SEARCHING FOR AN ELECTRICAL-GRADE GEOTHERMAL …jan.ucc.nau.edu/~pm8/PM_dwnld/Searching_N_AZ_GeoT... · and are thus likely to further DOE’s goal relatively easily. Other states,

Accepted for Presentation at the Geothermal Resources Council Annual Meeting in Morelia, Mexico, October 12-15, 2003, and Publication in Volume 27 of the GRC Transactions

SEARCHING FOR AN ELECTRICAL-GRADE GEOTHERMAL RESOURCE IN

NORTHERN ARIZONA TO HELP GEOPOWER THE WEST

Paul Morgan, Wendell Duffield, John Sass, and Tracey Felger Department of Geology, Northern Arizona University,

Flagstaff, Arizona, 86011-4099, USA

ABSTRACT The U.S Department of Energy’s “Geopowering the West” initiative seeks to double the number of states (currently 4) that generate geothermal electric power over the next few years. Some states, like New Mexico and Oregon, have plentiful and conspicuous geothermal manifestations, and are thus likely to further DOE’s goal relatively easily. Other states, including Arizona, demonstrate less geothermal potential, but nevertheless have sites worthy of additional investigation. The Arizona site with greatest potential is near Sunset Crater, a basaltic volcano less than 1,000 years old several kilometers northeast of Flagstaff. Several silicic volcanoes nearby are young enough to have still-hot intrusive roots. Moreover, there is geophysical evidence for magma or hot rock in middle to lower crust beneath the area. A high level of interest in this area by the geothermal industry during the 1970s waned because surface thermal indicators, such as high heat flow, hot springs, and fumaroles are absent. The absence of these features is probably the result of a well-documented deep and pervasive regional aquifer, which likely creates a thermal barrier to the rise of hydrothermal activity from depth. Surface based geological and geophysical studies are under way to evaluate further the geothermal potential of the area and to locate a site for a drill hole to explore for a hydrothermal system beneath the ground-water barrier. INTRODUCTION Most geothermal resources capable of powering turbine generators are located in areas of Quaternary volcanism. The fundamental reason behind this common association is simple. Magma that ponds within the crust transiently heats surrounding water-saturated permeable rock to temperatures sufficient for exploitation to generate electricity. Silicic (dacite and rhyolite) volcanic rocks are derived from basaltic parental magma (whether by partial melting of silicic crust resulting from protracted contact with basaltic magma, fractional crystallization of basaltic magma, etc.). Such derivation occurs in or around a crustal magma reservoir(s?). Thus, young silicic volcanism is a first-order guide in exploring for areas of promising geothermal-energy potential of electrical grade. Other factors being equal, the younger, more voluminous and less aerially dispersed such volcanism is, the greater the chance for discovering a developable geothermal resource in the root zone of the silicic vents.

