 156.2 550 160 175.4 74.2 540 156.2 600 160 194.9 77.5 550 156.6 625 160 224.1 80.9 560 156.7 650 160 253.4 84.8 570 157.2

272.8 87.5 580 157.7 303.1 92.0 590 157.6 322.5 94.7 600 158.5 350.8 98.4 610 158.7 370.3 101.4 630 159.9 389.8 104.3 640 161.2

650 162.5 660 163.3 670 163.9 675 163.9

a Temperatures in PLTG-l and PLTG-3 were measured by H. Rufenacht, Swissboring, Ltd. Temperatures in PLTG-2 were measured by J. Meen and D. Smith (1991-this volume); values given for PL TO-2 are from selected depths only. b KusterTool calibrated in ice water before use.

F. GOFF ET AL.

160°C persists all the way from 350 m to total depth, caused by heating of the rocks during flow in the wellbore.

The log listed for PLTG-2 was obtained about 2 weeks after completion of coring operations and is described in detail by Meert and Smith (I 991-this volume). The log displays two zones of linear (conductive) gradients separated by a zone with a small temperature reversal at 150 - 175 mdepth which is assumed to be caused by lateral flow of water around the casing at this horizon. The bottom hole gra­dient is 139°C/km.

The log from PL TG-3 was obtained only 2 days after completion of coring operations and before the well was flowed to clean out drilling fluids. The temperature of 163.9°C at total depth is considered to be slightly low because of insufficient time to let the well bore equilibrate. A more accurate log was never obtained because the core hole was killed with cold water after flow test operations were completed to in­stall the NQ liner (Table I). Once the liner was installed, the rig was released and no more temperature logs could be obtained. The estimated temperatures from the 459 m and the 622 - 635 m production zones are ~ 155°C and 165°C, respectively.

When temperatures from the three holes are compared with the boiling point curve for pure water (Fig. 5), it can be seen that subsurface temperatures lie well within the field for liquid water and that the reservoir has not boiled. The only exception to this condition is the large water entry at 26 m in PLTG-3, which had a temperature of 128°C. This point lies just below the boiling curve and suggests that sub­surface boiling is occuring in the shallow part of the geothermal system along the Quebrada del Agua Caliente fault zone. Temperature gra­dients in the lower portion of the two flowing wells show near-isothermal conditions that are independent of stratigraphic boundaries. There is no evidence for a well-defined cap rock to the system or for a sudden increase in temperature to underlie PLTG-I or PLTG-3. If

EXPLORATION DRILUNG AND RESERVOIR MODEL OFPLATANARES, HONDURAS III

a deeper reservoir at 210 to 240°C underlies the Platanares area, as suggested by chemical geothermometry (Janik et aI., 1991-this vol­ume), downward continuation of the tempera­ture gradient below 170 m depth in PL TG-2 in­dicates that the depth to this reservoir is at least 1.5 to 2.0 km (Meert and Smith, 1991-this volume).

Flow tests of PLTG-l and PLTG-3

Although the three exploration core holes were never intended to be production wells, PLTG-I and PLTG-3 encountered fractured zones in altered andesite and conglomerate that produce copious amounts of hot (- 160°C) water. Flow tests were performed on these bores in February - March 1987 (PLTG-I) and

TABLE 5

Pressure and flow rate data collected for pressure tran­sient analysis of core holes PLTG-I and PLTG-3, Platanares. Hondurasa

Step Po P w (kg/ em2) (kg/ em2 )

1lJ'1!2 Qb (kg/cm2)112 (lImin)

PLTG-l (y = 0.116) I 6.75 5.38 1.17 149 2 6.75 5.20 1.25 189 3 6.75 4.12 1.59 255 4 6.75 2.96 1.95 336 5 6.75 2.33 2.10 347 6 6.75 2.18 2.13 356

PLTG-3 (y = 0.126) I 8.65 3.57 2.23 556 2 8.65 3.98 2.16 504 3 8.65 4,69 1.99 456 4 8.65 5.10 1.88 440 5 8.65 5.61 1.74 398 6 8.65 5.30 1.83 436 7 8.65 4.19 2.11 487

a Po = shut in pressure; P w = flowing pressure; dj>1I2 = {Po-pw)ll2; Q = flow rate. b Values corrected for steam flash, y, using a formation temperature of 160°C and 165°C for PLTG-I and PLTG-3, respectively and a separation temperature of 98,5°C,

June- July 1987 (PLTG-3) to obtain informa­tion on aquifer fluid chemistry and flow characteristics. Preliminary reports on the fluid chemistry and flow rates of PL TG-I are given by Truesdell et al. (1987b) and F. Goff et al. (1987a), respectively, and a detailed evalua­tion of the fluid chemistry can be found in Janik et al. (l991-this volume).

