Ehni Enterprises, Inc. Geologic Consulting, Prospecting & Evaluation P. O. Box 4228, Carson City, Nevada 89702 • (775) 883-1107 FAX (775) 883-7279 email: [email protected]Geologic Evaluation of the Geothermal Potential of Wabuska Hot Springs Lyon County, Nevada By: Wm. J. Ehni Geologist November 24, 2008
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Ehni Enterprises, Inc. Geologic Consulting, Prospecting & Evaluation P. O. Box 4228, Carson City, Nevada 89702 • (775) 883-1107 FAX (775) 883-7279 email: [email protected]
Geologic Evaluation of the Geothermal Potential of Wabuska Hot Springs Lyon County, Nevada
By:
Wm. J. Ehni Geologist
November 24, 2008
McCabe/Fleeman, Wabuska Hot Springs Page 2 of 13
Ehni Enterprises, Inc. P. O. Box 4228 Carson City, NV 89702-4228 November 2008
Scope and Purpose
The purpose of this report is to provide a cursory geologic evaluation of the geothermal potential
of acreage located near Wabuska Hot Springs in sections 21 and 22 of Township 15 North Range
25 East (MDB&M), Lyon County, Nevada, with an emphasis on the McCabe/Fleeman land as
outlined on Figure 1. Information used to evaluate the prospect consisted of exiting public
domain data. A partial list of data and reports used for this process is included at the end of the
report. It is not within the scope of this report to provide a detailed explanation of the
commercial uses of geothermal resources or to provide a detailed discussion of the techniques
used for geothermal development. The user of this report should be casually familiar with the
project area and have a layman’s concept of geothermal systems.
McCabe/Fleeman, Wabuska Hot Springs Page 3 of 13
Ehni Enterprises, Inc. P. O. Box 4228 Carson City, NV 89702-4228 November 2008
Figure 1: Location map of Wabuska Hot Springs and the McCabe/Fleeman parcel in
sections 21 and 22 of T15N R25E; Lyon County, Nevada.
McCabe/Fleeman, Wabuska Hot Springs Page 4 of 13
Ehni Enterprises, Inc. P. O. Box 4228 Carson City, NV 89702-4228 November 2008
Background
Wabuska Hot Springs is located in Lyon County, Nevada. A power plant using geothermal brine
produced from two wells was built in 1984 and expanded in 1987 to its current configuration.
The Wabuska power plant (currently operated by Homestretch Geothermal LLC) has the
distinction of being the first geothermal power plant to be built in Nevada. In addition, the inlet
temperature and production rates of the wells have not declined over this period of time,
something that has plagued numerous other geothermal power plants in Nevada. However,
naturally occurring hot springs in the area have dried up indicating inadequate recharge to the
system. According to information available from the Nevada Division of Minerals
http://minerals.state.nv.us/ogg_nvgeorespro.htm the plant is currently generating approximately
0.7 net megawatts (6221 MWH in 2007). The inlet temperature is approximately 220 degrees
Fahrenheit. The binary electrical generators are supplied by hot water from two production
wells, PW1 and PW2. PW1 is 488’ deep, had an Initial Production (IP) rate of 700 gallons per
minute (gpm). PW2 had an Initial Production rate of 800 gpm and is 500’ deep. The combined
IP of the two wells is 1500gpm which equates to 2.16 million gallons per day. All produced
fluids are currently discharged on the surface. Current discharge rates are reported to be 1.9
million gallons per day. Prior to the startup of the Wabuska power plant, surface discharge from
natural hot springs in the area was approximately 50 gallons per minute (72,000 gallons per day).
Geology
A northeast trending fault running from Wabuska Hot Springs to Kribs Hot Spring (see Figure 2)
is the controlling structure for the Wabuska Geothermal System. The Wabuska Fault, as
identified in Figures 2 and 3, parallels the “Wabuska Lineament”. This fault is part of the
McCabe/Fleeman, Wabuska Hot Springs Page 5 of 13
Ehni Enterprises, Inc. P. O. Box 4228 Carson City, NV 89702-4228 November 2008
extensional tectonic setting associated with the northwest striking Wabuska Lineament and the
right lateral faulting associated with the Walker Lane. In 1988, Stewart recognized the northeast
trending “Wabuska Lineament” and other researchers (Trexler and others, GSA 2000) have
noted the relationship between the Wabuska Lineament and the right lateral Walker Lane fault
system.
The existing geologic model for the Wabuska geothermal system focuses on this east dipping
high angel normal fault as providing increased permeability and the conduit for hot water to
migrate from deeper higher temperatures as depicted in Figure 2 and on cross section A-A’ in
Figure 3. The existing production by Homestretch from production wells PW1 and PW2 is from
the footwall side of the fault as interpreted in this report. Production temperatures of PW1 and
PW2 are only slightly higher than the original surface temperatures of the hot springs as reported
by Mariner in 1974. Wabuska Hot Springs subsequently “dried” up after production from this
shallow outflow zone from PW1 and PW2 was initiated. Accurate historical groundwater table
elevation data is not available for the area; however, some wells that used to be artesian no
longer flow and therefore it appears that the water table is dropping due to production of
geothermal fluid in the absence of any recharge.
The existing production wells produce from a shallow outflow zone on the northwest side
(footwall) of the Wabuska fault zone. Using the east dipping geologic model for this geothermal
system, and assuming that the fault zone will be the best production zone, the optimum drill sites
should be in the east (hanging wall) side of the fault. Higher temperatures will be encountered at
shallower depths on the east side of the fault also. The geochemistry of the historical Wabuska
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Ehni Enterprises, Inc. P. O. Box 4228 Carson City, NV 89702-4228 November 2008
Hot Spring suggests that these are “mature” unmixed waters, supporting the interpretation that
springs emanate from this fault.
Figure 2: Contoured temperature gradient map of the Wabuska Geothermal System
McCabe/Fleeman, Wabuska Hot Springs Page 7 of 13
Ehni Enterprises, Inc. P. O. Box 4228 Carson City, NV 89702-4228 November 2008
In 1988, the operator of the power plant attempted to locate a recharge zone. Although it is not
clear in the existing documents, this effort probably coincided with decreased flow of surface hot
springs, and possibly the cessation of artesian flow from some wells. The attempt to identify a
useable injection zone to recharge the system was limited to a very small portion of the resource.
As a result of limiting the areal extent of the investigation, the operator was unable to find a
satisfactory injection zone and continued with surface discharge.