Preliminary Assessment of the Structural Controls of Neal Hot Springs Geothermal Field, Malhuer County, Oregon

Abstract

The Neal Hot Springs geothermal field is marked by hot springs that effuse from opaline sinter mounds just north of Bully Creek, in Malheur County, Oregon. Production wells have high flow rates and temperatures above 138C at depths of 850-915 m. On a regional scale, the geothermal field occupies a broad zone within the intersection between a regional, N-striking, normal fault system within the Oregon-Idaho graben and a regional NW-striking, normal fault system within the western Snake River Plain (Vale fault zone). Locally, the system appears to lie near the southern terminus of a major N-striking, W-dipping normal fault zone where it intersects at least one major E-dipping normal fault. Both fault zones appear to have accommodated Quaternary displacement (probably Pleistocene). Multiple fault splays associated with the fault intersection and horse-tailing end of the W-dipping fault zone probably account for increased permeability in the area thereby facilitating the geothermal activity.