Enthalpy measurements

A diagram of the flow-test equipment used for both tests is shown in Figure 6. The design of the equipment was based partly on the description of Mahon (1964, 1966) and the equipment was used to derive an independent estimate of the aquifer fluid enthalpy in PL TG­I and PL TG-3. A horizontal pipe 10.7 m long and 7.2 cm in diameter was connected between the wellhead and a muffler/weir tank fabri­cated in the field. A throttling valve was install­ed in the horizontal flow line 3.7 m from the wellhead and sampling ports were installed I m from either end of the flow line. During the tests, samples of water and steam were col­lected from small, single-stage centrifugal se­parators (miniseparators) at the sampling ports over a range of line pressures controlled by the throttling valve. The enthalpy of the produc­tion fluid can be calculated using the equations outlined in Truesdell et al. (I 987b) and Janik et al. (1991-this volume). This method of enthal­py measurements using detailed gas analyses was effective on PLTG-I (660 J/g versus 675 J/g obtained by Kuster Tool) but not as effec­tive on PLTG-3, possibly due to problems with proper adjustment of valves and the separator. The enthalpy of 660 J / g calculated geochemi­cally in PL TG-I may be lower than that calcu­lated from Kuster Tool measurements due to conductive cooling ofthe produced fluid in the upper part of the wellbore before surface sam­pling (the maximum wellhead temperature of PLTG-I never exceeded 145°C during the flow tests due primarily to flashing). On the other hand, the difference in the two enthalpies is

112 F. GOFFET AL

FLOW TEST EQUIPMENT PLATANARES

, ..... " """" PRESSURE LOCK!

lOOGING PIPE, WRAPPfD WITH ABERGlASS INSULATION fROM REDUCER TO 3" GATE VALVE

PIPE. 'NfW'PED WITH ABeAGI.ASS INSULATION FROM GATE VALVE TO END

2-1~ PIPE SAMPLING PORTS DETAil

DIMENSIONS

JOINTCAN BE \' PIPE

WELDEDORTHREADE0-U ' ~ ,"NlPPlENPTTHREADS

1" GATE VALVE

BAFA..E BOX: 40" ~ 80" x 25' TANK: 40' x 80" x ~ WEIR TANK: 40" x 40" x 40"

3" FLOW PIPE X·SECTION

Fig. 6. Diagram of flow test equipment (not to scale) used for obtaining geochemical samples and flow/pressure build-up measurements, PLTG-l and PLTG-3, Honduras. '

relatively small and may only reflect the range in errors of measurement by the two methods.

Flow tests of PLTG-l and PLTG-3

During February - March and June - July of 1987 short flow tests were conducted on PLTG-I and PLTG-3 to evaluate the perfor­mance and volume of the 160 to 165°C reser­voir fluids. Again, it should be stressed that these slim core holes are not production wells and that the data obtained are only indicative of what might be obtained from production wells.

Each test was initiated by a series of short flows at different throttling conditions by using the throttling valve on the flow-line (Fig. 6). Pressure and flow rate data were collected for about 35 minutes under each set of conditions in order to make pressure transient analyses of each core hole. From field observations and core analyses, the Platanares reservoir is fracture-dominated where both flow paths and fluid storativity are provided by fractures. In such a system, for the short duration flow tests reported here, it is not appropriate to use con-

ventional porous flow models. Hence, a lumped-parameter, fracture-dominated, vol­ume-impedance model was developed (F. Goff et aI., 1987a) based on experiments in the hot dry rock reservoirs at Fenton Hill, New Mexico (see also, Bear,1972, Chap. 11). Assuming that the flow is turbulent, the equations relating flow to pressure are described by:

t (I)

(VfJl~)

and

(2)

where Q = the production flow rate, Qo = the initial production flow rate, t = flowing time, V = reservoir volume tapped by the well, fJ = reservoir compressibility, It = turbulent im­pedance, P w = flowing pressure and Po = shut in pressure.

Equation 2 gives the maximum production flow rate that can be expected at any given con­stant value of the production pressure (p w)'

EXPLORATION DRILLING AND RESERVOIR MODEL OFPLATANARES, HONDURAS 113

2.

2.0

~ 6J' 1.5 E

~

PLTG-3 (SIOpe=3.40x10-~

"$. 1.0 PLTG-1 (slope=4.77x10-:; Il. <I

0.5

100 200 300 400 500

Qo(J/min)

Fig. 7. Plot of pressure transient data from Table 5 used to calculate turbulent impedance. It. for core holes PL TG-I and PLTG-3, Platanares, Honduras.

400 PLTG-1

C :§ 300 ~ SIGNIFICANT ~ SCALING BEGINS 0; a:

~ 200 u.

100 50 100 150 200

Elapsed Time (hrs)

600 PLTG-3

50 100 150 200

Elasped TIme (hrs)

Fig. 8. Plots of flow rate versus elapsed time during 8- and 6-day flow tests of PLTG-I and PLTG-3, Platanares, Honduras.

The data obtained during these tests are listed in Table 5 and plots of t1P1I2 versus Qo are shown in Figure 7 _ The turbulent impedance (It) is obtained from the slopes of the lines by linear regression_

Longer flow tests at mru.timum output were conducted for each well and the results are shown in Figure 8. An 8-day test of PL TG-I showed a gradual decrease in flow rate until about 100 hours and then a more rapid decrease in flow rate until the end of the test at about 190 hours. After this test, the surface piping was dismantled and aragonite scale (identified by XRD analysis) was found lining the inside of the wellhead (Fig. 9). This scale coated the 7.78-cm diameter casing from 0 to 50 m, with most occurring between 0 to 20 m. The diameter of the minimum restriction was 3.0 cm. Thus, the sudden and dramatic decrease in flow rate after 100 hours was caused by scaling. The core hole was then mechanically reamed to total depth to remove scale. Aragonite scaling occurs during flashing of the alkaline reservoir water (Table 6) and removal of CO2 according to the reac­tion:

114

Ca2 + + 2HC0:l ~ CO2 + CaC03 + Hp (3)

A six-day flow test of PL TG-3 showed an in­itial sharp drop in flow rate during the first 10 hours and a gradual decrease in flow rate to the end of the test at about 150 hours. The surface piping was dismantled after this test but no scale was found and no scale was encountered when the NQ-Iiner was run back into the hole after the tests. The lower Ca content of PL TG­I water compared to PL TG-3 water (Table 6) may reflect deposition of aragonite in the wellbore of PLTG-I prior to sampling. It is our opinion that the higher pressure and flow rates of PL TG-3 prevent significant flashing and scale formation until the fluid enters the muf­fler Iweirbox. No significant scale deposits

F. GOFFET AL

were found in the weirbox after tests of either well.