Morgan, Duffield, Sass and Felger

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Typically, areas of “young-and-hot” silicic volcanism also include one or more thermal manifestations, such as hot springs, fumaroles, mud pots, and anomalously high heat flow. Taken together, various combinations of these indicators provide compelling evidence of high potential for an underlying resource of commercial grade. When temperatures calculated from fluid geothermometry are sufficient to support development by binary, flash, or direct-vapor technologies, exploratory drilling is generally initiated to test the resource. However, when thermal manifestations are absent and heat flow is “normal” or lower, the presence of voluminous, youthful silicic volcanic rocks alone is viewed with less certainty about resource potential. This is precisely the situation for the San Francisco Volcanic Field (SFVF) in northern Arizona. We hope to bring this area into the exploratory drilling stage to add a new geothermal resource to the inventory of proven resources in the western U.S. GEOLOGIC SETTING: The SFVF ranges from Pliocene to Holocene in age (Wolfe, 1990). The field is located near the southern margin of the Colorado Plateau, and is built atop a roughly kilometer-thick horizontal sequence of Paleozoic and minor Mesozoic sedimentary rocks that cap Precambrian basement. The volume of volcanic rocks is about five hundred cubic kilometers, distributed over an area of roughly five thousand square kilometers. Rock composition spans the spectrum from basalt to rhyolite, but is dominantly basaltic. The volcanic field defines an east- to northeast-trending belt about one hundred kilometers long and thirty-to-forty kilometers wide (Figure 1). Most of the oldest rocks (6 Ma to 5 Ma) were erupted from vents within the western third of this belt; a centroid of volcanism subsequently migrated east northeastward at an average rate of about two centimeters per year (Tanaka et al., 1986). The most recent eruption created Sunset Crater basaltic cinder cone, two associated lava flows, and a surrounding blanket of fallout cinders, all about 1,000 years ago (Self, 1990), within the eastern third of the volcanic field. Native Americans watched this eruption, presumably in awe, as cinder fallout buried their pit houses and agricultural fields (Elson et al., 2002). From west to east (older to younger), silicic volcanism is concentrated at four principal centers (Bill Williams Mountain, Sitgreaves Mountain, Kendrick Peak, and San Francisco Mountain). A fifth silicic east-northeast trending zone extends eastward from San Francisco Mountain (Figure 2). We have selected this youngest part, a roughly nine by four kilometer finger of silicic-volcanic-rich real estate, as the target for our studies (Figure 2). With the exception of a forty-acre patented claim near the center of this target, the area is within the Coconino National Forest. GEOTHERMAL INDICATORS Geology: About 40% of the target area is underlain by rhyolite and dacite lava domes and flows, whose vents are also within the area. Additional young silicic lavas and their vents are located just outside the target area, to the north and west. Basaltic and andesitic cinder cones and their associated lava flows underlie most of the rest of the area. Erosional remnants of fallout cinders


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from the 1000-year-old eruption of nearby Sunset Crater volcano mask some of the bedrock geology. As part of a national assessment of geothermal resources completed during the 1970s by the U.S. Geological Survey, Smith and Shaw (1979) evaluated many Quaternary volcanic fields. They developed a geologic model wherein for every volume of silicic magma that erupts, ten volumes remain lodged in the crust. On the basis of this model, and with knowledge of the numerical age and approximate temperature of the most recent silicic eruption, Smith and Shaw calculated the present-day temperature and volume (and therefore content of thermal energy) of that crustal magma (or pluton, if sufficient time had elapsed for solidification) with reference to a spectrum of conductive and convective cooling scenarios. Their evaluation of the eastern (youngest) silicic part of the SFVF suggests a geothermal potential similar to other young fields such as those at Long Valley and Coso in California, and at the Valles Caldera of New Mexico (Figure 3).

Figure 3. Log age versus log volume for selected volcanic/magmatic systems in the western

United States (modified from Smith and Shaw, 1979). Thus, in terms of the amount and age of silicic volcanic rocks, our target area is quite favorable in terms of a Smith/Shaw analysis. Also of note is the fact that the SFVF is the only area of


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Quaternary volcanism in Arizona that includes silicic rocks (Smith and Shaw, 1979; Duffield et al., 2000). Because very young volcanic rocks blanket the target area, no faults are visible at the surface. Nonetheless, the area is strongly imprinted with an east-northeast trending structural fabric. For example, about a dozen vents are strung out along the axis of the area and immediately adjacent terrain. Moreover, immediately to the west, an elongate basaltic accumulation of cinders deposited along an eruptive fissure aligns with the axis of the target area. The first-order fabric of the map of Bouguer gravity also parallels the axis of the area. Obvious alignments of northwest-elongate vents (including Sunset Crater) project into the target area from the southeast, strongly suggesting zones of fault intersections that potentially host permeability that could be targeted for drilling. Hydrology: The SFVF ranges in elevation from about 5,000 feet to 12,630 feet. Average annual precipitation over the volcanic field and surrounding terrain ranges from about 25 inches to less than 10 inches. There are no perennial streams in this area. Deep canyons bound the volcanic field and the extensive horizontal sedimentary platform on which it is built. The Grand Canyon and the canyon of the Little Colorado River lie to the north, while the valley of the Verde River lies to the south. One result of this physiographic setting in an arid climate is that the regional water table beneath the SFVF is deep. Wells that tap this regional aquifer to supply the city of Flagstaff with potable water encounter the water table at about 800 feet deep at the shallowest to more than 2,000 feet at the deepest. Contoured elevations of the surface of the water table define a roughly southeast-northwest axis just south of Flagstaff (Sass and others, 1994; Bills and others, 2000) from which groundwater flows north and south, feeding springs that emerge in the walls of the bounding canyons. Some of these springs are thermal, which we interpret to reflect heat from a magma/hot-pluton body beneath the SFVF being swept laterally with the regional flow of groundwater. Slightly anomalous chemistry of water pumped from Flagstaff’s wells is consistent with a model of deep thermal water mixing with and being swept laterally by flow in the groundwater system. In particular, silica is present in concentrations higher than typical of non-thermal groundwater. On the assumption that such elevated silica concentrations do indeed result from a component of deep thermal water, Taylor (1997) calculated silica and magnesium-corrected sodium-potassium-calcium geothermometer temperatures and discovered that highest temperatures coincide with eastern part of the SFVF. Moreover, Taylor’s calculations define a thermal plume that extends northward from our target area, as would be expected if a subjacent heat source is pumping thermal energy (and fluid?) into the northward-moving part of the regional groundwater system. Using a technique developed to estimate heat flow from groundwater silica concentrations, Swanberg and Morgan (1979, 1985) concluded that the average heat flow within the eastern (youngest) part of the SFVF is about 75 mW/m2, nearly twice the value determined in the conventional manner from rock samples and temperature profiles in drill holes (Sass et al., 1994).


