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Joum of Hydrology, 56 (1982) 119- 136 119 Elsevier Scientific Publishing Company , Amsterdam - Printed in The Netherlands

VALLES CALDERA GEOTHERMAL SYSTEMS, NEW MEXICO, U.S.A.

FRASER GOFF and CHARLES 0 . GRIGSBY

Geoscien~es Division, Los Alamos Scientific Laboratory, Los Alam os, NM 8 7544 (U.S.A .)

ABSTRACT

Goff, F . and Grigsby, C.O., 1982. Valles Caldera geothermal systems, New Mexico, U.S.A. In : J . Lavigne and J.B.W. Day (Guest-Editors), Hydrogeothermal Studies- 26th Inter· national Geological Congress . J . Hydro!., 56: 119-136.

Valles Caldera is part of a Quaternary silicic volcano in northern New Mexico that pos· sesses enormous geothermal potential . The caldera has formed at the intersection of the volcanically active Jemez lineament and the tectonically active Rio Grande rift . Volcanic rocks of the Jemez Mountains overlie Paleozoic--Mesozoic sediments, anrl Precambrian granitic basement. Although the regional heat flow along the Rio Grande rift is ~2.7 HFU*, convective heat flow within the caldera exceeds 10 HFU. A moderately saline hot· water geothermal system (T > 260°C, ct.:::: 3000 mg/1) has been tapped in fractured caldera-fill ignimbrites at depths of 1800 m. Surface geothermal phenomena include central fumaroles and acid-sulfate springs surrounded by dilute thermal meteoric hot springs. Derivative hot springs from the deep geothermal reservoir issue along the Jemez fault zone, 10 km southwest of the caldera. Present geothermal projects are : (1) proposed construction of an initial 50-MWel power plant utilizing the known geothermal reservoir; (2) research and development of the prototype hot dry rock (HDR) geothermal system that circulates surface water through deep Precambrian basement (-5 MWth); (3) explo· ration for deep hot fluids in adjacent basin-fill sediments of the Rio Grande rift; and (4) shallow exploration drilling for hot fluids along the Jemez fault zone.

INTRODUCTION

Large Quaternary magma-hydrothermal systems and continental rifts have vast potential for the production of geothermal energy in the western U.S.A. (Muffler, 1979; Goff and Waters, 1980). Valles Caldera in northern New Mexico is a large Quaternary silicic volcanic complex adjacent to the Rio Grande rift that presents a well-defined geothermal target with several opportunities for exploration and development. Among these ventures are the prototype hot dry rock (HDR) geothermal system on the west flank of the caldera and a moderate-sized conventional hydrothermal system within it. The caldera also contains a relatively simple suite of natural thermal fluids that can be used to evaluate hydrothermal resources. The object of this paper is to review the geology and tectonics of the Valles region, differenti­ate the thermal waters, and describe four geothermal prospects.

* 1 HFU (heat flow unit)= 1 peal. s -I em - 2 = 41.67 mW m - 2.

0022-1694/82/0000-0000/$02.75 © 1982 Elsevier Scientific Publishing Company

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120

GEOLOGICAL OVERVIEW

Geologic and tectonic setting

Valles Caldera formed from catastrophic ignimbrite eruptions of Bandelier Tuff -1.1 Myr. ago (R.L. Smith and Bailey, 1966) and represents the culmi­nating phase of volcanism within the Jemez Mountains volcanic field (R.L. Smith et al., 1970). The Jemez Mountains (Fig. 1) lie at the intersection of the Jemez volcanic lineament and the Rio Grande rift tectonic province (Mayo, 1958; Chapin, 1979; Laughlin, 1981). Several volcanic fields have erupted along the lineament. They consist of Miocene-Quaternary age basalts and lesser quantities of more silicic lavas and tuffs. No systematic age or compositional variations can be correlated with direction along the lineament (Laughlin et al., 1976). By far the largest volume of in~rmediate-and silicic volcanics are found in the Jemez Mountains. ·

The Rio Grande rift is a continental rift, stretching from northern Colo­rado into Mexico, that began development -30 Myr. ago (Chapin, 1979). The rift is composed of three major segments that become progressively older to the south. Three en echelon basins compose the middle segment (east of the Jemez Mountains), and each basin is filled with thick middle­late Tertiary sedimentary sequences (Fig. 1). Ignimbrite eruptions from Valles Caldera poured over the western edge of the central basin to form the Pajarito Plateau (Figs. 2 and 3).