Reservoir storage capacity (V {3) can be calcu­lated using the values of flow impedance from Figure 7, the rate versus time data of Figure 8, and Equation I. Although these calculations are approximations based on short tests, they indicate that the storage capacity of the reser­voir is relatively large [9.7 x 106 l!(kg/cm2)). The approximate volume of reservoir fluid "accessible" (that is, the volume of reservoir fluid that influences the test) to each hole is estimated at about 0.1 km3 if we assume a reasonable value for the reservoir com­pressibility, {3( -I X 10-4 (kg/cm2)-I)(Mur­phy and Dash, 1985).

The wellhead pressure of PL TG-I returned

Fig. 9. Photo of argonite scale in 10.2 em (4 in.) ball valve dismantled from wellhead of PLTG·I after g·day flow test, Platanares, Honduras.

EXPLORATION DRILLING AND RESERVOIR MODEL OF PLATANARES, HONDURAS 115

TABLE 6

Average total flow composition of reservoir fluids pro­duced from exploration core holes, Platanares, Hon­duras.a Values in mg/kg except where noted

PLTG-1 PLTG-3

Formation temp. (OC) 160 165 Surface pH (field) 8.6 8.7 Downhole pH (calculated) 6.1 6.0 Si02 263 248 Na 271 272 K 30 33 Li 3.35 3.45 Ca 3.9 8.0 Mg 0.18 0.23 CI 29.9 29.0 F 11.1 10.6 HC03 (total) 478 496 SO, 206 201 B 14.3 13.8 Asb 0.83 0.85 NH, 15.3 14.5 CO, 639 1227 H,S 7.04 14.06 H, 0.022 0.041 CH4 3.95 7.44 N, 9.34 64.9 NH3 10.3 18.8 Ar 0.24 1.344 He <0.01 <0.01 Total CO, 984 1585 Total H,S 7.05 14.08 Total NHJ 24.7 32.5 oD. H,O (%0) -46.1 -47.7 0180, H,O (%0) -6.60 -6.48 Ol3C, CO, (%0) -9.08 -9.40 Tritium (T.U.) 0.52 0.34

a See Janik et al. (l991-this volume) for comparison with hot spring waters; data listed were obtained from miniseparator . b Arsenic values listed are from weirbox samples.

to a static pre-flow value of 6.75 kg/cm2 in on­ly 35 seconds after 8 days of flow. PL TG-3 returned to a static pre-flow value of 8.65 kg/cm2 in 20 minutes after 6 days of flow. The rapid initial decline in flow of PLTG-3 and the longer pressure-buildup time may indicate some "skin-damage."

Thermal power output

Estimates of maximum and minimum ther­mal power produced by the two core holes can be computed as:

P = Qe(Ml) (4)

where P = thermal power, Q = flow rate, e =

density of water at the temperature of flow-rate measurement (98.5°C) and tlH = the enthalpy change between the production zone ( 160 to 165°C) and ambient temperature (- 30°C). PLTG-I and PLTG-3 produce as much as 3.1 and 5.1 MW(t) , respectively, which is fairly significant for holes only 7.78 cm (3-1/ 16 in.) in diameter. Table 7 compares various parame­ters of holes PLTG-I and PLTG-3.

Reservoir residence time and volume

Tritium is useful in determining the relative ages of groundwaters because it has a short half-life of 12.43 yr and was produced in large amounts during atmospheric nuclear tests in the 1950s and 1960s (Fontes, 1980). Meteoric precipitation in Honduras presently contains 2 to 5 T.V. To make quantitative estimates of mean residence times of water in several Hon­duras geothermal reservoirs, F. Goff et al. (1987b, 1988b) derived analytical solutions as function of tritium content for two end­member types of reservoirs, piston-flow and well-mixed, following the methods of Pearson and Truesdell (1978). They concluded that any water having a tritium content greater than 5 T.V. must have a piston-flow component whereas any waters with tritium contents less than about 1.7 T.V. must have a well-mixed component. Because boiling hot spring waters in Honduras have tritium contents < I T.V., the mean residence times of water in the reser­voirs are best estimated using the equations describing the well-mixed case. Table 8 shows mean residence time versus tritium content for well-mixed reservoirs in Central America for the years 1985 -1990, 1995, and 2000 (see F.

116

Goff et al.. 1987b. 1988b for derivations and codes). This table is useful for evaluating mean residence times in all types of aquifers having low tritium concentrations in this region.

Produced fluids from PLTG-1 and PLTG-3 have an average tritium value of 0.44 ± 0.19 T.V. (n = 7). VsingTable 8 for the year 1987 when the core holes were flowed. water con­taining 0.44 T. V. has a mean residence time of roughly 450 yr assuming the reservoir is 100010 well-mixed. A crude estimate of reservoir volume can be calculateq. assuming that discharge from the reservoir is equal to recharge. using the equation:

(5)

where V = reservoir volume. q = steady-state discharge rate and T = mean residence time

TABLE 7

F. GOFF ET AL.

(Pearson and Truesdell. 1978). The "surface" discharge of the Platanares geothermal system has been measured at approximately 3300 IImin (F. Goff et al.. 1988b). but some subsur­face discharge of unknown volume occurs in a lateral flow system to the south . When equa­tion 5 is solved using T = 450 yr and q = 3300 IImin. the volume of the Platanares reservoir is estimated to contain ;;" 0.78 km3 of thermal fluid.