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Heat Flow: Viewed in the context of the southwestern United States (Figure 4), heat flow within the SFVF is anomalously low, despite being the site of very young volcanism. Most of the terrain comprising the Great Basin, the Basin and Range and the Colorado Plateau margin is characterized by moderately high to high heat flow. Of special note, the Eureka heat-flow low of Nevada (Figure 4) has been interpreted to be the result of regional inter-basin ground-water flow that mines subsurface heat “upstream” and delivers it to the surface “downstream” at hot springs and as evapo-transpiration (Sass and others, 1971).

Figure 4. Heat flow map of the southwestern United States. Flg = Flagstaff; SBR = Southern

Basin and Range; CP = Colorado Plateau; EL = Ely Low; SRP = Snake Rive Plain; CR = Cascade Range.

When values of heat flow and contours of the ground-water table are mapped together (Figure 5), a similar hydrologic explanation for the low heat flow in the SFVF is suggested. Ground water


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levels are highest just south of Flagstaff, and drop off steeply to the north, beneath the SFVF, including our target area. Thus, ground water flowing northward and downward sweeps much of the heat (that would be otherwise be conducted vertically to the surface) laterally to be discharged at springs and seeps along the canyon of the Little Colorado River, and/or to be dissipated via evapo-transpiration in low-lying areas.

Figure 5. Heat flow and water-level elevation contours for the eastern San Francisco volcanic

field and adjacent areas (from Sass and others, 1994). The 24 mW m-2 heat flow value immediately east of the San Francisco Peaks is from the well SCC-1 discussed in the text and Figure 7.


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During heat-flow surveys, conductive temperature gradients measured in wells commonly are extrapolated upward to estimate a ground-surface temperature (To) undisturbed by diurnal weather cycles and longer-term patterns of climate change. These To temperatures are then plotted against well-head elevation to determine what is called the atmospheric lapse rate, which is an indicator of the effect of elevation on temperature. Worldwide, lapse rates are in the range of 4 to 6 °C/km. Sass and others (1982) have shown that the lapse rate for the SFVF is nearly 12 °C/km (Figure 6), far greater than the worldwide value. One implication of the SFVF lapse rate is that temperature at sea level and deeper, beneath the volcanic field, is higher than would be expected from a simple downward extrapolation of conductive gradients. This implication, in turn, suggests that the near-surface thermal regime within the SFVF is detached from the deep regime. Lateral ground water flow in the regional aquifer is a logical candidate to explain such detachment.

Figure 6. Extrapolated surface temperature versus elevation of wellhead for twelve wells in the

eastern part of the San Francisco volcanic field. The equation is for the best fit to the data, T temperature and z elevation.