Basins of the rift are bounded on the west by extensive normal faults

I I

UTAH I COLORADO -----~------+--- -----------

ARIZONA NEW MEXICO

, :

D Iff/ rfi

•'•

<

' . . ,

, .....

:

50 100

, ~ Miocene·Holocene ~ Volcanic Rocks

/ , Beeins of the Rio Grande

Fig. 1. Location map of northern New Mexico showing relationship of Jemez lineament and Rio Grande rift to the Jemez Mountains volcanic field.

Fig. geoU

whi< faul1 shee thes the Jem

H whe con'

Geo

T ture geo1 the Lau simJ avai by 1

1

·tions of Bandelier 1resents the culmi­>lcanic field (R.L. ;he intersection of tectonic province lcanic fields have ~uatemary age Is. No systematic irection along the 1e of intermediate

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ment

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Basins of the Rio Grande

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Jemez Fault Zone

TOLEOO CALDERA

/'-los Alamos L • 1Aiternate Energy Well

Geothermal System (Union Oil Co. I

Pajarito Fault

E

10

KUometer•

121

Santa Fe •

20

Fig. 2. Map showing Valles Caldera region, Jemez fault zone, Pajarito fault and various geothermal targets discussed in text. Cross-section of Fig. 3 follows a W-E line.

which cut volcanics of the Je!llez Mountains. Two of the more important faults are the Pajarito and Jemez fault zones (Figs. 2 and 3). Ignimbrite sheets of Bandelier Tuff are offset roughly 200m and 50 m, respectively, by these fault zones. The Jemez fault zone, which trends northeastward toward the center of Valles Caldera, is locally a surface tectonic expression of the Jemez lineament (Goff et al., 1981).

Heat flow along the west margin of the Rio Grande rift averages 2. 7 HFU whereas heat flow within Valles Caldera locally exceeds 10 HFU because of convection (Reiter et al., 1975, 1976).

Geologic section

The following discussion describes selected aspects of geology and struc­ture which elucidate the setting of natural thermal waters and the various geothermal targets within the Jemez Mountains. The reader should refer to the superb geologic map of R.L. Smith et al. (1970) and the review paper by Laughlin (1981) for more details. The cross-section of Fig. 3 is an over­simplification but includes recent data from many deep wells that were not available to earlier workers. A more conventional interpretation is presented by the cross-section of Eichelberger and Westrich (1980).

The Jemez Mountains volcanic rocks overlie Precambrian granite, gneiss

Canon De San Diego

Jemez Springs Well

Rio Guadalupe (Bend)

W Motors Derivetive DHp Wer.r

Fenton Hill Wells

(Band I

Jemez Mountains

Valles Caldera

Sulphur Springs (Bend)

Baca*4 Well (Projected)

Thermel MetfiOrlc Spring RelufJient Dome

~ Rlnr mcfllre

Rio Grande Rift, Pajarlto Plateau

Alternate Energy Well Rio 'Grande

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2000

1000

See Levol

·1000

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~ Moat Rhyolites, 0.1 ·1.0 m.y.

0 Bandelier Tuff. Includes Intra-caldera deposita, 1.1 · 1.4 m.y.

!:ill Tshicoma Rhyodacltes, 3 • 7 m.y.

0 Paliza Canyon Volcanics, 8 • 10 m.y.

0 Santa Fa Formation, Miocene

~ Mesozoic and Paleozoic Sediments

0 Precambrian

0 Stratigraphy Unknown

0 5 10

Kilometer•

Fig. 3. Simplified cross-section through the Jemez Mountains showing major structures and stratigraphic units. Geologic settings of natural thermal waters and geothermal targets are discussed in text. The white area beneath the eastern portion of Valles Caldera indicates where structure and stratigraphy are not well defined by geophysics or drill'holes.

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123

and schist, and also the relatively undeformed Paleozoic-Mesozoic marine carbonates, sandstones, shales and evaporites of the Colorado Plateau. As shown in Fig. 3, these basement rocks are progressively down-faulted east­ward into the Rio Grande rift. Unconsolidated to weakly indurated Tertiary sediments overlie the fault blocks and thicken to the east into the rift. Older Miocene-Pliocene volcanic sequences of the Jemez Mountains (i.e. Paliza Canyon and Tschicoma Formations) overlie and interfinger with the Tertiary sediments.