Age of hydrothermal system

As mentioned above. there appear to be at least two separate hydrothermal events that have affected the Platanares area. The first event or series of events is associated with the gold- and antimony-bearing quartz veins of the

Comparison of PLTG·I and PLTG·3 Core Holes. Platanares. Honduras

Parameter

Date

Total depth Flow zone depth Core hole diameter, flow zone Bottom hole temperature Static shut-in pressure (gauge) Flowing wellhead temp. Flowing pressure, max. flow (gauge) Maximum wellhead temp .. after shut-in Time of pressure buildup (static shut-in pressure) Total volume flowed, Vtol (includes all clean-up

and flow tests) Turbulent impedance, It Time of longest test, t Initial flow rate, Qo Final flow rate, Q Storage capacity, Vj3 Estimated compressibility, J3 Reservoir volume, V Enthalpy change. Ilif Fluid density. e (98.5'C) Minimum power, P Maximum power, P

a Before significant scaling began.

Units

m m em 'C kg/em2

'C kg/em2 'C sec

I (kg/cm2)112/l/min min tlmin tlmin 1/(kg/em2) (kg/em2)-1 km3

J/g g/em3

MW(t) MW(t)

PLTG-I

Feb.-Mar. 1987

650 625 to 644 7.57 160 6.75 138 2.18' 145 35

8.6 x UP 4.77 x 10-3

16.36 x ui' 356 327' 9.64 x UP I x 10- 4

0.096 550 0.959 2.89 3.12

PLTG-3

Jun.-Jul. 1987

679 459; 622 to 635 7.57 165 8.65 150.5 4.19 158 1200

10.4 x 10" 3.40 x 10- 3

8.58 X 103

563 487 9.77 x 106

I X 10-4

0.098 572 0.959 4.47 5.12

EXPLORATION DRILLING AND RESERVOIR MODEL OFPLATANARES, HONDURAS 117

TABLE 8

Tritium concentration and residence times for selected base years in Central America, well-mixed case (from Goff et aI., 1988b)

Residence Tritium concentration (T.V.) time (yr) 1985 1986 1987 1988 1989 1990 1995 2000

1.0 1.70 1.46 1.25 1.07 0.92 0.79 0.37 0.17 2.0 1.91 1.63 1.40 1.20 1.03 0.88 0.41 0.19 3.0 2.19 1.88 1.61 1.38 l.l8 1.01 0.47 0.22 4.0 2.58 2.21 1.89 1.61 1.28 l.l8 0.55 0.25 5.0 3.02 2.59 2.22 1.90 1.63 1.40 0.65 0.30 7.5 4.01 3.49 3.04 2.64 2.30 1.99 0.98 0.47

10.0 4.63 4.09 3.61 3.19 2.81 2.48 1.30 0.67 12.5 4.91 4.40 3.93 3.51 3.13 2.79 1.55 0.85 15.0 5.00 4.51 4.07 3.66 3.30 2.97 1.73 0.99 17.5 4.96 4.51 4.09 3.71 3.36 3.05 1.84 1.09 20.0 4.86 4.44 4.05 3.69 3.37 3.06 1.90 l.l6 22.5 4.72 4.33 3.97 3.63 3.33 3.04 1.93 1.21 25.0 4.57 4.20 3.86 3.55 3.26 2.99 1.93 1.23 27.5 4.40 4.07 3.75 3.46 3.18 2.93 1.92 1.24 30.0 4.24 3.93 3.63 3.35 3.10 2.86 1.90 1.24 40.0 3.66 3.41 3.17 2.95 2.74 2.55 1.75 l.l9 50.0 3.18 2.98 2.78 2.60 2.43 2.27 1.59 l.l1 75.0 2.38 2.24 2.10 1.98 1.86 1.74 1.26 0.90

100.0 1.89 1.78 1.68 1.58 1.49 1.40 1.03 0.75 150.0 1.33 1.26 l.l9 l.l3 1.06 1.01 0.75 0.56 200.0 1.03 0.98 0.92 0.87 0.83 0.78 0.59 0.44 300.0 0.71 0.67 0.64 0.60 0.57 0.54 0.41 0.31 400.0 0.54 0.51 0.48 0.46 0.44 0.41 0.31 0.24 500.0 0.43 0.41 0.39 0.37 0.35 0.33 0.25 0.19 750.0 0.29 0.28 0.26 0.25 0.24 0.23 0.17 0.13

1000.0 0.22 0.21 0.20 0.19 0.18 0.17 0.13 0.10 1500.0 0.15 0.14 0.13 0.13 0.12 0.11 0.09 0.07 2000.0 0.11 0.11 0.10 0.10 0.09 0.09 0.07 0.05 3000.0 0.07 0.07 0.07 0.06 0.06 0.06 0.04 0.03 4000.0 0.06 0.05 0.05 0.05 0.05 0.04 0.03 0.03 5000.0 0.04 0.04 0.04 0.04 0.04 0.03 0.03 0.02 7500.0 0.03 0.03 0.03 0.03 0.02 0.02 0.02 0.01

10000.0 0.02 0.02 0.02 0.02 0.02 0.02 om 0.01 15000.0 0.01 0.01 0.01 0.01 om 0.01 om 0.01

