This research is a continuation of the assessment of low-temperature geothermal resources of Cache Valley, Utah, initiated by the Utah Geological and Mineral Survey. The study area is the southern part of a narrow, north-trending graben located in north-central Utah and southern Idaho.

Tools used for evaluating the low-temperature geothermal resources are ground-water temperatures, thermal gradients, silica and Na-K-Ca geothermometers, and mixing models for estimating reservoir temperatures.

Ground-water temperatures range from 9.9 to 50.0°C, with a background temperature of about 13.0°C. The three areas in the study area with anomalous water temperatures are: (1) North Logan, 16.0 to 25.1°C; (2) Benson, 13.5 to 23.0°C; and (3) Trenton, which has evidence for past and present warm spring activity, with temperatures ranging from 22.9 to 50.1°C.

Thermal gradients generally range from 15° to 52°C/km, approximately the same as the average thermal gradient for the Basin and Range province, 35°C/km.

The silica and Na-K-Ca geothermometers, when applied to the ground-water of Cache Valley, Utah, show varying degrees of agreement, with estimated reservoir temperatures averaging 50 to 100°C. When the mixing models are applied to the water in the Trenton area, estimated reservoir temperatures are 30 to 200°C.

The function, F (T) = log (Na/K) + B log (<rad>Ca/K</rad>), is used as an exploration tool to indicate a possible geothermal anomaly when the value is less than 2.00. The Trenton area is a possible geothermal anomaly located using this method.

Warm water in Cache Valley, Utah, appears to be the result of groundwater which has migrated from depth, is warmed by the normal thermal gradient, rises quickly along permeable fault zones, and either mixes with near surface recharge water or is forced to flow horizontally because of a less permeable confining layer. Both of these models mask the higher temperatures at depth.

Considering measured surface temperatures, calculated reservoir temperatures, thermal gradients, and the local geology, most of the Cache Valley, Utah, area is unsuitable for geothermal development. However, the areas of North Logan, Benson, and Trenton have anomalously warm ground water in comparison to the background temperature of 13.0°C for the study area. The warm water has potential for isolated energy development but is not warm enough for major commercial development.

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