Rift structures formed during the late Tertiary have been overprinted by caldera structures formed in Quaternary time during eruption of Bandelier Tuff and subsequent caldera events. Eruption of ~ 500 km3 or more of ignimbrite suggests existence of a relatively shallow silicic magma body beneath Valles Caldera of almost batholithic size (R.L. Smith and Bailey, 1968). The "floor" of the caldera collapsed several thousand meters during and after these eruptions producing a series of ring fracture faults. After collapse, the central floor presumably was uplifted by resurgent (structural) doming according to mechanisms outlined by R.L. Smith and Bailey (1968). Finally, the moat area between the resurgent dome and caldera walls was partially filled by rhyolite domes, flows and small-volume tuffs which are -l.O-Q.1 Myr. old (Doell et al., 1968).

Fig. 3 shows a large area beneath the eastern caldera where stratigraphy and fault displacements are unknown. Several structural configurations could be drawn in this space (and have been) but no deep drill hole and little geophysical data exist to confirm them. For example, a detailed gravity survey of the caldera presented by Dondanville (1971) shows a gravity low of >25 mGal, centered over the eastern side. This could be a reflection of thicker ignimbrite' fill on a more deeply down-faulted block of pre-caldera (rift) origin. On the other hand, the gravity low could suggest extensive vents for ignimbrite eruptions concentrated on the east side of the caldera. Other interpretations that consider variable densities of caldera-fill materials caused by hydrothermal alteration are also possible.

Drilling of 24 deep drill holes through the resurgent dome into Bandelier Tuff and pre-caldera rocks does not verify structural uplift of the dome, as conceived by R.L. Smith and R.A. Bailey. Drill-hole information shows general down-faulting of Precambrian basement to the east towards the rift. Tertiary rift sediments increase in thickness to the east, as well (Slodowski, 1976; R. Denton, Union Oil Co., person. commun., 1980). Examination of cuttings by Union Oil Co. has not revealed any evidence for injection of shallow silicic plugs, dikes, sills or laccoliths into the resurgent dome from the underlying batholith.

Although the resurgent dome is cut by a multitude of normal faults (not shown in Fig. 3) to form a "keystone" graben, these faults are on trend with the Jemez fault zone, southwest of the caldera. This suggests that structures within the resurgent dome are controlled by pre-existing rift faults.

Several important questions could possibly be answered by a deep bore-

124

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NATURALTH.

Setting, simp/

Three typ< possesses dis chemical and acid-sulfate; ( types. Severa and by Dond;

Acid-sulfate z.

Waters of they dischari largest zone ( fractures int~ Springs is a J

pots, gaseous seasonal with the late sumn

Chemical < extremely hi (Fig. 4A) she the bubbling H2 S and sub gas (Table II carbonated 1

Springs. Alth not allow m (Table I) and

Spring sys vapor-domin; and C02 are gases reach densed acid

125

hole (~5-8 km deep) on the east side of the caldera: ( 1) the depth to pre­volcanic rocks would teli us if the caldera floor is indeed asymmetric due to rift tectonics; (2) the resulting stratigraphic information would control parameters used in geophysical models; (3) the eastern limit of the hydro­thermal system (see p. 129) could be better ascertained; and (4) strati­graphic and structural boundaries between Valles Caldera and the earlier Toledo Caldera (Fig. 2) could possibly be resolved.

NATURAL THERMAL WATERS

Setting, simplified hydrology and geochemistry

Three types of natural thermal water are found in Valles Caldera. Eac~ possesses distinct geologic and structural controls, and displays unique chemical and isotopic characteristics. These waters are herein named: (1) acid-sulfate; (2) thermal meteoric; and (3) deep geothermal and derivative types. Several of the ideas that follow were first suggested by Trainer (1975) and by Dondanville (1978).

Acid-sulfate waters

Waters of this type are restricted to the interior of Valles Caldera where they discharge from faults and fractures within the resurgent dome. The largest zone of activity is at Sulphur Springs (Fig. 3) where several faults and fractures intersect (Summers, 1976; Goff and Gardner, 1980). Sulphur Springs is a hydrothermally altered area of small-volume acid springs, mud pots, gaseous cold springs and fumaroles. Flow rates and temperatures are seasonal with the lowest flow rate and highest temperature occurring during the late summer and early fall.

Chemical data for two samples from Sulphur Springs reveal them to have extremely high S04 , low pH, and more K than Na (Table 1). Isotopic data (Fig. 4A) show that the flowing acid spring is isotopically meteoric whereas the bubbling mud pot is shifted towards enriched values of 18 0. Although H2 S and sublimed sulphur occur here, C02 is by far the dominant insoluble gas (Table II). Related gaseous springs, which discharge what we have named carbonated meteoric waters, discharge around the periphery of Sulphur Springs. Although these waters bubble C02 freely, their relative acidity does not allow much HC03 to remain in solution. Chemically they are dilute (Table I) and isotopically they are meteoric (Fig. 4A).

