Results of Exploration Drilling in the East Branch of the Harlem Hot Springs KGRA, California

Abstract

Geothermal waters heated during circulation ascend along fault zones in the southern range front of the San Bernardino Mountains of California. The Harlem Hot Springs Geothermal Area contains mixtures of low temperature geothermal and cooler groundwaters in sedimentary aquifers adjacent to the range front faults. Temperature logs and geochemical data were used to define a drilling target and site a geothermal exploration well on the inferred trace of the F2 fault zone in the east branch of this KGRA. An exploration well (PGT1) drilled in early 1990 penetrated 287 m of sediments and 91 m into the basement complex. The highest temperature measured in the PGT1 is 46°C in aquifers at 200-220 m and all waters deeper than 170 m are >44°C. The F2 fault is believed to be 75-200 m north of the PGT1 based on the distribution and temperatures of geothermal aquifers. Geochemical evidence supports the conclusion that at <350m deep geothermal well drilled directly into or just down gradient of the F2 fault zone will produce 50-60°C waters from sedimentary aquifers and >65°C waters from shear zones in the crystalline basement complex.