There is only one observation well near our target area. It (well SSC-1) was drilled to provide water to the Sunset Crater Volcano National Monument offices and visitor center. The water table was encountered at about 2,000 feet below the surface. Productivity of the well is so low that it was abandoned years ago. The temperature profile of well SSC-1 (Figure 7) is typical of others that have been measured in the SFVF. The average thermal gradient is low, and there is no sharp temperature discontinuity at the water table. The thermal gradient below the water table is somewhat lower than that above,


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and it is slightly concave-upwards near the bottom of the well, indicating downward water flow. The mean heat flow from SSC-1 is 24 mWm-2, hardly indicative of a geothermal resource under ordinary circumstances.

Figure 7. Temperature profile for well SSC-1, near Sunset Crater. Crustal Structure: Additional evidence for residual heat in the crust beneath the eastern portion of the SFVF has been derived from a variety of geophysical data. Data from distant earthquakes recorded on a northwest-southeast line of seismographs across San Francisco Mountain indicated significant delays for ray-paths traveling through the crust beneath the area, which Stauber (1982) interpreted to indicate a low-velocity body in the crust approximately 6 km wide at depths between 9 and 34 km below sea level. The decrease in velocity in this body was more than 6%with respect to the surrounding rocks, and this result is consistent with partially molten and/or anomalously hot rock in the mid to lower crust. A more recent, shallow seismic study detected what has been interpreted to be the upward extension of this hot body into the upper crust (Durrani et al., 1999). This experiment used explosive sources shooting side shots (“fan shooting”) into linear arrays of receivers and the primary phase recorded was the upper crustal arrival (Pg) so that the study was sampling the


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upper 5 km. The seismic-velocity image obtained indicated a high-velocity zone at upper crustal depths beneath the San Francisco Mountain area, with P-wave velocities 6% higher than the surrounding basement rocks (Durrani et al., 1999). This high-velocity zone appears to lie immediately above the low-velocity middle and lower crustal body of Stauber (1982). Durrani et al. (1999) suggest that they could be the same body, the inversion in velocity contrast perhaps resulting from cooling upward within the body and compositional changes in the host rock. At depth the body is hotter, has a higher percentage of partial melt, and is more silicic than its host rock, resulting in a negative velocity contrast. At shallow depths the body is not so hot, has little or no partial melt, and is more mafic than its most rock, resulting in a positive velocity contrast. Unfortunately, the ray paths in this study are just outside our study area. Mickus and Durrani (1996) carried out gravity and magnetic studies of the SFVF. Their data are interpreted to be basically consistent with both the shallow and deep crustal seismic studies, but also show additional upper to mid crustal bodies (Mickus and Durrani, 1996). We are currently reworking these data and collecting more closely spaced gravity data. A preliminary result of a new terrain-correction analysis of the existing gravity data is that there is a strong trend in the gravity anomalies parallel to the alignment of young silicic volcanism, suggesting that there may be subsurface structures and plutonic bodies associated with the volcanism. Relatively old geomagnetic variation studies also detected parallel telluric current systems at depths shallower than 10 km, consistent with thermal phenomena (Towle, 1984) CONCLUSIONS: Reevaluation of earlier studies and consideration of our newly gathered data leave us in the position of having several indications that a geothermal resource exists beneath the study area of young silicic volcanic rocks, but of not being able to estimate directly subsurface temperatures from the compositions of hot springs. Such geothermometry is common in most volcano-related resource areas and is very useful in making the tough decision of whether to invest in deep drilling. Nonetheless, in view of the several encouraging indicators of a resource, we believe that the potential for positive payoff justifies the level of risk involved with drilling through the regional aquifer into what may well be a substantial hydrothermal system in fractured Precambrian basement rocks. It appears that such deep drilling is the only way to answer the question that has tantalized geothermal scientists and developers for the past several decades. This area of youngest silicic volcanism in the SFVF holds the most promise for adding Arizona to the list of states that can contribute to GeoPowering the West. REFERENCES: Bills, D.J., M.Truini, M. E. Flynn, H. A. Pierce, R. D. Catchings, and M. J. Rymer, 2000.

“Hydrogeology of the Regional Aquifer fear Flagstaff, Arizona, 1994-97.” U.S. Geological Survey Water-Resources Investigations Report 00-4122, 143 pp.