Spring systems such as Sulphur Springs display all the characteristics of vapor-dominated geothermal systems (White et al., 1971). Water vapor, H2 S and C02 are distilled from an underlying boiling-water table. When these gases reach the surface, H2 S is oxidized to form sulfuric acid. This con­densed acid water effectively leaches and attacks surface rocks. If enough

...... TABLE I ~

O'l

Field data, chemical analyses and isotopic data from thermal, mineral and non-thermal natural waters of the Valles Caldera region and selected analyses of fluids from the Los Alamos hot dry rock (HDR) geothermal demonstration project (chemical analyses reported in mg/1; isotope analyses reported in per mille relative to SMOW)

Name Symbol Temperature Field Flow Rock*' Si02 Fe Ca Mg Na (Fig. 4) (oC) pH (I/ min.) type

Surface meteoric water:

Agua Durme Springs *3 m 16 7.8 400 v 64 - Iii ;) 13 Fish hatchery spring* 3 m 11.5 - - v - - II 1 9

Carbonated meteoric water:

Bubbling pool em 0 4.5 5 v -14 1.3i 14.1 2.i5 5.H Bubbling seep em 6.7 5.2 I v 51 0.28 83.5 12.0 32.8

Thermal meteoric wale>:

Spence Springs tm 45 6.7 60 V (P) 66 <0.04 !'),!) 1.9 GO San Antonio Springs tm 42 6.8 160 v 79 <0.04 2.3 0.30 21

Acid·sul(ate water:

Sulphur Springs, mud pot a 78 2.5 0 v 103 .,1,!)5 2.1 1.0 2.1 Sulphur Springs, acid spring a 62.8 2.4 2 v 229 26.0 !JO.H 16.2 1 l.(j

Deep geothermal fluid and derivatives:

Valles Caldera (Baca 4)* 4 d - 9.0 (lab.) - v 820 lr. 0.0 0.0 2,100 Valles Caldera *5 d -260 7.2 - V and P 599 - 1fi - 1,il9 Soda Dam Springs d 48 6.4 60 p 50 0.1·1 3·10 2·1.-1 !J:JH Main Jemez Springs d 55 7.0 20? AI (P) 93 0.20 152 5.4 6fi6 Travertine mound spring* 6 d 70 6.3 4 AI (P) 93 0.15 182 4.6 614

Hot dry rock (HDR) fluids:

Make-up well hm 20 7.9 38 v 68 <0.01 39.3 3.9 12.2 Initial sample hi 155 6.6 750 G 245 1.4 51 1.1 1,072 "Steady state" hs !55 6.3 380 G 236 1.0 -IH 0.9 690 GT·2a Madera Formation* 3 d ,;;;76 8.8 - p - 1.2 2 2 6,300* 2

GT·2b Madera Formation*' d 56 7.4 - p 115 - 78 42 550

TABLE I (continued)

Name K Li HC03 so. Cl F B oo o"o

"Steady state" GT-2a Madera Formation* 3

GT-2b Madera Formation*'

TABLE I (continued)

Name

Surface meteoric water:

Agua Durme Springs*3

Fish hatchery spring* 3

Carbonated meteoric water:

Bubbling pool Bubbling seep

Thermal meteoric water:

Spence Spring. San Antonio Springs

Acid-sulfate water:

Sulphur Springs, mud pot Sulphur Springs, acid spring

hs d d

Deep geothermal fluid and derivatives:

Valles Caldera (Baca4)* 4

Valles Caldera 05

Soda Dam Springs Main Jemez Springs Travertine mound spring* 6

Hot dry rock (HDR) fluids:

Make-up well Initial sample "Steady state" GT·2a Madera Formation* 3

GT-2b Madera Formation • 7

tr. =trace.

155 ,;;;76

56

K

-2.4

4.5 7.9

1.3 1.7

8.2 18.7

777 370 183

74 75

4.1 85 59

350 -

Li

--

0.04 0.08

0.66 <o.o2

0.02 0.05

--

13.2 10.1

8.2

<0.02 21 11 25 -

6.:J 8.8 7.4

HC03

--

88 62

0 178

144 56

0 0

323 127

1,514 711 723

118 373 415

6,820* 2

1,230

3HO

so.

3 !)

