| Title | Assessing Structural Controls on Geothermal Fluids from a 3D Geophysical Model of Warner Valley, Oregon USA |
|---|---|
| Authors | Jonathan M.G. GLEN, Gerry G. CONNARD, John CASTEEL and Patrick WALSH |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Crump geyser, Warner Valley, Oregon, hot springs, geothermal, faulting, geophysics, gravity, magnetics, 3D modeling |
| Abstract | Warner Valley in southern Oregon (USA) is the site of a geothermal system that hosts several hotsprings in addition to Crump geyser – a geysering well that soon after being drilled in 1959 underwent frequent eruptions of boiling water. This, and thermochemical studies that estimate reservoir temperatures of 150°C, have prompted ongoing geological and geophysical investigations. Warner Valley is situated in a tectonically complex region in the northwest corner of the Great Basin - a basin and range province characterized by east-west extension. The regional geology consists predominantly of Neogene volcanics that have been faulted by a series of obliquely oriented NW and NNE-trending extensional faults. The valley forms an asymmetric graben, with the NNE-trending range front fault along the west Warner escarpement exposing over 600m of section. Warner Valley seems to be similar to other extensional geothermal systems in the Great Basin, which arise from deep circulation of meteoric water along major normal faults. This is evident in the proximity of Crump Geyser to the principal rangefront fault. Localization of surface hydrothermal features and patterns of borehole temperatures and flow suggest that secondary fault interactions may play a role in controlling geothermal fluids along the rangefront, but their presence within the basin is obscured by the basin fill. The large contrast in properties (density and magnetic susceptibility) between the basin sediments and volcanic rocks render potential field methods (gravity and magnetics) particularly well-suited to mapping and modeling subsurface geologic structures such as faults that juxtapose contrasting rock types and lead to distinct gravity and magnetic anomalies. We have performed geophysical studies in Warner Valley and surrounding regions, collecting high-resolution gravity and ground magnetic data along several detailed transects around Crump Geyser as well as regionally to characterize intra-basin and basin-bounding faults, constrain basin geometry, study fault interactions, identify areas favorable to hydrothermal flow, and ultimately to guide exploration of the area’s geothermal system. We have also performed density, magnetic susceptibility, and magnetic remanence measurements on samples from several stratigraphic sections in the hills surround the valley as well as on two ~1000m cores recently drilled near Crump Geyser. We are using these rock-property measurements, along with borehole geophysical logs, to correlate subsurface and outcrop stratigraphies. These data, along with recent 1:24,000-scale geologic mapping and newly-acquired seismic and airborne magnetic surveys, place critical constraints on 2D and 3D potential field models we are developing of the subsurface around Crump Geyser. Our results reveal buried intra-basin structures that intersect the rangefront at Crump Geyser that we suggest are principally responsible for promoting permeability and facilitating hydrothermal flow within the Crump Geyser geothermal system. |