U.S. Geological Survey

Hydrogeologic appraisal of the Klamath Falls geothermal area, Oregon

By by E.A. Sammel

USGS Professional Paper 1044-G (published 1980)

Abstract

Geothermal phenomena observed in the vicinity of Klamath Falls include hot springs with temperatures that approach 96°C (the approximate boiling temperature for the altitude), steam and water wells with temperatures that exceed 130°C, and hundreds of warm-water wells with temperatures ranging from 20° to 40°C. Although warm water is produced from wells scattered throughout most of the 350-square-mile area studied, waters with temperatures exceeding 60°C are confined to three relatively restricted areas, the northeast part of the City of Klamath Falls, Olene Gap, and the southwest flank of the Klamath Hills.

The hot waters are located near, and are presumably related to, major fault and fracture zones of the Basin and Range type. The displaced crustal blocks are composed of basaltic flow rocks and pyroclastics of Miocene to Pleistocene age, and of sedimentary deposits and basalt flows of the Yonna Formation of Pliocene age. Dip-slip movement along the high-angle faults may have been as much as 6,000 feet at places.

Average annual precipitation in 7,300 square miles of the upper Klamath River basin surrounding the study area is about 18.2 inches, of which between 12 and 14 inches is estimated to be lost through evapotranspiration.

Ground water of local meteoric origin moves through the shallow sedimentary deposits and volcanic rocks at relatively slow rates. Within the older basaltic rocks of the area, hydraulic conductivities are greater than in the shallow sediments, and ground water may move relatively freely parallel to the northwest-southeast structural trend. A small amount of ground water, perhaps 100,000 acre-feet per year, leaves the area in flow toward the southwest, but much of the ground water is discharged as evapotranspiration within the basin.

The local meteoric water that is assumed to be the source of the thermal water in the area has low concentrations of dissolved-solids in which calcium and bicarbonate are the dominant ions. During its passage through the geothermal reservoir, concentrations of dissolved solids increase to about 900 milligrams per liter, and sodium and sulfate become the dominant ions. Chloride concentrations remain relatively low, and silica concentrations increase from an average of about 35 milligrams per liter to about 100 milligrams per liter.

The evidence from cation ratios, silica concentrations, and oxygen and deuterium isotopes in the hot waters indicates that temperatures in the near-surface geothermal reservoir are relatively low. The estimated minimum reservoir temperature, based on the quartz geothermometer and mixing models, is 150°C. In contrast to these indications, the sulfate-oxygen isotope geothermometer indicates that, at some time in their history, the thermal waters have been exposed to temperatures approaching 200°C, possibly in a deeper reservoir.

Temperature distributions and heat flows in the shallow rocks of the area are strongly influenced by convective flow of water. Most observed temperature gradients are unreliable indicators of depths to the geothermal reservoir. Heat flow in the vicinity of the geothermal areas has not been determined, but evidence from temperature profiles suggests that heat flow in the Lower Klamath Lake basin is about 1.4 microcalories per square centimeter per second (1.4 Heat flow units), a value that is near the minimum expected for the Basin and Range province.

A conceptual model of the geothermal system at Klamath Falls suggests that most of the observed phenomena result from transport of heat in a convective hot-water system closely related to the regional fault system. Temperatures at shallow depths are elevated both by convective transport and by the blanketing effect of rocks of low thermal conductivity. Circulation of meteoric water to a depth of 15,000 feet could account for the estimated temperatures in the thermal reservoir, assuming conductive heat flow and a temperature gradient of 30°C per kilometer in the shallow crustal rocks. Circulation to shallower depths may be sufficient to warm the water to the required temperatures under the more probable conditions of convective transport of heat and the insulating effect of overlying sediments.

Heat content in the shallow hot-water system (< 10,000 feet depth) is probably in the range 15 x 1018 to 190 x 1018 joules, but is likely to be in the lower part of this range. This thermal energy may be stored in two or more separate reservoirs having a total volume not much greater than 12 cubic miles

This report is out of print. It may be inspected at:

U.S. Geological Survey, 10615 S.E. Cherry Blossom Drive, Portland, OR 97216 (ph: 503-251-3201, e-mail info-or@usgs.gov),

and is also available at many large libraries.