109 254

16 7

786 2,110

66 64 38 41 36

13.3 520 820

... ,110 200

u p p

~Ju l.V

1.2 115

Cl F

6 0.6 4 0.4

4.9 0.23 7.2 0.23

H 0.55 2 0.08

2.48 6.36 3.72 0.61

3,618 15 3,061 -1,503 3.67

904 5.19 829 5.21

19.0 0.07 1,502 8.57

613 10.0 3,500 0

400 3.1

B

2 78

-0.02

<0.1 <o.1

0.15 <0.05

<0.1 <0.1

38 23 13.82

7.86 7.83

0.02 41.6 16.2 25.0

*1 Rock type: V =Plio-Pleistocene volcanics; AI= Alluvium; P =Paleozoic rocks; G =Precambrian granodiorite; ( ) =rock type at depth. *2 Sample probably contaminated with Na 2 C03 from drilling mud. *lTrainer (1978). *4 Dondanville (1971 ). *5 Dondanville (oral commun., 1978) (average of 5 analyses). *6 Isotope data from Trainer ( 197 8 ). *7 Purtyman et al. (1974).

2 42

liD

-8o.o -96.o

-97.3

-H6A -92.0

·-·50.2 -60.7

--84.9 -82.3 -82.1

-90.0 -77.8 -81.0 -78.9

6,300* 2

550

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-12.05 -13.14

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1-' 1:\:1 --.J

128

TABLE II

Gas compositions associated with fluid analyses from Table I (gas compositions expressed as percent of total gas)

Gas Sulphur Springs area Jemez fault zone* 1

carbonated acid sulfate Soda Dam M. Jemez Springs meteoric

C02 94.5 71.9 90.2 89.7 N2 4.2* 2 23.2* 2 1.7 1.5 02 0.8* 2 4.6* 2 1.0*3 0.88* 3

H2 S 0.53 0.3 tr. n.d.

tr. =trace; n.d. =not determined. * 1 Samples GT235 Irs 72 and GT237 IB 72, U.S. Geological Survey files. *2 Probably atmospheric contamination. *3 Including argon.

Hot dry rock

57 39

4 tr.

surface groundwater is mixed with the acid condensate, flowin& springs result. Because the condensed water from the bubbling mud pot has a pro­nounced 18 0 shift, considerable rock-water isotopic exchange has occurred at depth, suggesting that the boiling-water table may be hotter than the boiling point at the surface (93°C at 2800 m). Two exploratory wells, Baca Nos. 1 and 3, failed to intersect the boiling-water table at depths of -650 m beneath Sulphur Springs but achieved temperatures in excess of 100°C.

Thermal meteoric waters

This type of water is found in the moat region of the caldera, where it discharges from fractures and from the contacts of rhyolite and underlying volcanic and Paleozoic units (Fig. 3). Some water wells have encountered this type of water at shallow depths in Quaternary alluvium and lacustrine units, filling the southwest section of the moat.

Thermal meteoric water is nothing more than heated near-surface ground­water. It is chemically dilute and of near-neutral pH (Table I). Trace ele­ments such as Li and B are barely enriched above background levels. Iso­topically the water is meteoric (Fig. 4). However, low concentrations of 3 H (Dondanville, 1971) indicate that this water has a relatively longer residence time underground than most near-surface waters of the region.

Deep geothermal and derivative waters

Water of this type was first discovered by deep exploration drilling into the resurgent dome east of Sulphur Springs (i.e. Baca 4, see Fig. 3). Deep geothermal fluids are localized along faults and fractures of the keystone

graben in silici lying volcanic: ploited in the depth is -15( eastern limit c shallow-hole g that the therm

Chemically, solids (TDS) topically, thi.! rock-water i: 2 H suggests t from underlyi

Derivative along the Jen Isotopically, geothermal fl tion diagram! of Na, and B out of theca before it dis1 solve conside Table I) and

To date, 1

hydrotherma deep fluids o

GEOTHERMA

Baca project

To date l central and deep geothE rate vapor-< (1979) has i

A 50-M~ that anothE modeled th charge calc producing exploited a resurgent d

·xpressed

dry rock

springs .>a pro­ccurred 1an the Is, Baca ~650m

vhere it ierlying untered ~ustrine

ground­·ace ele­els. !so­lS of 3 H ~sidence

ing into ). Deep eystone

129

graben in silicified ignimbrite (Dondanville, 1978). Producing zones in under­lying volcanics, Tertiary sediments and Paleozoic formations may be ex­ploited in the future (R. Denton, pers. commun., 1980). Average producing depth is ~1500 m at temperatures of 260°-300°C. Fig. 3 shows that the eastern limit of the deep geothermal system is not clearly known. However, shallow-hole geothermal gradient data acquired by Union Oil Co. indicate that the thermal anomaly does not extend to the eastern caldera wall.