San Andres and El Quetzal mines, respectively, are commonly fine-grained in texture, black in and includes cinnabar mineralization at San color, show several periods of deposition, and Andres. Quartz veins containing visible stibnite contain up to 3 ppm gold (Heiken et al., 1991-cut altered andesite exposed in the Quebrada this volume). The operators of the San Andres del Agua Caliente upstream of PLTG-I and Mine also report anomalous gold concentra-chalcopyrite and stibnite are found associated tions in the altered andesites exposed in the ca-with fractured, mineralized andesite in the up- nyon bottom upstream of PL TG-I. According per250m ofPLTG-l. Quartz veins in PLTG-I to the fluid inclusion work of Bargar (1987,

118

1991-this volume), the highest temperatures ex­perienced by the altered rocks of PL TO-l are > 250°C and he reports an Ar40/39 date of 37.5 Ma on hydrothermal biotite deposited in small open fractures at 138.9 m that were later filled completely by quartz.

An attempt was made to estimate the age range of this earlier event by obtaining whole­rock KI Ar ages on the altered andesite exposed just west of the San Andres Mine and in the creek bottom of the Quebrada del Agua Caliente, and on an altered diorite dike in­truding Paleozoic gneiss at EI Quetzal Mine (Table 9). The dates obtained on the andesites of 21 to 25 Ma and on the diorite of 98 Ma should be regarded as minimum ages for these rock units due to alteration. Possibly the age of Au, Sb, and Hg mineralization is < 14.5 Ma because quartz veins and silicification are observed cutting the Miocene tuffs exposed along the Quebrada del Agua Caliente below the waterfall. Thus, dating techniques and geologic inference indicate that alteration ex-

F. GOFF ET AL.

tends from 37.5 to < 14.5 Ma and that andesites of the Matalgalpa Formation are pro­bably Oligocene in age.

The andesites are offset by <! 450 m along normal faults that step down to the east be­tween the San Andres Mine and PL TO-I. Fault gouge composed of shattered vein quartz, altered andesite, and Paleozoic schist is observ­ed along La Bufa fault in the San Andres Mine, and the total offset along this fault may exceed 1000 m. This much tectonic activity is not likely to have occurred in the last I Ma, thus the tim­ing of the first series of hydrothermal events is probably» I Ma.

The second hydrothermal event is associated with the active geothermal system. It is also associated with at least some of the calcite veins observed in the PLTO-I core hole based on lower salinities and lower equilibration temperatures determined on some calcite fluid inclusions (Bargar, 1991-this volume). Calcite veins can be observed cutting quartz veins in rocks exposed along the Quebrada del Agua

Potassium-argon ages from three hydrothermally-altered rocks, Platanares, Honduras (Geochron Laboratories, Cam­bridge. Massachusetts)

Sample no. Location

Rock type Material 0J0 Kb Radiogenic 40 ArC

Map oO,a

a

F87-29 Quebrada Agua Caliente

150 m northwest of core hole PL TO-l

Andesite W/R 1.311 ± 0.001

4.828 ± 0.085 52.0 ± 3.6 21.1 ± 1.1

b

F87-32 Large, unnamed arroyo

112 km west of San Andres Mine

Andesite W/R 0.827 ± 0.145

3.595 ± 0.185 43.1 ± 3.4 24.9 ± 1.5

c

JAM87·2 EI Quetzal antimony mine,

2 km north of San Andres Mine

Diorite dike W/R 0.796 ± 0.002

14.013 ± 0.520 60.2 ± 13.8 98.8 ± 4.5

(x 10- II mol/g) 070 Radiogenic 401 Ar Age (Ma)d Comments Sample is hydrothermally altered; should be considered minimum age, only

a Map numbers shown on Fig. 2. b Average of two potassium analyses. C Average of two argon analyses. ct)., ~ 0.581 x IO-JO yr -',A{3 ~ 4.962 X IO-JO yr -',4oK/K ~ 1.193 X 10-4

EXPLORATION DRILLlNG AND RESERVQ[R MODEL OF PLATANARES, HONDURAS 119

Caliente upstream of PLTG-I, and a large calcite vein occurs within a fault strand of La Bufa Fault where it crosses the quebrada.

An attempt was made to date the calcite veins using the U/Th disequilibrium method (Table 10). A succinct description of the principles and limitations of this method .as applied to dating calcite veins has been given by Sturchio and Binz (1988). Because the solubility of uranium in geothermal water is typically low, large samples (14 to 47 g) were required to obtain enough uranium for analysis. Two of the samples have analytical problems that result in no age determination or require that the age be disregarded. Sample F87-5 from the San An-

TABLE 10

dres Mine has low uranium content and a 234U /238U value that is much greater than I even though 230Th/234U is less than 1. This sample is probably recrystallized and should be disregarded. Also, this vein occurs > 450 m above the present hot springs and is, thus, not part of the present hydrothermal system. Sam­ple F87-13 from a vein in the Quebrada del Agua Caliente also has low uranium content and both the 234U/238U value and the 230Th/234U value are > > I. The age of this sample is impossible to calculate. The remain­ing three samples yield satisfactory analytical results and ages ranging from 250 to about 50 ka. Thus, the maximum age of the present

Uranium-thorium disequilibrium ages from five calcite veins, Platanares, Honduras (T.-L. Ku, University of Southern California)

Map no.a

PLTG·I PLTG-I d e f

Sample no. 71·7 130-lB F87·5 F87-9 F87-13 Location PLTG-I core PLTG-I core San Andres Mine, Trail from Quebrada Agua

hole at hole at 30 m west of Platanares to Caliente, 249.25 m 493.7 m rock crusher La Bufa, 112 km north

