Fault-Dominated Geothermal Reservoirs

Earl D. James

Geothermal fields in three geologic provinces of the western United States are being developed utilizing major fault systems as the primary reservoirs: Heber, California, in the Salton Trough rift basin; Beowawe, Nevada, in the northern Basin and Range; and Roosevelt Hot Springs, Utah, at the eastern boundary of the Basin and Range. Geologic definition of the fault system controlling the thermal water plume was complicated in each case by lateral flow of thermal waters in shallow aquifers and by skewing of the rising thermal water plume by regional meteoric water movement. Spacing of development wells in faults is determined by permeability as measured during well testing. Parallel strands of faults within a zone may not be in good pressure communication even though tied o the same ultimate source of geothermal fluids at depth. The final size of development is determined by the area of the fault that is permeable and carrying waters of commercial temperatures.

The key elements of the Heber geothermal field are: impermeable capping clays at 1,000 to 2,000 ft, a shallow matrix permeability reservoir in deltaic sandstones from 2,000 to 4,500 ft, and a major north-northeast-trending normal fault carrying thermal waters through highly indurated sediments below 4,500 ft. The normal fault controlling the system is a tensional structure formed in an area subject to dextral shear bounded by northwest-trending strike-slip faults. Thermal waters rise at temperatures greater than 390°F (199°C) from south to north in the normal fault. Outflow into permeable sandstones above depths of 4,500 ft is spread to the north-northwest by regional ground-water flow. These structural and hydrodynamic features were defined by dipmeter data, core analysis, porosity logs, temperature data, lost circulation zones encountered during drilling, and production zones identified on spinner surveys. A seismic line across the southern end of the field is particularly useful in identify ng the major features of the geothermal system.

The Beowawe geothermal area is being developed in the east-northeast-striking Malpais fault system which bounds the Whirlwind Valley and is the production and injection zone. Thermal waters rise from west to east in the Malpais system, tilted by regional meteoric water movement. The maximum temperature encountered to date is 420°F (216°C). Two production wells are completed in separate faults below 8,000 ft near the source region of the rising geothermal waters to the west in the fault system. The wells communicate strongly along strike to the east with shallow wells (2,000 ft) completed beneath the geothermal surface manifestations on the sinter terrace. The production wells do not communicate with each other across strike of the fault system and are apparently completed in isolated faults.

The Roosevelt Hot Springs geothermal system is one of the hottest in the western United States with maximum temperatures of 520°F (271°C). Roosevelt is near the eastern edge of the Basin and Range province and is associated with rhyolitic volcanism less than 500,000 years old. Fractures in Tertiary granitic and Precambrian metamorphic rocks provide the permeability for production. Production is found at shallow depths near the intersection of major north-northeast and east-west-trending fault systems. The shallow depths to production may be due to active movement on the faults. Ongoing subsidence at rates up to several inches per year and low seismicity are interesting features of the area.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.