Chemically, the deep fluid is distinct, having ""7000 mgfl total dissolved solids (TDS) and significant concentrations of Na, Cl and B (Table I). Iso­topically, this water is enriched in 18 0 resulting from high-temperature rock-water isotopic exchange (Fig. 4A). However, distinct enrichment in 2 H suggests these waters may contain some connate fluids derived possibly from underlying Paleozoic rocks (White et al., 1973).

Derivative geothermal fluids containing surface meteoric water discharge along the Jemez fault zone at Soda Dam and Jemez Springs (Figs. 2 and 3). Isotopically, the derivative waters fall between meteoric waters and the deep geothermal fluid, Baca 4 on the Craig meteoric line (Fig. 4A). Chloride varia­tion diagrams (Fig. 4B) show that the derivative waters have identical ratios of N a, and B to Cl as the deep fluid. Apparently the deep fluid leaks laterally out of the caldera along the Jemez fault zone and mixes with surface water before it discharges (Trainer, 1975; Goff et al., 1981). The deep fluids dis­solve considerable Paleozoic limestone during flow to Jemez Springs (Fig. 3; Table I) and form copious travertine deposits where they issue.

To date, nothing is known about the age and chemical evolution of the hydrothermal system, or the variation in temperature and rate of leakage of deep fluids out of the caldera.

GEOTHERMAL TARGETS

Baca project

To date Union Oil Co. of New Mexico has drilled 24 deep wells into the central and west portions of the resurgent dome at Valles Caldera to tap the deep geothermal reservoir. Although Union Oil initially thought that a sepa­rate vapor-dominated zone existed at depth in the geothermal field, Grant (1979) has indicated that only a liquid-dominated reservoir exists (Fig. 3).

A 50-MWe1 power plant is now planned, and Union Oil Co. originally felt that another 350 MWe1 could be extracted. Bodvarsson et al. (1980) have modeled the longevity of the Baca reservoir, using recharge, volume and dis­charge calculations, and think that the real capacity may be less. However, producing zones in rocks underlying fractured ignimbrite have not been exploited and the zone underneath Sulphur Springs on the west flank of the resurgent dome is not well explored.

130

Jemez fault zone

The community of Jemez Springs (Fig. 3) drilled a 255m deep explor­ation well in 1979 to try and tap a hot reservoir to use for space heating. This venture encountered the hottest fluid (72°C) at 30m in Paleozoic lime­stone and finally bottomed in Precambrian granite (Goff and Kron, 1980). Available data indicate that the thermal fluids originate from the deep geo­thermal system in Valles Caldera (Goff et al., 1981). Maximum volume is expected to be ~600 1/min. along the Jemez fault zone (Summers, 1976), and hotter reservoirs may be found in the direction of the caldera. Recent resistivity surveys in the area (Pearson and Goff, 1981) verify that these small reservoirs exist at depths of 50 m or less, thus modest ventures to develop space heat appear to have good potential.

Prototype hot dry rock (HDR) system

Robinson et al. (1971) and Smith et al. (1976) proposed that heat could be extracted from HDR by drilling into low-permeability rock to a depth where the temperature is high enough to be useful, creating a reservoir by hydraulic fracturing, and then drilling a second hole to intersect the hydrau­lically fractured region (Fig. 5). Thermal energy would be extracted from this system by injecting cold water down the first hole, forcing the water through the hot fractured system, and then returning the hot water to the surface where the thermal energy would be converted to electrical energy or used for other purposes. System pressures would be maintained so only one phase, (liquid) water, would be present in the reservoir and the drilled holes.

This concept is being investigated at Fenton Hill, New Mexico, (Fig. 2) by the Los Alamos National Laboratory. The location of these experiments on the west lip of Valles Caldera is a favorable one because the thermal gradient in Precambrian granodiorite and granitic gneiss is high (55°C/km) but intense Quaternary faulting and fracturing due to caldera collapse is non-existent. A vast area exists on the west flank of the caldera where this method could be utilized. The first deep borehole, Geothermal Test-2 (GT-2), bottomed in granitic rock at a depth of 2.929 km where the temperature was 197°C. A second borehole, Energy Extraction-1 (EE-l) was drilled towards a hydraulic fracture made by pressurizing the GT-2 well bore in order to complete a cir­culating heat-extraction system (Fig. 5).