112 km north of core hole of core hole PLTG-I PLTG-I

Weight sample (g) 14.2 35.8 27.0 46.8 39.2 Residue (wl."Io) 8.8 0.2 2.3 32.2 15.6 U (Ppb) 38.5 ± 1.2 27.6 ± 0.6 3.9 ± 0.3 11.8 ± 1.0 1.6 ± 0.3 234U/238U 1.13 ± 0.06 1.01 ± 0.04 1.46 ± 0.17 1.16 ± 0.13 1.94 ± 0.46 23"Th/234U 0.934 ± 0.049 0.796 ± 0.036 0.685 ± 0.072 0.406 ± 0.039 1.895 ± 0.277 23"Th/232Th 0.459 ± 0.021 0.365 ± 0.014 1.158 ± 0.120 9.200 ± 1.681 2.994 ± 0.276 Age e3"Th/234U)b

ka 256, +66-42 171, +21-17 115, +22-19 56, +7-6 Comments Part of complex Vein 2 to 4 em Vein 1.6 m wide, Vein 0.5 to 1.0 m Vein 0.5 m- wide

of quartz-cal- wide in altered strike NlOoW, wide~-_ strike strike N 40 0 W, cite veins in andesite. dip 65°W, in N30oW. dip dip 300 S, in . altered andesite altered andesite 75°E; along altered andesite from 249 to breccia. fault between 253 ID. andesite (west)

and tuff (east).

a Map numbers shown on Fig. 2. b Ao = 9.217 X 10- 6 yr-', A4 = 2.795 X 10- 6 yr- 1, uncertainties are one standard deviation (10-) derived from counting statistics.

120

Platanares system is estimated to be about 250 ka.

Reservoir model and conclusions

A reservoir model of the Platanares system is depicted in the schematic cross-section of Figure 10 adapted from Heiken et al. (l991-this volume) and other surface investigations (F. Goff et ai., 1986, 1987b). The geochemistry of the reservoir fluids and core hole data indicate that the geothermal water equilibrates in sedimentary rocks, probably the Cretaceous to Eocene Valle de Angeles Group (Janik et ai., 1991-this volume). Our work and various other investigations reported in this volume show that> 95"70 of reservoir fluids discharge along the northwest-trending Quebrada del Agua Caliente fault zone but that some reservoir fluids issue from faults and fractures throughout the length and width of a structural

NW

F. GOFF ET AL.

graben in the Platanares area. The northern limit to the (shallow?) system is bounded by the major northeast-trending La Bufa fault that juxtaposes Paleozoic schist (north) against Ter- , tiary volcanic rocks and Cretaceous to Eocene red beds. Other limits to the reservoir are not structurally defined but it appears to underlie a zone having an area of at least 3.5 km2 as shown in Figure 2.

From constraints supplied by stable isotope data on the waters, recharge to the Platanares system is fairly local and may come from regions of higher elevation to the north and northeast. Tritium data suggest that percola­tion is relatively slow and that the mean residence time of the fluid in the reservoir is about 450 yr old. When residence time is com­bined with surface discharge data, the volume of the Platanares reservoir is estimated at ~ 0.78 km3 of water. After heating and equilibration with rocks, geothermal fluid rises

s La Bufa F~h

PLTG-1 C_ del Agua PLTG-3 PLTG-2 Rio Lara Caliente (Bend)

SW Edge Q. Del Agua Cal. Fault Zone

~ With Abundant Boiling Hot Spgs.

Tpm

Fig. 10. Schematic northwest - south cross-section of Platanares geothermal site; rock symbols same as on Fig. 2 except Qal = Quaternary alluvium and Qt = Quaternary terrace gravels (adapted from Heiken et al.. 1991-this volume). A postulated reservoir model is shown as are the temperatures of major fluid entries in the core holes (see text for detailed explanation). Thickness of the Valles de Angeles red beds (KTva) is unknown but is believed to exceed 1000 m (thickness at type locality may exceed 3000 m; Carpenter. 1954). Mesozoic formations underlying KTva do not have surface exposures in the Platanares area.-Evidence for slow meteoric recharge is presented in Janik etal. (1991-thisvolume)_ Horizontal scale is 7 em = 1 km.

EXPLORATION DRILLING AND RESERVOIR MODEL OF PLATANARES, HONDURAS 121

convectively along the many faults and frac­tures cutting the reservoir rocks. Temperature data from the large fluid entry at 26 m in PL TG-3 and the localized presence of fumaroles indicates that subsurface boiling oc­curs in the shallow part of the geothermal system. Thermal gradient from PL TG-2 sug­gest that there is a lateral flow system to the south toward the Rio Lara (F. Goff et aI., 1987a, 1988a). Chemical geothermometers in­dicate that the source reservoir has an equilibration temperature of 225 to 240°C (Janik et aI., 1991-this volume). Dates on calcite veins by the U /Th disequilibrium method suggest the maximum age of the geothermal system is roughly 250 ka.

Downward continuation of the conductive thermal gradient in PLTG-2 (139°C/km at 400 m and 104°C) would suggest that the depth to a reservoir of 225°C is roughly 1.2to 1.5 km. However, the gradient of this well may be in­fluenced by convecting hydrothermal fluids at greater depths, thus, this estimate should be regarded as a minimum depth (Meert and Smith, 1991-this volume). The reservoir fluid in the upflow zone along the Quebrada del Agua Caliente displays near-isothermal condi­tions of 130 to 165°C at 26 to 680 m. There is no known structural or stratigraphic reason to suggest that the temperature gradient should rise suddenly below the 165°C zone at 680 m and that a 225°C reservoir should lie at depths above 1.2 km. The available data do not in­dicate if there are discrete shallow and deep reservoirs or if there is merely one displaying conductive cooling, mixing, and shallow boil­ing as fluids ascend.