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Duffield, W., P. Morgan, and J. H. Sass, 2000. “Untapped Potential? The San Francisco Volcanic Field, Arizona.” Geothermal Resources Council Bulletin, v. 29, no. 3, p. 97-99.

Durrani, B. A., D. I. Doser, G. R. Keller, T. M. and Hearn, 1999. “Velocity structure of the upper

crust under the San Francisco volcanic field, Arizona.” Bull. Seism. Soc. Am., v. 89, p. 239-249.

Elson, M.D., M. H. Ort, J. Hesse, and W. A. Duffield, W.A., 2002. “Lava, Corn, and Ritual in

the Northern Southwest.” American Antiquity, v. 67, no. 1, p. 119-135. Mickus, K. L., and B. Durrani, 1996. “Gravity and magnetic study of the crustal structure of the

San Francisco volcanic field, Arizona, USA.” Tectonophysics, v. 267, p. 73-90. Sass, J. H., A. H. Lachenbruch, S. P. Galanis, Jr., P. Morgan, S. S. Priest, T. H. Moses, Jr., and

R. J. Munroe, 1994. “Thermal Regime of the Southern Basin and Range Province: 1. Heat Flow Data from Arizona and the Mojave Desert of California and Nevada.” Jour. of Geophys. Res., v.99, p. 22,093-22,119.

Sass, J. H., C. Stone, and D. J. Bills, 1982. “Shallow Subsurface Temperatures and some

Estimates of Heat Flow from the Colorado Plateau of northeastern Arizona.” U.S. Geological Survey Open File Report, 82-994, 112 pp.

Sass, J.H., A. H. Lachenbruch, R. J. Munroe, G. W. Greene, T. H. and Moses, Jr., l97l. “Heat

flow in the western United States.” Jour. Geophys. Res., v. 76, p. 6376-64l3. Self, S., 1990. “Sunset Crater, Arizona.” p. 280-281 in Volcanoes of North America, edited by

Charles A. Wood and Jurgen Kienle, Cambridge University Press, New York, 354 pp. Smith, R.L., and H. R. Shaw, 1979. “Igneous-Related Geothermal Systems.” P. 12-17 in

Assessment of Geothermal Resources of the United States – 1978, edited by L.J.P. Muffler, U.S. Geological Survey Circular 790, 163 pp.

Stauber, D. A., 1982. “Two-dimensional compressional wave velocity structure under San

Francisco volcanic field, Arizona, from teleseismic P residual measurements.” Jour Geophys. Res., v. 87, p. 5451-5459.

Swanberg, C. A., and P. Morgan, 1979. “The linear relation between temperatures based on the

silica concentration of groundwater and regional heat flow: a new heat flow map of the United States.” Pure Appl. Geophys., v. 117, p. 227-241.

Swanberg, C. A., and P. Morgan, 1985. “Silica heat flow estimates and heat flow in the Colorado

Plateau and adjacent areas.” Jour. Geodynamics, v. 3, p. 65-85. Tanaka, K.L., E. M. Shoemaker, G. E. Ulrich, and E. W. Wolfe, 1986. “Migration of Volcanism

in the San Francisco Volcanic Field, Arizona.” Geol. Soc. Am. Bull, v. 97, p. 129-141.


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Taylor, J.A., 1997. “Hydrothermal Regimes of the San Francisco Peaks Volcanic Field and the Southern Verde Valley, North-Central Arizona.” M.S. Thesis, Northern Arizona University, Flagstaff, 95 pp.

Towle, J. N., 1984,. “The anomalous geomagnetic variation field and geoelectric structure

associated with the Mesa Butte fault system, Arizona.” Geol. Soc. Am. Bull., v. 95, p. 221-225.

Wolfe, E.W., 1990. “San Francisco, Arizona.” P. 278-280 in Volcanoes of North America, edited

by Charles A. Wood and Jurgen Kienle, Cambridge University Press, New York, 354 pp.

Documents

SEARCHING FOR AN ELECTRICAL-GRADE GEOTHERMAL …jan.ucc.nau.edu/~pm8/PM_dwnld/Searching_N_AZ_GeoT... · and are thus likely to further DOE’s goal relatively easily. Other states,