Reservoir performance was ·evaluated by two flow tests in which hot water from the production well, GT-2, was directed to the water-to-air heat ex­changer. Then the water was cooled to ~25°C before reinjection. The cooled water, in addition to the make-up water that was required to replace down­hole losses to the rock surrounding the fractured region, was then pumped down the injection well, EE-l, and through the fracture system. Heat was transferred to the water by means of conduction within the nearly imper­meable rock of the fracture, and the heated water was withdrawn by means

ELECTR,

SEOIMEN. VOLC:

CRYSTALL BASEMENl

PROOl: WEL

HYDRAUU FRACTl

0.1 to 5 THICKN

200 1o 1·

Fig. 5. Scher tion and pro posed binar) heat.

of the pro< and circuh 1980). A I

pilot HDR Samples

analyzed c' Table I. G1 for the tw< that most pre-existin1 ture systen in the ran~ corrosion }·

Com par and the ret glance, the fluid; how derived by

.or-ng. :J.e-0). eo-! is 6), :!nt ese to

Jld >th by lU­

>m ter he or ne s. by :ln ·nt tse A be m A

lie rr-

er x­:!d n­~d

as ·r-1S

I I

ELECTRIC GENERATION PL"ANT

CONDENSER

HOT DRY ROCK RESERVOIR CONCEPT

131

Fig. 5. Schematic of Hot Dry Rock Geothermal Energy Reservoir concept showing injec­tion and production wells connected by a vertical hydraulic fracture. Also shown is a pro­posed binary cycle electric generation scheme for producing electricity from low-grade heat.

of the productiori. well. The circulating volume of the reservoir is "'2.8 ·105 I and circulation rate is "'380 1/min. (Tester and Albright, 1979; Murphy, 1980). A 60-KWe1 binary electric generator is presently connected to this pilot HDR system.

Samples of the produced, injected and make-up fluids were collected and analyzed during both tests. Examples of these analyses are presented in Table I. Graphs of Si02 vs. time and Cl vs. time are shown in Fig. 6A and B for the two tests. Modeling of the chemical behavior of the system indicates that most of the chemical effects are due to addition of small amounts of pre-existing (NaCl) pore water to the fluid as it circulates through the frac­ture system. The circulating fluid chemistry is essentially benign - the pH is in the range of 6.5-7.5 and TDS are less than 2500 ppm- and no scaling or corrosion has been observed in the surface piping system.

Comparisons of the natural thermal fluids from the Valles Caldera region and the recirculating fluid at Fenton Hill are shown in Fig. 4A and B. At first glance, the Fenton Hill fluids appear to be related to the deep geothermal fluid; however, the B concentrations in the Fenton Hill fluids cannot be derived by mixing the deep geothermal fluid from the Valles with surface

132

300

24 DAY EXPERIMENT 0 250

E c. c. <{

75 DAY EXPERIMENT (.) :J Ui

50

0 0 10 20 30 40 50 60 70 80

TIME. days

1800

® 1500

~ 1200 c. w· 0 900 a: 0 ....1 24 DAY EXPERIMENT I (.) 600

300

0 10 20 30 40 50 60 70 80

TIME, days

Fig. 6. Comparison of: (A) measured monomeric Si02 ; and (B) measured Cl concentra­tions as a function of time for the first reservoir (75-day experiment) and for the larger second reservoir ( 24 -day experiment).

meteoric water (Fig. 4B). On the other hand, it is clear from Na/Cl and B/Cl ratios that the overlying, natural fluid GT-2a and GT-2b circulating through Paleozoic limestone (Madera Formation) at Fenton Hill is derived from the deep geothermal fluid. Work in progress at Los Alamos will provide further insight into the origin of the chemical variations observed in the Fenton Hill fluids.

For the two experimental tests briefly described here, an average of 3-5 MWth were extracted. Fluid losses were low and there was only minor in­duced seismic activity (Richter magnitude< -1). For further details of the system geometry, operation and results during these tests, consult Tester and Albright (1979) and Murphy (1980). A more detailed presentation of the

fluid che1 underwa~ >4km a commerc

Pajarito I

The R geotherr amos :K gations 2 and 3 fluids fo

Data: seismic 1 used tor Plateau ( ciates, 1~ depth>: and/or:><

No th Pajarito Pajarito caldera i Pajarito (1974) ~ at deptr temper< a deep~

Alth< 2.7 HFl potablt: neath : saline · stone. Pre car

Lo:, target I was sevt Bandelie rift. Higl lems cau has not i

centra­i! larger

j B/Cl rough m the .1rther m Hill

f3-5 or in­:>f the ~rand

Jf the

133

fluid chemistry will be found in Grigsby et al. (in prep.). Drilling is currently underway at Fento~ Hill to construct a 20-MWe1 HDR system at depths of >4 km and temperatures >300°C which will be used to demonstrate the commercial viability of HDR geothermal systems.