Flow test data from PLTG-I and PLTG-3 demonstrate that fluids of 160°C to 165°C are easily produced from fractured andesite and metaconglomerate along the Quebrada del Agua Caliente fault zone. Fractures with permeabilities of 300 to 1500 md occur sporadically throughout an interval from 250 to 650 m depth and porosities vary from 2 to 607. over the same interval. Using a simple,

fracture-dominated, volume-impedance mo­del, a reservoir storage capacity of about 9.7 x 1()6 l/(kg/cm2) was calculated from data for each bore. Maximum thermal power output is about 3.1 and 5.1 MW(t) for PLTG-I and PLTG-3, respectively. The downhole fluid composition of both holes are very similar. The fluids are relatively dilute « 1200 mg/kg TDS) and slightly alkaline and are not expected to be corrosive in any geothermal applications. Scaling of aragonite in the upper casing of PLTG-I resulted from flashing and release of CO2 because of lower flow rates and flowing pressures compared to PLTG-3. Use of downhole pumps and/or a binary-cycle heat extraction system, or injection of scale in­hibitors could probably prevent scaling in any electrical applications of the relatively shallow 160°C resource.

Acknowledgments

The authors gratefully thank the people at ENEE, Tegucigalpa, Honduras, who helped make this project possible; particularly, Rodrigo Paredes, Napoleon Ramos, Ricardo Perdomo, and Miguel Lewa . Thanks also to the alcalde of Platanares, Ricardo Portillo, and to Doug Blum and co-workers at the San An­dres Mine. Tomas Hirschman of Swissboring, Ltd., Guatamala City, was instrumental in development of the coring program. Pat Tru­jillo and Dale Counce, Los Alamos National Laboratory performed the chemical analyses on water. Cathy Janik of the U.S. Geological Survey and C.O. Grigsby and J. Wilfred Gutierrez of Los Alamos were of great assistance during much of the project. Mary Stallard of the U.S. Geological Survey helped with the field work and sample collections dur­ing the flow tests. Geochron Laboratories (Cambridge, Massachusetts), G.H. Ostlund (University of Miami), J. Borthwick (Southern Methodist University), T.-L. Ku (University of Southern California), and Terra Tek Research (Salt Lake City, Utah) provided various

122

isotope and physical property data cited in some of the tables. Dick Benoit of Oxbow Geothermal Co., Reno, Nevada and Cathy Janik reviewed the paper. This project was funded by the U.S. State Department, Agency for International Development.

References

Aiken, C,L.,Ander, M.E., de laFuente, M. and Hoover, D.B., 1991. Geophysical investigations of the Platanares geothermal site, Honduras, Central America. J. Volcanol. Geotherm. Res., 45: 71-90.

Aldrich, M.J., Adams, A.!, and Escobar, C., 1991. Struc· tural geology and stress history of the Platanares geothermal site, Honduras: implications on the tec­tonics of the northwestern Caribbean plate boundary, J. Vokano!' Geotherm. Res., 45: 59-69.

Bargar, K., 1987. Fluid inclusion data for drill hole PLTG-l, Platanares geothermal area, Honduras. Geotherm. Resour. Coune. Trans., 11: 225-230.

Bargar, K .• 1991. Fluid inclusions and preliminary studies of hydrothermal alteration in core hole PL TO-l, Platanares geothermal area, Honduras. J. Volcanol. Geotherm. Res., 45: 147 -160.

Bear, J., 1972. Dynamics of Fluids in Porous Media. Elsevier, New York, N.Y., 764 pp.

Carpenter, R.H., 1954. Geology and ore deposits of the Rosario Mining District and the San Juancito Moun­tains, Honduras, Central America. Geol. Soc. Amer. Bull., 65: 23 - 28.

Eppler, D., Fakundiny, R. and Ritchie, A., 1986. Recon· naissance evaluation of Honduran geothermal sites. Los Alamos Nat'!. Lab. Rept. LA-10685·MS, IZ pp.

Flores, W., 1980. Geology of the Platanares, Annex V. In: Report of the activies and interpretation of results of the geothermal project of Honduras 1979-1980: Energy Programme of the Central American Isthmus, Dept. RLA/76/OlZ (ENEE), v. I.

Fontes, J. Ch., 1980. Environmental isotopes in ground­water hydrology. In: P. Fritz and J. Ch. Fontes (Editors), Handbook of Environmental Isotope Geochemistry, vol. IA: The Terrestrial Environment. Elsevier. Amsterdam, pp. 75-140.

GeothermEx, Inc., 1980. Honduras, Central American Energy 'program Phase II, reconnaissance geochemical survey and interpretation of thermal spring waters and gases. Consult. rept. Richmond, Calif., 140 pp. (unpub!.).

Goff, F., Shevenell, L., Janik, C., Truesdell, A., Grigsby, C. and Paredes, R., 1986. Hydrogeochemistry and preliminary reservoir model of the Platanares geother­mal system, Honduras, Central America. Geotherm.