Pajarito Plateau

The Rio Grande rift contains deep sediment-filled basins that may have geothermal fluids at depth. In order to investigate this possibility, Los Al­amos National Laboratory conducted geophysical and geochemical investi­gations to evaluate geothermal prospects beneath the Pajarito Plateau (Figs. 2 and 3). Los Alamos National Laboratory is interested in using geothermal fluids for space heating of laboratory facilities.

Data acquired from gravity surveys (Budding, 1978; Cordell, 1978) and seismic lines (Los Alamos National Laboratory, unpub. data, 1979) were used to model the sub-surface structure and stratigraphy beneath the Pajarito Plateau (Fig. 3). A time-domain electric survey (Williston, McNeal & Asso­ciates, 1979) found a 3-7 ll-m resistivity low east of the Pajarito fault at a depth >2000 m. The obvious conclusion was that this low is caused by warm and/or saline fluids within probable Paleozoic limestones at depth.

No thermal springs of any kind are found east of Valles Caldera on the Pajarito Plateau. If a deep thermal aquifer does exist at depth beneath the Pajarito Plateau, it is probably independent of geothermal fluids within the caldera by virtue of groundwater barriers such as ring fracture faults and the Pajarito fault (Fig. 3). Hydrologic investigations by Purtyman and Johansen (1974) show that an extensive potable aquifer underlies the Pajarito Plateau at depths of 600 m and that it slopes gently eastward. This aquifer has a temperature of 3o-35°C and contains barely any geochemical indicators of a deep geothermal component (Goff and Sayer, 1980).

Although the conductive heat flow along the west margin of the rift is 2. 7 HFU, convective effects due to flushing of cool water through overlying potable aquifers is expected to lower the normal temperature gradient be­neath the Pajarito Plateau. A conservative estimate suggests that moderately saline waters exist at a temperature ;<:90°C at a depth of 3-3.5 km in lime­stone. The possibility of drilling through the high-conductivity zone into Precambrian basement to try HDR methods also has been entertained.

Los Alamos National Laboratory's first attempt to reach this geothermal target by means of an alternate energy well (Fig. 3) was a failure. Drilling was severely hampered by lost circulation zones in fractured and jointed Bandelier Tuff and by caving in unconsolidated Tertiary sediments of the rift. High cost.<> for drilling mud and cement made necesssary by these prob­lems caused drilling to stop at a total depth of only 600 m. A second attempt has not been planned.

134

TABLE III

Summary of data for geothermal targets in the Valles Caldera region, New Mexico

Site Depth Temper· R~servoir Use Comments (km) ature rock

(OC)

Baca project Valles Caldera 1-3 26Q-300 ignimbrite* 1 electricity 50 MW el planned generation

Jemez fault zone Jemez Springs .;;:0.25 72 limestone space no plans

Hot dry rock Fenton Hill 3-5 20Q-300 granodiorite electricity 10-20 MWel planned system generation for 2nd loop

Pajarito Plateau Los Alamos 3-~ ;;.go limestone* 2 space no plans

*I Reservoirs in underlying formations not evaluated. ·~Possible hot dry rock reservoir in underlying Precambrian rock.

CONCLUSIONS

Valles Caldera and surrounding areas possess enormous geothermal poten­tial due to: (1) a large Quaternary magma-hydrothermal system; and (2) an active continental rift. Natural thermal waters in the Jemez Mountains dis­play distinct chemical and isotopic characteristics, and geologic settings that can be used as exploration tools for geothermal reservoirs. Four geothermal targets exist in the Valles region (Table III). The Baca project is a conven­tional magma-hydrothermal prospect that may eventually produce 400 MWe1 whereas the Jemez fault zone is a low-temperature hydrothermal prospect of limited volume. Prospects beneath the Pajarito Plateau are still hypothetical. In contrast, the prototype HDR system has the greatest geothermal potential (if the method becomes economical), because the Valles Caldera region con­tains. vastly more heat in near-surface impermeable crystalline rock than in all the hot fluids combined.

ACKNOWLEDGMENTS

This research was funded by U.S.D.O.E., Office of Basic Energy Science and Department of Geothermal Energy. A.W. Laughlin and A.C. Waters reviewed the manuscript. We thank Dale Counce, Los Alamos National Laboratory, for chemical analyses of thermal waters, and L. Merlivat, Saclay, France, for isotopic analyses. John Paskiewicz and Andrea Kron drafted the figures and Barbara Hahn prepared the manuscript.

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