F. GOFF ET AL.

Resour. Counc. Trans., 10: IZ5 - 130. Goff, F., Shevenell, L., Kelkar, S., Smith, D., Meert, J.,

Heiken, G., Barger, K., Ramos, N., Truesdell, A., Stallard, M. and Musgrave, J., 1987a. Stratigraphy, temperature profiles, and flow test data from the PLTG·I and PLTG-Z core holes, Platanares geother­mal system, Honduras. Geotherm. Resour. Counc. Trans., 11: 253 -Z59.

Goff, F., Truesdell, A., Grigsby, C., Janik, c., Shevenell, L., Parades, R., Gutierrez, J., Trujillo, P. and Counce, D., 1987b. Hydrogeochemical investiga­tion of six geothermal sites in Honduras, Central America. Los Alamos Nat'!. Lab. Rept. LA-10785-MS, 170 pp.

Goff, F., Shevenell, L., Gardner, J., Vuataz, F. and Grigsby, C., 1988a. The hydrothermal outflow plume of Valles caldera, New Mexico and a comparison with other outflow plumes. J. Geophys. Res., 93(B6): 6041-6058.

Goff, F., Truesdell, A., Shevenell, L., Janik, C., Grigsby, C., Parades, R., Trujillo, P., Counce, D., Gutierrez, J., Adams, A., Urbani, F. and Perdomo, R., 1988b. Hydrogeochemical report of the second Honduras sampling trip, January-February, 1986. Los Alamos Nat'!. Lab. Rept., !O5 pp. (unpub!.).

Goff, S., Riifenacht, H., Laughlin, A., Adams, A., Plan­ner, H. and Ramos, N., 1987. Geothermal core hole drilling and operations, Platanares, Honduras, Cen­tral America. Geotherm. Resour. Coune. Trans., 11: 37.

Goff, S., Laughlin, A., Riifenacht, H., Goff, F., Heiken, G., Adams, A., Musgrave, J., Planner, H. and Ramos, N., 1988. Exploration geothermal gradient drilling, Platanares, Honduras, Central America. Los Alamos Nat'!. Lab. Rept. LA-I 1349-MS, 29 pp.

Heiken, G., Eppler, D., Wohletz, K., Flores, W., Ramos, N. and Ritchie, A., 1986. Geology of the Platanares geothermal site, Departamento de Copan, Honduras, Central America. Los Alamos Nat'l. Lab. Rept. LA-10634-MS, 24 pp.

Heiken, G., Duffield, W., Wohletz, K., Priest, S., Ramos, N., Flores, W., Eppler, D., Ritchie, A. and Escobar, C., 1987. Geology of the Platanaresgeother· mal area, Copan, Honduras. Geotherm. Resour. Counc. Trans., 11: 263 - 266.

Heiken, G., Ramos, N., Duffield, W., Musgrave, J., Wohletz, K., Priest, S., Aldrich, J., Flores, W., Rit­chie, A., Goff, F., Eppler, D. and Escobar, C., 1991. Geology of the Platanares geothermal area, Depar­tamento de Copan, Honduras. J. Volcano!' Geotherm. Res., 45: 41- 58.

Hoover, D. and Pierce, H., 1988. Electrical geophysical studies of the Platanares geothermal area, Honduras. U.S. Geo!. Surv. Rept., Denver, Colo., 24 pp. (un· pub!.).

EXPLORATION DRILLING AND RESERVOIR MODEL OFPLATANARES, HONDURAS 123

lanik, C.l., Goff, F., Truesdell, A.H., Shevenell, L., Stallard, M., Trujillo, P. and Counce, D., 1991. A geochemical model of the Platanares geothermal system, Honduras. 1. Volcanol. Geotherm. Res., 45: 125 -146.

Mahon, W .A.l., 1964. Sampling of geothermal drill-hole discharges. Proc. U.N. Conference on New Sources of Energy, Rome, 1961, v. 2, pp. 269-278.

Mahon, W .A.l., 1966. A method for determining the en­thalpy of a steam-water mixture discharged from a geothermal drill hole. N.Z.l. Sci., 9: 791- 800.

Meert, 1.0. and Smith, D.L., 1991. Heat flow at the Platanares, Honduras geothermal site. J. Volcanol. Geotherm. Res., 45: 91- 99.

Murphy, H. and Dash, Z., 1985. The shocking behavior of fluid flow in deformable joints. Geotherm. Resour. Counc. Trans., 9(11): 115-118.

Pearson, F. and Truesdell, A., 1978. Tritium in the waters of Yellowstone National Park. U.S. Geol. Surv.

Open-file Rept. 78-701, 4 pp. Roberts, R. and Irving, E., 1957. Mineral deposits ofCen­

tral America. U.S. Geol. Surv., Bull. 1034, 205 pp. Sturchio, N.C. and Binz, C.M,. 1988, Uranium-series age

determination of calcite veins, VC-l drill core, Valles calders, New Mexico. J, Geophys, Res" 93: 6097 -6102.

Truesdell, A., Janik, D., Goff, F., Shevenell, L., Trujillo, P., Counce, D., Kennedy, B. and Parades, R" 1987a. The origin of thermal waters of Honduras and puzzl­ing variations in spring chemistries. In: Proceedings of the Ninth New Zealand Geothermal Workshop, Univ. Auckland, Geotherm. Inst., pp. 79 - 88.

Truesdell, A., Stallard, M., Trujillo, p" Counce, D" Janik, C., Winnett, T., Goff, F, and Shevenell, L., 1987b, Interpretation of fluid chemistry from the PL TG-l exploratory drill hole, Platanares, Hon­duras. Geotherm, Resour. Counc, Trans., 11: 217-222.

• ,'I'

1 I